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Digital Sqrt edited this page Oct 10, 2021 · 20 revisions


Welcome to the Little Monster wiki!


The Little-Monster was designed to meet the need for a low cost high power driver for 3d printer Hotend or and heated beds. We wanted a good quality product that you can rely on.

If you would like to know our design process please see our Inscructable Article written by our Design Engineer Adrian West. (HERE)

We recognize that 60Amps is is more than over kill for most heated beds. More like nuking a dead horse. But there in lies some of the problem with specifying current draw for a device like this. The real test is total resistance of the contacts when the MOSFET is on! The Little-Monster has less then 2mOhms of on resistance at 20 Amps. That is why we don't need a large heat sink. The power loss across the MOSFET is very small.

Theory of Operation

The Little-Monster uses an optically isolated transistor to turn on the MOSFET. This allows the MOSFET to be turned on in a variety of ways. It also allows the heated bed to operate at a different voltage than the main control board.

When the trigger on the Little-Monster is supplied with 5mA, the output Little-Monster will turn on. If you notice U1 (TLP182) it has 2 LED’s going in opposite directions. This makes the polarity of the heated bed output unimportant. One of the 2 LED’s is going to turn on the output driver no matter how you wire it.

When the LED turns on, the light from the LED causes Q1 to conduct.

Basically, instead of the heated bed being the load on the controller board, the Little-Monster's trigger circuit is. Most users will be using this configuration.

Ttigger connector

Note: Some users are confused at the size of green trigger connector. It is a very tiny connector. Previously, the heated bed output had large wires installed. Now these very tiny wires are being used. This is perfectly fine to do. Because the max draw is 20mA from the controller board.

Even though the output on the Printer Main Board was designed for several amps it will only deliver what is needed.

Power Supply Configurations

There are two basic power supply configurations for the Little-Monster. Single-Supply and Duel-Supply.

Dual Supply Operation

Below the power is configured with two power supplies. One of the power supplies is connected to the Printer Main Board and the other is connected to the Little-Monster.

Dual Supply

Notice that the ground of the Little-Monster and the Printer Main board are completely different. That is the advantage of optical isolation. Large power surges (like switching a heated bed) don’t affect the Printer Main Board.

The second advantage is that the Little-Monster can use a voltage that is greater than the Printer Main Board’s specified value.

Single supply Operation

Below is how you would configure the printer with one supply.
Sigle Supply

The down side of this configuration is both the Little-Monster and the Printer Main Board must be rated to use the voltage supplied.

Over Driving Your Heated Bed

Overdriving your heated bed is using a 12v bed and running it at more then 12v. Doing so greatly reduces warm up times and increases top end temperature for the bed. In fact, the top end temp will double if you double the voltage.

But therein lies one of the big issues. If your heated bed can reach 130 degrees C at 12v; now it will be doing 260 degrees C. Lead-free solder melts at 188 degrees C. This sounds like a fire hazard to us. We really don’t recommend doing this.

Now; if you have a heated bed that is toping out at 90 degrees C with 12v. You may want to run it with 16v. Your top end would be right around 120 degrees C. Not nearly as scary; but you could be exceeding the spec’s of your heated bed.

However, we also recognize the Little-Monster lends itself to this kind of activity. With that thought, there are some issues that need to be addressed.

  1. Your temperature-sensing device can’t save you from disaster. It provides feedback to the control board. If it breaks, the control board can turn on the heated bed to full. If your firmware supports detecting a damaged sensing unit, TURN IT ON!

  2. Add a fuse to the power line going to the Little-Monster. This is a good idea anyway. But if things go badly, hopefully the fuse will blow before something else bad happens.

  3. Add a thermal switch. This is an extra sensor to provide redundancy. Again, if both sensors break then you are back to the original problem.

Again, we don’t recommend this. You are on your own. If your printer catches fire ……. we told you so.

Wire Sizing

Wire is probably the most miss-understood topic in the DIY 3D printer market. We should talk about how wire is rated and how to use the ratings.

Voltage – This is really specifying how good the insulation is. The higher the voltage, the thicker the insulation is going to be.

Current – This is the one you need to watch. Any given length of wire at a specific gauge is going to have a fixed resistance. As the current increases, more ,and more voltage is dropped across the wire. This also causes a heating effect in the wire. I^2 x R = the energy that is being used to heat the wire. This is the same principle that your heated bed works on.

Consider this. If you have a heated bed that is rated for 12v and it draws 12A, its resistance is 1 ohm. Now let’s say the wire that was used to power the bed is .1 Ohms. Not much but lets do the math. Instead of 1 ohm you have 1.1. Instead of drawing 12A the system will draw 10.9Amps. 12v / 1.1 = 10.9A not the expected 12A. Power = I^2*R so 10.9^2 * 1ohm = 118.8Watts at your heated bed. You should have 144Watts. That is HUGE. 17.5% of the power intended to heat the bed is lost. Using the right size wire is very important.

  What size wire should you be using? That is easy. Consult an automotive wire gauge chart.

The automotive charts suggests a wire between 14-10 AWG. That depends on the length of the wire used. With a 14 AWG wire, there are 6 feet of length before needing to move to the next size gauge. (That’s 3 feet of power and 3 feet of ground.) If your power supply, Little-Monster, and heated bed are all very close to each other, 14 AWG will work.

We recommend 12 AWG wire. It’s a good balance between line loss and size.



The Mosterfet has a similar mounting patter and size to many of the common MOSFT board in the market. Below are the dimensions of the Little-Monster board. It will fit in a good many different brackets found on Thingiverse. Such as

Installation Wiring

If you have our wiring kit then you should have everything you need to wire the Little-Monster. We solder the terminals on the ends of the wire. However these same terminal can be crimped. The heat shrink we provide is not shown so you can see the connections.

How to wire Little-Monster

Little-Monster Electrical Specifications

MOSTERFET Electrical Specifications