I wanted a variable load for 24VAC that would go up to at least 12VA. Before I would connected power resistors in series but that gets troublesome when you want to change the load with a fine resolution. There are variable power resistors or rheostats but they are big and clumsy and from what I can tell expensive. So I decided to design a little variable AC load that would simulate a resistive load and that would also work with DC. After a bit of work and one board spin I came up with this:
It works like the standard DC load that you can find all over the web with an op-amp that drives an N-Channel MOSFET with a current sense resistor but on this one I added a P-Channel MOSFET in series. This way during the positive half of the AC waveform the body diode of the P-Channel MOSFET conducts and the N-Channel MOSFET provides the load and then during the neagtive half of the AC waveform the the body diode of the N-Channel MOSFET conducts and the P-Channel MOSFET provides the load.
To turn of the MOSFETs the op-amp needs to drive the N-Channel FETs gate positive and the P-Channel FETs gate negative so I added a simple +/- 10V supply. First I bring in power through USB just because that is a simple and easy was to get 5V, also I thought I might redesign this with a micro-controller at some point that will be able to do more complex loads. The 5V is fed into a voltage doubling charge pump and then that is fed into an inverting charge pump.
When the voltage to be loaded is connected to the device it is divided by R1 and the potentiometer R3 which provides the set-point for the control loop this makes the load dependent on the voltage which means this simulates a resistive load unlike most DC loads which are constant current loads.
I had a few issues with this design to begin with:
- First originally I had one gate resistor that was shared between both mosfets but I was getting some very high frequency oscillations when ever the N-Channel FET turned on, isolating the MOSFETs with separate gate resistors solved that issue.
- Next I had trouble getting the control loop stable; originally I used the jellybean part TS321 for the op-amp but no matter what combination of gate drive resistor and compensation capacitor I couldn’t get it stable. I changed the op-amp to the fancier LM7321 which is a higher current output and unlimited capacitive load op-amp and with that I got the control loop stable.
One thing I would do if I do another revision of this design would be to add more protection. I have damaged it a few times by connecting the load power without connecting the 5V.
Anyway this was a fun little project that I wanted to share :)