Heated Bed MOSFET Power Expansion Module
As Rep-Rap style printers are getting larger their heated beds require more and more power. Also; more "exotic" filament materials are requiring higher temperatures and heated build spaces. (check out MIT's 3D printer that prints Glass) In addition, The voltage required to meet the power requirements are climbing. The heated bed resistance can only go so low before the cabling becomes an issue. It's not uncommon to find 3dprinters that are using 24 volts instead of 12.
Rather than ditch the current set of electronics an "adapter" board can be used. It acts as a middle-man between the lower powered control electronics and the heated bed.
Large transistors can be used to accomplish the task and a fairly low cost. However, wiring and mounting a free standing transistor can become troublesome.
Others are using solid state relays to do the same thing. With this solution wiring is easy and the bed power becomes isolated from the control board. The downside to this solution is the cost. Another negative to this is that the solid state relays must be driven at amuch lower speed than a fet. As a result, the heated bed has a much wider temperature variation.
The Little Driver is a Heated Bed Power Module. It solves the ease of wiring, temperature variation, and cost problems assoiated with the realy solution.
Little Driver Features
The Little Driver can have loads as large as 900 Watt (30v x 30A) of power. It can be controlled by an existing heated bed output or logic levels from the control board CPU.
- Full documentation / Not just a single pic, actual documentation HERE)
- Upgrade any 3D printer control board
- Max 900 Watt delivery to any non-inductive load
- Runs on 12v-30v
- Drives up to 30Amps (don't be fooled by claim's of 210 or 50-60 amps, go HERE to find out more)
- Uses existing output as control
- Alternate control from 3v or 5v logic
- Optically Isolated (Printer Control Board and Heated bed can have different power supplies)
- Auxiliary fan connection
- On board air flow through heat sink
- Opensource Hardware design
- 50% Smaller than the BIQU Heat Bed Power Module Expansion
- 30% Smaller than our original design.
- Unpolarized control lines. Just plug it into your Heated bed output, The +/- no longer matter.
Theory of Operation
Using The Trigger
The Little Driver 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 Driver is supplied with 4mA, the output MOSFET will turn on.
The circuit below is a model of how the Little Driver works.
Note: The output power MOSFET is not shown in this diagram. The LED and Q1 are in the same package on the board. This LED is not the indicator LED.
There are some things that are worth mentioning about this circuit. The ground (or reference) is not the same ground as on the Little Driver. In the following example, both devices are powered by a completely different power supply. Your main control board may only be designed for 12V, but you want to use a 24V 20A heated bed. The Little Driver is perfect for that situation.
When the LED turns on, the light from the LED causes Q1 to conduct.
Circuit example: The LED’s forward voltage is 1.4v or less. The CPU logic is3v. We set R3 to300 ohms. Voltage across R3 = 3-1.4. I1 =VR3 /300 = 5mA. For the same example, a 5v CPU I1 = 5–1.4/300 = 12mA.
Once Q1 is conducting the voltage across R2, causes the Power MOSFET to conduct (not shown in the diagram). It also acts as a bleed off resistor when Q1 is off.
It is also possible to use the main control board's supply as the supply for the Little Driver. The control board and the Little Driver would share a ground. This does not change the operation. However, the heated bed, Little Driver board, and your control board must be rated to use the same voltage.
Having 2 power supplies is not the only reason for optical isolation. The trigger can be used in another configuration.
Consider the configuration below.
Basically, instead of the heated bed being the load on the controller board, the Little Driver’s trigger circuit is. Most users will be using this configuration.
The calculations are very similar to the previous configuration. Some assumptions will need to be made. We will assume that the voltage drop across the controller boards MOSFET is 0V. We will also assume that the controller’s power supply is 12v and R3 is 1.99K. Then I1 = (12v – 1.4v)/1.99K = 5.3mA. If the supply voltage was 24v then we would have I1 = (24v – 1.4v)/1.99K = 11.4mA.
Remember, 4mA is the minimum required current to get the power MOSFET to turn on. At 30mA the part has a premature death.
If you noticed, we did change the value of R3 from the previous example. When JP1 is installed on the Little Driver, R3 is 300 ohms. When it is removed, R3 is 1.99K.
An important note: If the Little Driver is being controlled by the heated bed output, JP1 should NOT be installed. If it is, you will destroy the Little Driver’s optoisolator.