Voltmitten

is a configurable, modular, chainable DC voltage monitoring device that will cut off all power to a load if the voltage on any of several power supply channels goes too high or too low. Voltmitten is meant to protect vintage electronics if their old power supplies malfunction and attempt to send damaging voltages into sensitive circuits.

It may not be surprising that "overvoltage" failures like this one involving an HP9825T desktop calculator or this one in an Intel MDS-800 power supply can destroy electronic components like integrated circuits. "Undervoltage" failures (e.g. one of the power supply outputs shutting down altogether) can also be destructive, as this video explains. Voltmitten aims to protect against both failure types by monitoring and switching up to four power supply channels. Two of those channels can supply negative voltages. Up to eight channels (with up to four negative voltages) may be protected if you chain two Voltmittens together.

Voltmitten is

• modular -- individual power supply channels are handled by separate modules that plug into a backplane, allowing for simple reconfiguration.

• configurable -- power supply channel modules may be customised for a wide range of high and low threshold voltages.

• chainable -- chain multiple Voltmittens together for power supplies with more than four channels.

Fair warning

This hardware design and all other materials distributed alongside it are made available for free with NO WARRANTY OF ANY KIND. Any system or device that interfaces with hardware built from this design could suffer malfunction, data loss, physical damage, or other harms. Some of these effects could be permanent and/or unrepairable. If you're not prepared to risk these consequences, don't use these resources.

Table of contents

• Is Voltmitten ready for my application?
• An overview of Voltmitten in pictures
• Assembly notes
• Operation
  • Setup
  • Usage
    • An ordinary usage cycle
    • Exceptions to ordinary usage
    • Ganged operation
• How does Voltmitten work? (A tour of the schematic)
  • Positive voltage channel module
  • Negative voltage channel module
  • The backplane
• Other notes
• Acknowledgements
• Revision history

Is Voltmitten ready for my application?

Voltmitten was designed as a hobby project by an enthusiast without formal training in electronics. There are many things that would be good to know about Voltmitten before you depend on it to safeguard a precious device. If you can find or design a more trustworthy device than Voltmitten, you should buy or build that one instead. If not and Voltmitten is your only option, here's what you should know, along with what's not known:

Voltmitten has never had a "save". So far, a pair of Voltmittens have been monitoring the +12V, +8.5V, +5V, -5V, and -12V supply inputs to the main logic assembly inside an IBM 5100 portable computer. The decades-old power supply in this computer hasn't failed yet, so there's been no demonstration of Voltmitten protecting an actual electronic device. Of course Voltmitten was tested in an experimental test setup with artificial loads, where it worked as intended.

Voltmitten timing remains uncharacterised. Although Voltmitten uses only solid-state parts that sense and switch quickly, nobody has measured (for example) how fast Voltmitten responds to voltage excursions, or whether its response time differs between modules that switch power supply channels with different voltages. The designs of Voltmitten positive and negative channel modules differ considerably, so there is reason to expect that switch-on and switch-off times may also vary between these modules in particular. This may be extremely important to your application: for further thoughts, see SEQUENCING.md.

Voltmitten's upper limits are unknown. Although certain performance bounds can be inferred from the datasheets for Voltmitten's components, no effort has been made to determine whether Voltmitten can work for voltages larger than the fairly ordinary ones required by the IBM 5100 mentioned above, nor has anyone determined what Voltmitten's current, power, or thermal limits are.

Voltmitten's mechanical shortcomings are unknown. As a modular design with separate connectors for inputs, outputs, and operating power, Voltmitten may be susceptible to malfunction if it is shaken or jarred, or it could grow less reliable as its connectors wear and tarnish over time. Nobody has investigated how great these risks could be.

It's not known how well Voltmitten tolerates non-resistive loads. There are three main categories of electric loads, where each refers to a way that the load reacts to changes in electrical current or voltage. A resistive load is the simplest type, as it has no significant reaction to such changes (so, basically, it behaves like a resistor). The other two types are more complicated to deal with, since their reactions to changes can include temporary current or voltage spikes. Voltmitten switching electrical power on or off certainly yields a change in voltage or current, and if the load has a big enough reaction, it could damage certain Voltmitten components.

Roughly speaking, devices that are "mostly electronics" (ICs and similar devices) are probably mostly resistive loads, whilst loads that involve motors, solenoids, or capacitor banks are more likely to have non-resistive characteristics.

Voltmitten won't protect against voltage inversions. If something goes very wrong in your power supply and a positive voltage channel becomes negative with respect to ground, or if a negative voltage channel becomes positive with respect to ground, then Voltmitten won't be able to block the resulting reversed current flow --- even in the likely case that the electronic switching devices on the modules are open, or even if Voltmitten is powered off! These switches contain reverse-biased diodes that allow current moving in the "wrong" direction to pass through regardless of the switch's state.

Voltmitten has not been used for very long. There's no test like the test of time, and Voltmitten simply hasn't had a chance to pass it yet.

An overview of Voltmitten in pictures

This image shows two Voltmitten devices in a tandem arrangement: together they can monitor up to eight separate voltage channels. Voltmitten comprises long, narrow "voltage channel modules" that plug into a rectangular backplane. The backplane hosts monitoring and control logic, the power supply, and the "user interface" (i.e. four LEDs and a pushbutton). (The backplane actually sits on top of the voltage channel modules.) Prominent features from top to bottom include:

• Power connectors. These connectors receive power from the power supply and send switched output power to the load. Each voltage channel module is responsible for one of the power supply's supply voltages, and each has two identical connectors for greater current carrying capacity.

• Heat dissipation zone. Voltmitten's solid-state power switches can generate heat when they pass a significant amount of current. The areas of bare metal radiate much of this heat. Note: each of these areas is also a conductor for one of the inputs from the power supply, and so different voltage channel modules have bare conductors at different potentials. Do not allow stray pieces of metal or other conductive materials to short any of these areas together!

• Solid-state switches. The large black components are the switching devices that either block current or allow it to pass through Voltmitten. Voltage channel modules that switch negative voltages happen to use a larger device than positive modules, but their current-carrying capacities are not very different.

• Backplane connectors. Two header connectors at an unusual angle (not seen here, but the places where they are soldered to the backplane are visible) connect each voltage channel module to the backplane.

• USB power. Voltmitten's monitoring and control circuitry is powered by externally-supplied +5V power delivered through a USB micro-B connector.

• LED indicators. From left to right, when illuminated, these four LEDs mean:

  • Ready: Voltmitten is receiving USB power.
  • On: Voltmitten is allowing power to flow from the power supply to the load.
  • Voltage OK Tₗₒ < |V|: No undervoltage condition exists.
  • Voltage OK |V| < Tₕᵢ: No overvoltage condition exists:

• On/Reset button. Press this button to tell Voltmitten to allow current to begin flowing from the power supply to the load. Voltmitten will obey as long as no overvoltage condition exists. As long as no undervoltage condition exists either, Voltmitten will continue to allow current to flow after the button is released.

Further details from a close-up view of the Voltmitten backplane:

• Cat hair. Design and testing assistance from Shrigley the cat is gratefully acknowledged.

• Voltage monitoring IC. The LTC2914 quad voltage monitor, which detects voltage excursions, is the heart of Voltmitten. It's U1, the chip at bottom centre.

• Other support ICs. Remaining ICs perform some basic logic operations on the output of the LTC2914, control the "user interface", and generate the "power is OK" control signal for the voltage channel modules.

• Bus connectors. Right-angle header connectors on the left and right sides of the PCB allow you to gang multiple Voltmittens together for monitoring more than four voltage channels.

Here is an even closer view of the parts of the voltage channel modules, showing the components responsible for switching, configuring overvoltage and undervoltage thresholds, and interfacing with the backplane. A module that handles positive power supply voltages appears on the left; the other module is for negative voltages. Features from top to bottom:

• Test points. TP+1 and TP-1 are test points for monitoring the module's input from the power supply and its output to the load. From top to bottom, the pins are: ground, switched output to the load, and input from the power supply. These test points are optional and may be omitted along with R+3 and R+4 or R-4 and R-5.

• Solid-state switches. U+1 and Q-1 are the heavy-duty switching ICs that pass current from the power supply to the load. Positive modules use ST's VN7007AH driver; negative modules use their VNB35NV04.

• Voltage divider series resistors. R+A1, R+B1, and R+C1 on positive modules, and R-A1, R-B1, and R-C1 on negative modules, are resistors that you select in order to set the overvoltage and undervoltage thresholds for the power supply channel switched by the module. The LTC2914 datasheet, details a procedure for selecting these resistors. Note that R-A1, R-B1, and R-C1 are not fitted on the negative module shown above.

• Overvoltage and undervoltage monitoring enable/disable jumpers. On a positive module, use JP+1 to enable or disable detection of undervoltage conditions for the power supply channel; likewise, use JP+2 to enable or disable undervoltage detection. For negative modules, this association is reversed: JP-1 is for overvoltage detection and JP-2 is for undervoltage detection. An easier way to remember for both might be: jumper 1 controls whether Voltmitten cares if the voltage is too negative; jumper 2 controls whether it cares if it's too positive. In the photo, overvoltage and undervoltage detection are both enabled for the positive module and disabled on the negative module.

• Module identification jumpers. J+1 and J-1 identify the modules to the backplane as positive and negative modules respectively. Note their slightly different locations relative to the adjacent header sockets.

• Backplane connector header sockets. JSA+1 and JSB+1, and JSA-1 and JSB-1, connect the voltage channel module to the backplane.

Assembly notes

This section is minimal: it will receive more content if it becomes apparent that people are interested in building Voltmittens of their own.

Voltmitten makes extensive use of surface-mount components, and unfortunately it may offer a poor assembly experience to many hobbyists. Some of its components may be difficult to install with an ordinary soldering iron, particularly the solid-state switching devices on the voltage channel modules. These devices have a metallic tab that affixes to a large PCB "pour" that's designed to dissipate heat, and a soldering iron may not get this joint hot enough for a good bond.

Solder paste and a solder paste stencil are recommended for easier mounting of surface-mount parts. If you don't have access to a reflow oven to melt the paste, a hot plate "preheater" like the Miniware MHP30 may be an economical alternative: use the hot plate to apply heat to different areas of the voltage module and backplane PCBs.

The trickiest part of placing the through-hole components is making certain that all of the pin headers and sockets are well-aligned. Here are some tips:

• Start with the backplane "side" right-angle connectors that allow you to chain Voltmittens together. First, plug pairs of headers and sockets into each other (each forming a kind of "wide U" shape): unlike individual connectors, these "Us" can sit on a surface in an upright orientation without tipping over. Now arrange the Us so that one connector in each U matches its final location in the backplane (refer to the close-up backplane image. Rest the backplane on the connectors and solder, or for even better alignment results, arrange two backplanes side-by-side on six sets of Us and affix the connectors on both boards at the same time.

• Next, affix the pin headers and sockets that connect the voltage channel modules to the backplane. Solder the sockets to the boards first, then plug the pin headers into the sockets as a jig for soldering the headers to PCBs.

Operation

Setup

1. Begin by collecting the overvoltage and undervoltage limits for each power supply channel in your application. This example shows the ranges of acceptable DC voltages for the logic circuitry inside a IBM 5100 computer:

   

   (From the IBM 5100 Maintenance Information Manual)

2. Assemble a positive or negative voltage channel module for each channel you wish to monitor. You will need to configure each module for its channel's voltage limits by placing correctly-valued resistors at locations R+A1, R+B1, and R+C1 on positive modules or R-A1, R-B1, and R-C1 on negative modules. Use the algebraic procedure described in the LTC2914 datasheet under "3-Step Design Procedure" in the Applications Information section to determine which resistor values to use.

   • Note that there are two slightly different procedures for positive and negative voltages.
   • For accurate thresholding, use resistors with low tolerances: the smaller the better.
   • Multiple resistor configurations can be specified for the same voltage limits: if 50kΩ, 10kΩ, and 450kΩ configure the module for the limits you need, then any scalar multiple of those values will work so long as the current draw through the three resistors is not excessive. You may wish to scale these values until you find a convenient set that matches the resistors that you have to hand or can find for sale. Note that excessive current draw may be a more critical concern for negative voltage channel modules, since the LTC2914's REF pin can pass a maximum of 1mA. See the Negative voltage channel module technical discussion for more information on how negative modules use REF.

3. Plug your assembled, configured voltage channel modules into the Voltmitten backplane. Any negative modules must be loaded from "left to right": in position 4 if you have only one negative module, or in positions 4 and 3 if you have two of them. A single Voltmitten device can host a maximum of two negative modules; if you need more, you'll have to use multiple Voltmittens ganged together.

   Use the two jumpers on each voltage channel module to enable or disable overvoltage or undervoltage protection as desired.

   All of the module positions on a Voltmitten backplane must be populated by a voltage channel module. If you have fewer than four voltage channels that you wish to monitor, populate any unused channels with a module whose configuration jumpers are set to disable overvoltage and undervoltage detection. Configuration resistors do not need to be fitted on these modules (see the negative module in the voltage channel module close-up images above).

4. You'll need to construct a wiring loom that connects your power supply to your load through Voltmitten via the connectors at the ends of the voltage channel modules. This part will likely be bespoke to your application, and to help you build it, here are the pinouts of those connectors when viewed as shown in the image above:

   On both connectors:
     Pins 1-2: Input from the power supply
     Pins 3-4: Switched output power to the load
     Pins 5-8: Ground
   
                         ###########
                         ###########
        #############################################
        #          #          #          #          #
        # SWITCHED # SWITCHED #  SUPPLY  #  SUPPLY  #
        # TO  LOAD # TO  LOAD # FROM PSU # FROM PSU #
        #          #\        /#\        /#          #
        #          ##\      /###\      /##          #
        #############################################
        #          #          #          #          #
        #          #          #          #          #
        #  GROUND  #  GROUND  #  GROUND  #  GROUND  #
        #\        /#          #          #\        /#
        ##\      /##          #          ##\      /##
        #############################################
   
   
   
        #############################################
        ##/      \##          #          ##/      \##
        #/        \#          #          #/        \#
        #  GROUND  #  GROUND  #  GROUND  #  GROUND  #
        #          #          #          #          #
        #          #          #          #          #
        #############################################
        #          ##/      \###/      \##          #
        #          #/        \#/        \#          #
        #  SUPPLY  #  SUPPLY  # SWITCHED # SWITCHED #
        # FROM PSU # FROM PSU # TO  LOAD # TO  LOAD #
        #          #          #          #          #
        #############################################
                         ###########
                         ###########
   

   Both connectors have the same pinout but are oriented at 180° to one another. The connectors themselves are 8-position ATX-style power sockets, sometimes used on computer motherboards for providing additional power to the CPU.

   The eight ground positions all connect to the same common ground shared throughout Voltmitten. Technically it's only necessary to make a single connection between any Voltmitten ground pin on any module and the ground shared by the power supply and the load, but it may be preferable for obscure "real world electronics" reasons to match the incoming and outgoing power wires with the same number of ground wires going to the same places.

   It isn't necessary to use all 16 connector positions on a voltage channel module. For low-current channels, one single wire connecting to the PSU and another connecting to the load, along with paired ground wires to match, can work.

5. Voltage channel modules that pass more than four or five amps of current may benefit from extra cooling devices. You can affix adhesive-backed heat sinks to the power dissipation areas on a module dealing with high current; a cooling fan may also help in extreme cases. Be certain that your cooling solution does not establish an electrical short between the power dissipation areas of two or more modules.

6. Lastly, connect Voltmitten to USB power via the USB micro-B connector. The "Ready" LED should illuminate; other LEDs may also illuminate in ways discussed in greater detail just below.

Usage

A good place to start is a configuration where

• the power supply and the load are connected to Voltmitten via the harness you built in Setup step 4,
• the power supply is off, and
• Voltmitten is receiving power over its USB port.

Voltmitten's user interface comprises four LEDs and a momentary pushbutton:

Here again is what each LED means, from left to right:

• Ready: Voltmitten is receiving USB power.
• On: Voltmitten is allowing power to flow from the power supply to the load.
• Voltage OK Tₗₒ < |V|: No undervoltage condition exists.
• Voltage OK |V| < Tₕᵢ: No overvoltage condition exists:

In this starting configuration, the Ready LED should be on. Because the power supply is off, then unless you've disabled undervoltage detection on all of the voltage channel modules, an undervoltage condition should exist. That means the On LED should be dark --- Voltmitten's switches are all open --- and the Voltage OK Tₗₒ < |V| LED should be dark as well. We're unlikely to find an overvoltage condition with the power supply off, so the Voltage OK |V| < Tₕᵢ should be on.

An ordinary usage cycle

Starting from the configuration described above, the next step is to turn on the power supply. Assuming it produces voltages within the ranges you've configured on all of the voltage channel modules, the Voltage OK Tₗₒ < |V| LED will illuminate: no undervoltage or overvoltage condition is present. Only the On LED is dark.

Tap the On/Reset button and Voltmitten will allow current to flow through the voltage channel modules. The On LED should illuminate: now all LEDs are on. The load receives power.

There is no way for you to use Voltmitten to cut power to the load (except for removing the USB cable, which is not recommended). When you're ready to shut things down, simply turn off the power supply. Voltmitten will likely detect and react to voltage excursions as the supply stops working (especially undervoltage), but removing power from the load is desirable at this point. Once power is off, you should be back in the starting configuration.

Exceptions to ordinary usage

Power supply malfunctions without a load: Some power supplies regulate poorly or even trigger their own built-in shutdown mechanism if they aren't connected to a load. This means the usage cycle described above won't apply: the input voltages to Voltmitten will never start within an acceptable range.

If the power supply malfunctions by making voltages that are too low, it's possible that things will still work out normally even though an undervoltage condition exists at first. Voltmitten disables all undervoltage detection anytime the On/Reset button is depressed. This means that when you press the button, Voltmitten will connect the underperforming power supply to the load, which may bring voltages back into a normal range. Once the load begins to operate, release the button to re-enable any undervoltage detection that you have configured on the voltage channel modules.

If the power supply responds to no-load conditions by making voltages that are too high, or if it engages its own safety mechanisms and shuts down, try turning on the power supply while holding down the On/Reset button. Since undervoltage detection will be disabled, Voltmitten will be ready to pass current into the load as soon as the power supply turns on. This situation is not too different from turning on the power supply when it's connected directly to the load: assuming no problems, it should bring the voltage channels up into a normal range. As above, once the load begins to operate, release the button to re-enable any undervoltage detection that you have configured on the voltage channel modules.

SEQUENCING.md has some notes about how this style of usage might affect the power-on timing for negative voltage channel modules.

Powering up Voltmitten when the power supply is already providing power within bounds: Voltmitten can't allow current to flow through the voltage channel modules unless it's receiving USB power. That said, if Voltmitten is powered up when the power supply is already producing voltages within the modules' configured ranges, Voltmitten may or may not immediately allow current to flow. Voltmitten's design has no mechanism to force it to be "on" or "off" when powering up. (This may be a feature worth adding.) The random behaviour can be a particular nuisance when undervoltage detection is disabled on all voltage channel modules, since even the (approximately) 0V voltages produced when the power supply is off will be within bounds.

A real power supply fault occurs: One very important kind of nonstandard operation happens when one of the power supply channels goes beyond upper or lower voltage limits --- here, Voltmitten should do its job and block all current from flowing through the voltage channel modules. Voltage remains blocked until the user presses the On/Reset button and no overvoltage condition exists. (What about an undervoltage condition? See "Power supply malfunctions without a load" above: in that case, Voltmitten is only waiting for the On/Reset button.)

Ganged operation

Using the "side" connectors on the backplane, two Voltmittens may be connected together to create a device that behaves like an eight-channel Voltmitten. The first image in An overview of Voltmitten in pictures shows this configuration. It's important to align the backplanes evenly when joining the two devices together: misalignment is unlikely to lead to damage to the Voltmittens, but they won't work together as intended.

Once assembled, the conjoined Voltmittens have two USB power sockets, two On/Reset buttons, and two sets of LED indicators. All of these duplicated components have an identical function, and you can use whichever button, socket, or lights you like. The Voltmittens share the power received by whichever device you plug the USB cable into, so you don't need to supply power to both of them (though nothing bad should happen if you do).

If you require more than eight total voltage channels or more than four negative voltage channels, you can chain three or more Voltmittens together. So far nobody has tried to use more than two Voltmittens in a ganged configuration, but the best estimate so far is that you could link up to around ten of them before approaching the power consumption limits of the isolating DC/DC converter that Voltmitten uses. Note that if you somehow have the desk space to chain more than ten devices, providing power to more than one USB socket is not guaranteed to work: the DC/DC converters have no way to communicate with each other about how to share the electrical load evenly.

There is no difference between the way you operate a single Voltmitten and the way you operate multiple ganged Voltmittens. You do not have to configure or use voltage channel modules any differently in a ganged setup.

How does Voltmitten work? (A tour of the schematic)

We'll examine the three major components of Voltmitten separately.

Positive voltage channel module

The positive voltage channel module is described by this portion of the Voltmitten schematic:

At top left are the symbols for the socket connectors that connect Voltmitten to one of the channels of your device's power supply and onward through to the device's load. The two connectors are duplicates of each other and together give the module more current-carrying capability; for smaller currents it is only necessary to use a single connector.

The pins marked A_SUPPLY receive a positive voltage from the power supply, specifically the voltage that this module is configured to handle. The pins marked A_SWITCHED connect that voltage to the load. Current along the switched voltage channel can only flow from A_SUPPLY through to A_SWITCHED if Voltmitten is "on". The remaining pins tie the power supply ground to the module ground (which the same ground as everywhere else in Voltmitten). While this connection is only used to establish the 0V reference voltage for overvoltage and undervoltage detection --- and therefore a small "tap" on the ground wire connecting the power supply directly to its load may suffice --- the interconnected socket pins allow you to use the same "through" wiring that you used for the positive voltage wiring.

At bottom left are the symbols for the connectors that connect the module to the backplane. These connectors are not duplicates of each other; together, they carry six separate voltages and signals.

VCC is the +5V power supply to the module from the Voltmitten backplane, and GND is the ground common to all of Voltmitten, the power supply, and the load. PWR_OK is a +5V active-high signal indicating that the module should allow current to flow from A_SUPPLY through to A_SWITCHED.

The remaining three signals are some of the inputs and outputs of the backplane's LTC2914 voltage monitoring IC. REF is a fairly precise +1V voltage reference that is not used by positive voltage modules. This leaves VH1 and VL1, which are analogue signals generated by the voltage divider series resistors shown at top centre:

The LTC2914 voltage monitoring IC detects an overvoltage condition for voltage channel 1 if a voltage greater than 0.5V is present on its VL1 pin. This detection will cause Voltmitten to cut off all power to the load across all channels. Similarly, it detects an undervoltage condition for that channel if a voltage below 0.5V is present on its VH1 pin. (Likewise VL2 and VH2 for channel 2, and so on.)

By selecting precise resistor values for the voltage divider series resistors R+A1, R+B1, and R+C1, you can take the critical voltage limits for a positive power supply channel --- for example, a +5V channel that must never exceed +5.5V or fall below +4.5V --- and transform them into VL1 and VH1 voltages that will trip the LTC2914's +0.5V-based overvoltage and undervoltage detection conditions. So, in our example, the correct choice of resistors will mean that if the power supply input voltage A_SUPPLY exceeds +5.5V, then VL1 will exceed +0.5V.

An algebraic procedure that tells you what resistor values you'll need for a specific pair of overvoltage and undervoltage limits appears in the Applications Information section of the LTC2914 datasheet under the "3-Step Design Procedure" heading. See also the note about choosing resistor values under the Setup subsection.

With the JP+1 jumper, the user can connect VH1 to Voltmitten's +5V power supply, which can't ordinarily fall below +0.5V. This disables the module's undervoltage detection. Similarly, JP+2 allows the user to connect VL1 to the shared ground, which can't exceed +0.5V by definition; thus, the module's overvoltage detection is then disabled.

Finally, test point pairs TP+A1 and TP+A2, TP+B1 and TP+B2, and TP+C1 and TP+C2 "straddle" the three voltage divider series resistors. These test points are ordinarily unused, but they do allow through-hole resistors to be fitted instead of SMT parts, or conceivably even potentiometers.

At bottom right the switching IC appears:

The PWR_OK signal from the backplane connector controls this device directly. A_SUPPLY goes in and A_SWITCHED comes out. The device possesses a few additional feature pins (CS and SEN), but these are not used.

The remaining items in the schematic include a test point header that you can use to measure voltages:

You may not have fitted this assembly if you didn't see a need for it when you were building Voltmitten, but if you did, the two 1K resistors you placed on A_SUPPLY and A_SWITCHED are there to limit the consequences of an accidental short-circuit across the test pins.

Finally, there is this "polarity selection jumper":

This header engages a particular part of a 2x2 socket on the backplane, completing a connection that tells the LTC2914 IC that the module is a positive module. It performs no electrical function for the module itself. For more on how these jumpers work, refer to the backplane section.

Negative voltage channel module

The negative voltage channel module is described by this portion of the Voltmitten schematic:

Many parts of the negative module are virtually identical to their counterparts on the positive module: all of the connectors and the test points are the same. The polarity selection jumper is also very similar but is placed in a slightly different location to its position on the positive module so that it engages a different row of the 2x2 backplane socket mentioned earlier.

The first substantial difference appears in the arrangement of voltage divider series resistors:

The operating principle behind this voltage divider is basically the same as the one for the positive module, but some details are different. Notice that instead of connecting to ground, one end of the resistor series connects to the +1V REF reference voltage. This positive voltage allows the LTC2914's +0.5V-based overvoltage and undervoltage threshold to work with negative A_SUPPLY voltages, because the voltage divider can be configured so that VH1 and VL1 will "straddle" +0.5V as long as A_SUPPLY remains within the bounds that you require. As with positive modules, an algebraic procedure for designing this configuration to your preferred overvoltage and undervoltage limits appears in the Applications Information section of the LTC2914 datasheet under the "3-Step Design Procedure" heading.

Note: when considering those limits and applying the procedure, remember that "overvoltage" for negative modules means a voltage that is excessively negative, while "undervoltage" means a voltage that isn't negative enough. Intuitively, "overvoltage" for both modules means "too strong", and "undervoltage" means "too weak". The polarity selection jumpers on positive and negative modules tell the LTC2914 IC how to apply the correct interpretation to VH1 and VL1 voltages that cross +0.5V, since those events have opposite meanings in both modules.

As on the positive module, negative modules have jumpers that allow you to disable overvoltage or undervoltage detection for the module. The JP-1 jumper can disable overvoltage protection by tying VH1 to +5V, while the JP-2 jumper can disable undervoltage detection by tying VL1 to 0V. This arrangement is opposite to how it works for positive modules.

The next substantially different portion of the negative module is the switching mechanism:

On the negative module, the PWR_OK signal can't be used to control the driver IC (at right) directly: the voltage between PWR_OK and A_SUPPLY could be quite large. If that voltage were passed directly into the driver's gate input, the driver would try to protect itself with an internal voltage clamp, and that could pull PWR_OK well below +0V. Instead, PWR_OK feeds into an optoisolator (left) that switches a separate voltage into the driver's gate input. The voltage comes from a small voltage regulator (top) that creates a voltage up to +5V higher than A_SUPPLY, an acceptable input to the gate.

In this arrangement, negative modules will undoubtedly not be able to switch voltages weaker than a certain value reliably: besides the fact that the threshold gate voltage may need to be at least 2.5V above A_SUPPLY, the voltage regulator will also trim a bit of the difference between GND and A_SUPPLY, and so will the optoisolator. -3.3V is probably about as small as you'd want a negative voltage to be but has not been tested; -5V has worked reliably so far.

Remaining discrete components have miscellaneous functions. C-1 and C-2 are required by the voltage regulator. R-1 is a current limiting resistor and the precise value listed is not required: a 1kΩ resistor as used in many other places on Voltmitten will do. R-3 is specified by the driver datasheet and R-2 is there to pull down the gate input if PWR_OK falls.

The backplane

The Voltmitten backplane is described by the left half of the Voltmitten schematic:

The top part is the voltage monitoring and control apparatus. Below, connectors are to the left, and accessories are to the right. We'll begin with the accessories:

These components at lower right are Voltmitten's power supply. A USB micro-B connector accepts a +5V input and feeds it into an isolating DC/DC converter that produces the +5V power used throughout Voltmitten. An isolated +5V supply allows Voltmitten to use the same ground reference as the power supply it monitors; it also reduces the risk of a ground loop that could disrupt the LTC2914 IC's voltage measurements.

The On/Reset button at centre right is a momentary switch that pulls down the active-low ~START control signal. It likely matters very little, but a 24kΩ pull-up resistor is fitted instead of a more typical 10kΩ because ~START could be pulled up by multiple Voltmitten devices ganged together. See the description of the bus connectors below for more details.

Next to this are the 2x2 jumper sockets associated with slots 3 and 4 on the Voltmitten backplane, the two slots that can host negative voltage channel modules. These sockets receive the polarity selection jumper header pins on the modules. A positive module will bridge pins 3 and 4 of J3 or J4, while a negative module bridges pins 1 and 2. Note that two negative modules will pull the LTC2914 IC's SEL signal to ground, two positive modules will raise it to +5V, and any other configuration will leave it floating. The LTC2914 interprets this final condition as a negative module in position 4 and a positive module in position 3. This is the reason that negative modules must be installed on the backplane from "left to right".

The final accessories are these two LEDs:

The "Power lamp" indicates whether Voltmitten is powered; the "Active lamp" illuminates when Voltmitten is allowing current to flow between the power supply and the load. This lamp is driven by the same PWR_OK signal that tells all of the voltage channel modules when to operate.

Although multiple Voltmitten devices can be ganged together, each Voltmitten generates PWR_OK independently. If you notice a disagreement between the Active lamp states on chained Voltmittens, a malfunction has occurred. One place to investigate might be this part of the control circuit:

Voltmitten control logic takes two active-low input signals, ~START and ~PWR_FAIL, and generates the active-high output signal PWR_OK. A (CMOS) 555 timer performs this task, which may be more complicated than necessary, but may also allow some quick-and-dirty timing hacks in a pinch. As configured, the timer operates in bistable mode to act as a simple SR latch with inverted inputs. An NPN transistor buffers the Q output to generate PWR_OK.

If ~PWR_FAIL is active, PWR_OK will always be low. Otherwise, if it's not on already, PWR_OK will rise and remain high if ~START pulses low. This condition persists until ~PWR_FAIL goes low.

When you gang multiple Voltmittens, ~START and ~PWR_FAIL are common to all of them, but each device generates its own PWR_OK signal. ~START originates from the On/Reset button described above, while ~PWR_FAIL comes from here:

The LTC2914 voltage monitor IC appears at left, receiving VHx and VLx inputs from all four voltage channel modules (as well as the SEL input that indicates how to interpret them; see above). If all voltages are within bounds, it pulls up the ~OV (overvoltage) and ~UV (undervoltage) signals; otherwise, either signal (or both) is pulled down as appropriate. These must be combined into the single ~PWR_FAIL signal that feeds into the 555. A 74LS33 quad NOR buffer IC accomplishes this while driving two indicator LEDs (which turn off if an overvoltage or undervoltage condition arises). The same IC also allows ~START to disable undervoltage detection when active for reasons described in the Operation section.

The 74LS33 is an open collector device, so if multiple Voltmittens are ganged together, the 'LS33 on a Voltmitten where all voltages are OK won't fight an 'LS33 on a different Voltmitten that detects a problem and lowers ~PWR_FAIL. R5 is a relatively weak 47kΩ pull-up for the same reason.

On to connectors:

The backplane has four sets of header pin connectors like these that are used to connect voltage channel modules. The PWR_OK, REF, VCC, and GND lines are as described in the positive module section and are common to all four module connector sets. The VHx and VLx are connected directly to the LTC2914 voltage monitor IC and are not shared with any other module connector.

Finally, the left and right sides of the backplane both have a pair of header connectors for ganging multiple Voltmitten devices together:

As mentioned, ~START and ~PWR_FAIL are shared active-low signals that any Voltmitten device can pull down. VCC is also shared, so only one Voltmitten needs to receive USB power in order for the chained devices to work. (Indeed, powering more than one Voltmitten is not recommended.)

Although mechanically this arrangement allows for chaining together an unlimited number of Voltmittens, realistically there must be some limit to the number of devices you can gang before overall operation becomes unreliable. No effort has been made to determine what this limit might be, but here are some factors that could determine where it lies:

• The combined effect of multiple R5 47kΩ pull-up resistors could overpower the 74LS33 IC on a Voltmitten that attempts to detect and signal a voltage problem by pulling down ~PWR_FAIL. Note however that the 'LS33 doesn't have trouble sinking the current passing through an LED and a 300Ω resistor, so this constraint alone likely still allows chaining dozens of Voltmittens.

• Similarly, too many R6 pull-up resistors could conceivably overpower any one Voltmitten's On/Reset button. This too is likely no practical limitation.

• The isolating DC/DC converter PS1 has a 600mA maximum rated output current. Voltmitten power consumption peaks at around 50mA, so hypothetically this places the limit somewhere near 12 devices, or 96 separate voltage channels. You might choose to undershoot this limit a certain amount to provide a bit of a power margin.

• A long chain of Voltmittens may take up more space than you have to spare. Two units connected in tandem have a width of about 220mm; the twelve mentioned above would extend over 1.3m, or much of the width of a full piano keyboard.

Whatever the limit is on the number of Voltmittens you can chain together, remember that Voltmitten has no mechanism to determine whether you've exceeded it. So far two Voltmittens have been shown to work well together, but ganging any more than two remains untested.

Finally, when ganging Voltmittens, it's recommended that USB power be supplied only to a single unit. The isolating DC/DC converters that Voltmitten uses are not designed to share a load, and it's possible that if multiple powered units together encounter a current draw that exceeds what a single converter can supply, an unbalanced loading could cause one of the powered converters to overload and shut down. The remaining converters would likely shut down soon afterwards. This would leave you no better off than if you had tried running Voltmitten off of a single USB cable in the first place.

Other notes

To the fullest extent possible, this design and any documentation that accompanies it are released into the public domain. Nobody owns these resources.

Acknowledgements

This project would not have been possible without

• Technical advice from Andrew M.,
• Ken Shirriff's technical writings about power supplies and older integrated circuits,
• Jerry Walker's videos mentioning the risks of incorrect power supply sequencing for certain kinds of ICs,
• CuriousMarc's exploding HP9825T power supply, the inspiration for this project, and
• bitsavers.org's archived technical documentation.

Revision history

13 November 2021: Initial release (Tom Stepleton, stepleton@gmail.com, London)