A couple weeks ago, I had a discussion with one of my co-workers about electronic equipment. I complained that alot of the equipment that I want is very expensive. I told him some of the requirements that I’m looking for. After a couple minutes talking, he said to me “Why not build it yourself? The requirements are not that stiff.” “Crap, he’s right”, I said. After couple weeks researching the topic and brushing up on Mosfets, I built a very simple one. There is a alot of refinement that could be done on the circuit, but it demonstrates the topic nevertheless.Today’s post will talk about how this electronic load circuit works and how to build one yourself.
What is an electronic load?
An electronic load is a device design so a power supply can draw a certain amount of current without desipating tons of heat. Electronic loads are useful for testing a power supply’s efficiency, current limit, etc.To understand how the circuit works, I’ll talk about two crucial things involving mosfets and op amps. Let’s talk about the mosfet aspect first.
Although the picture above looks like a bunch of lines for hobbyists, this VDS vs Id characteristics shows how the mosfet will behave depending on the voltage applied at the gate and source (VGS for short) Mosfets have 3 modes of operations: cutoff, active, and saturation. When the mosfet is in cutoff operation, it does not turn on. This is due to the voltage applied to the gate and source not high enough to turn it on. Active is a state in which the mosfet’s gate has enough voltage to allow the mosfet to behave as a variable resistor. Finally, saturation is when the mosfet behaves like a switch. For this load to work, we need it to enter active mode since we can set the mosfet to draw a certain amount of current.
Now let’s talk about one common configuration using an op amp: a unity amplifier. The sole purpose of a unity amplifier is to make sure that the voltage seen at the non inverting pin of the op amp (+ pin) appears at the output. Although it sounds simple, these amplifiers are useful to prevent the input source from getting affected from output impedance. However, we will be using this configuration for another reason.
Now let me explain how the circuit works. First the user sets the voltage using a potiemeter (R2 in the schematic) and gets set at the inverting pin (- pin). Although the op amp looks like its configured as a comparator, it’s actually set as unity amplifier due to the mosfet. To make sure the voltage at the non inverting pin appears at the inverting pin, the op amp must set a certain voltage to the mosfet gate. Depending on the gate voltage, the mosfet will draw a certain current until the voltage at R1 is equal to the voltage at the non-inverting pin.
How to select your mosfet?
Unlike my other tutorials, there’s a high possibility you’ll damage the mosfet in this tutorial, if you’re not careful. When looking at the datasheet for a mosfet, remember to pay attention to the drain current (ID) and drain to source (VDS) ratings. ID determines how much current the mosfet can handle before exploding and VDS determines the maximum voltage that can be applied for safe operation.
Now for the little known parameter you should REALLY look at when reading mosfets datasheets: the safe operating range (SOA). Keep in mind, mosfets liked to be switched on and off. Rarely do you want the mosfet to work with analog voltages. The SOA tells you the recommended voltage and current that the mosfet can handle before exploding.
Build Your Own!
The following circuit is designed to handle 20V from 0A-5A. You’ll need the following to build this circuit
1x 10k Poteimeter
1x Buz11 Mosfet
1x 10Watt TO-220 Resistor
1x 741 Op Amp
2x .1uF Capacitors
1x Cooling Fan (optional)
Consequentially, you’ll need the following tools for this circuit
1x +15V/-15V power supply
1x +32V/5A Power Supply
Now assemble the circuit as shown in the picture below. Remember to attach the heatsinks to the resistor and mosfets! THEY WILL GET DAMAGED WITHOUT THE HEATSINKS! If you have a cooling fan lying around, turn it on and direct it towards the resistor and mosfet. The colder these components run, the less likely they will get damaged.
Before Turning The Circuit On.
Do not apply 20V to the mosfet just yet.You want to make sure the voltage at the poteimeter is 0V, otherwise your power supply will go into current limit. Apply power to the op amp/poteimeter. Make sure the voltage at the poteitmeter is 0V. Now apply power to the mosfet and start turning the knob. You should start seeing the circuit consuming a certain amount of current will keeping the voltage of the power supply the same.
Well that’s it for today! Thank you guys for reading this post and if you have any questions, comments or concerns, please post a comment. Also, be free to follow me on twitter. Have a wonderful week!