#MintTinPS_882

A simple power supply board for "standard" mint tins. Examples: Altoids, Penguin Mints, Mintz. The board is sized/shaped to fit in the tin while still allowing enough room for up to three AA batteries in an open holder and a mini-toggle to switch the power supply on/off.

[Note: There are current limitations with this design that drove me to design a more powerful board: the MintTinPS_604 which uses a MAX604 regulator instead of a MAX882 regulator. I recommend you skip this board and go with that one instead. I'll leave these files here because why not...]

This version of the mint tin power supply uses a Maxim MAX882 linear voltage regulator, which gives us the following output voltage options:

• Default voltage: 3.3V
• Adjustable voltage: 1.25V to 11V

The adjustable voltage is activated by setting a jumper (ADJV). If the jumper is open, you get 3.3V.

##Parts

• Maxim MAX882 voltage regulator (8 pin DIP)
• 8 pin DIP socket (optional, but smart)
• 0.1uF capacitor (ceramic, C2 on the board)
• 2.2uF capacitor (polarized, C1 on the board)
• 10k Bourns trimmer potentiometer (3306)
• 2-pin male header (2.54mm/0.1") & jumper

Optional for power connection:

• 2 x 2 male header (2.54mm/0.1"), right-angled
• Two 1 x 2 female headers (2.54mm/0.1") for power wires

Other:

• "Standard" size mint tin (ex: large Altoids/Mintz/Penguin Mints tin)
• .125" spacers (I'm guessing adhesive, nylon) if spacers are your thing
• Something else to fully insulate the bottom of the board

##Setting the adjustable voltage

The level of the adjustable voltage is set with a trimmer potentiometer on the board. Connect the output power wires (POW|out) to a multimeter, set the multimeter to read DC voltage, then turn the potentiometer until you have your desired voltage. You are limited by the voltage that is put out by your batteries (there is no boost circuitry in this power supply). Also, make sure you are not powering any components with the power supply when you adjust the voltage.

##Implementation

I intentionally left the power connections (POW|in/out) generic so you can connect whatever battery you wish to connect in the manner in which you'd like to connect it, as long as you keep in mind that your power source has to be between 2.7V and 11.5V.

Until 5V goes out of fashion, I'm using a 9V battery with this. No combination of inline AA or AAA batteries will get high enough voltage to provide >5V and still fit in a mint tin. When 5V goes away, three AA batteries should work.

The use of a jumper for switching voltages was intentional. Anything that can switch from a lower to a higher voltage should involve some user experience "friction." If the voltage switch is exposed on the outside of the tin, it could be accidentally bumped. Depending on what voltage was switched to, this could result in fried components. Keeping the voltage switching experience inside the tin makes more sense.

There are no LEDs for power status, either. Feel free to add them if you like. I wanted all power to go to the circuit I'm designing, not blinkenlights.

##Limitations

• Input voltage range: 2.7V to 11.5V
• Output voltage range: 1.25V to 11V (input voltage limits upper value)
• Non-adjustable voltage: 3.3V
• No voltage boost capabilities
• Maximum current: 200mA
• Linear regulators are not the most efficient

This is a perfectly capable power supply for breadboarding. If you need a power supply for something else, this isn't it.

##Motivation

I wanted to have a small breadboard/power supply combo that would be compact, self-contained, and offer two voltages. There are projects on the Internet that do this already but they either skimp on the voltages (only offer one, typically 5V) or they default to 5V and 9V (I have never needed 9V on a breadboard before, and I probably never will).

I also wanted to future-proof the power supply. Right now the common component voltages seem to be 3.3V and 5V, but 5V components are becoming less common. It makes more sense to have a power supply that offers 3.3V plus one adjustable voltage, so when 5V goes away, I'll still have 3.3V and one other up-and-coming voltage (looks like 2.5V may be the next one).

##Miscellaneous Notes

If you want the power supply to default to 5V instead of 3.3V, the Maxim MAX883 should be a drop-in replacement for the MAX882. This chip would give you everything the MAX882 offers but the non-adjustable voltage would be 5V. The adjustable voltage is handled identically to the MAX882.

The first run of PCBs from DirtyPCBs has been tested and they work fine. With the jumper out I get 3.3V and with the jumper in I get my adjustable, dialed-in voltage.

##VERSIONS

0.1 - Initial version.

0.2 - Fixed arrangement of power pins, mount holes now .125", fixed silk on bottom, fixed C1 on schematic.