Voltage controlled current source

Design

I needed a voltage controlled current source (VCCS) to test some current limiting devices. A friend who knows far more than I about analog electronics recommended using a power op-amp, which is an op-amp capable of outputting a high current. I didn’t have one of these to hand, so used a regular low power op-amp to drive the gate of an N-type MOSFET. I think that a power op-amp is just a regular op-amp with some meaty FETs inside and a package capable of disappating more heat than a regular puny op-amp. I use the word puny having watched too much Electroboom on YouTube.

Please see a screen shot of the simulation of the circuit I ran using the falstad circuit simulator. The code for this simulation can be found at the end of this post. The op-amp needs a +15V positive voltage and to be grounded on the negative rail to enable about a 580mA current through the FET, from the +5V rail connected to the drain of the FET.

Using Falstad to simulate the VCCS circuit.

How does it work

The FET source follows the voltage applied to the positive input to the op-amp, providing among other things that there is enough voltage supplied to the positive supply of the op-amp. So the current flowing through the device under test will be V/R, where V is the +5V rail in this example and R is 8 Ohm in this example. Well, 8.2 Ohm if the part under test (PUT) has a resistance of 200 mOhm as shown in the simulation above.

Assembly

I used an lm324 op-amp and a buz73 FET as I had these on hand. I lashed it up on some breadboard. I’m not testing at high frequency, so the bread board was suitable. The completed circuit can be seen in the photo below.

VCCS circuit, testing some polyfuses.

I used a Hameg PSU that was in the lab to power both the op-amp and to supply the +5V rail to the FET drain. I used my Analog Discovery 2 to ‘scope the output and to generate the input for the positive input to the op-amp.

VCCS circuit connected to an Analog Discovery 2 function generator and oscilloscope and a fancy power supply.

Results

The current through the PUT, which I measured with my rinky dinky shiney new FLIR DM66 was as expected. It topped out at about 580mA due to the limitations of the design and components. This was all the current that I needed. I applied a 0-5V ramp over 5 seconds to the positive supply of the op-amp and measured the voltage over the 8 Ohm load resistor to see how the PUT behaved. Overall, it seemed to work as designed. Nobody is more surprised than I am!

One example is shown below where a 0-5V signal was applied to the positive input to the op-amp over 5 seconds and the result ‘scoped over the 8 Ohm resistor for an fpf2123 current limiter IC. We can see that the IC does not turn on for a voltage below about 1.7V. Once the current goes too high, it turns off. But it turns on again every 160ms for 10ms. This is all as it should be according to the data sheet. With this circuit I was able to check the current at which it turned off.

Voltage across 8Ohm load resistor when a 0-5V input is swept across a fpf2123 current limiter IC.

Falstad code

As promised at the start of the post, here’s the code you can cut and paste into the falstad circuit simulator to play with the design.

$ 1 0.000005 10.20027730826997 69 5 50
r 272 112 384 112 0 10000
w 272 112 272 160 0
a 272 176 384 176 8 15 0 1000000 2.89985920119757 2.9 100000
f 480 176 528 176 32 1.5 0.02
w 480 112 576 112 0
w 576 112 576 224 0
w 576 224 528 224 0
w 528 224 528 192 0
g 528 448 528 480 0
r 528 320 528 368 0 8
w 528 320 528 304 0
r 400 176 464 176 0 100
w 384 176 400 176 0
w 464 176 480 176 0
w 480 112 384 112 0
172 176 192 176 256 0 7 2.9 5 0 0 0.5 Voltage
w 176 192 272 192 0
172 528 16 448 16 0 7 5 5 0 0 0.5 Voltage
w 528 16 528 160 0
370 528 384 528 432 1 0
w 528 368 528 384 0
w 528 432 528 448 0
r 528 224 528 304 0 0.2
x 356 269 510 272 4 24 part\sunder\stest