I have been taking an online course in electronics called Contextual Electronics with Chris Gammell as the instructor.
We currently are working on a small board that acts as a current sink. The idea is that by using a potentiometer, op amp, and N channel Mosfet, we can control the amperage that goes into a load ( LED, battery testing, etc. ).
So far we have been creating the schematic, picking parts, and laying out the PCB. We have been able to get this done in about a month so far. Next step is to order the boards, probably via OSH Park, order the parts ( Digikey ) for me, assemble, and then test.
My desire to understand how this works better and my eagerness to see something in the physical got the better of me this past weekend. As a result, I replicated the current control portion of the schematic on a breadboard.
The “active” parts I used are:
| Type | Part |
|---|---|
| Op Amp | MCP601 |
| N Channel Mosfet | IRF510 |
These are not the same parts we are using for the course, but they are what I had on hand and wanted to try to understand how the circuit works by being able to probe for voltages, measure current, and “twiddle” the power supply knob to see the impact of higher and lower input voltages.
What I learned is that by adjusting the input voltage to the Op Amp, it would cause the FET to allow current ( and voltage ) to pass through. Once the return voltage to the Op Amp reached the input voltage, the Op Amp would not need to let current through. Then, the return voltage would decrease and the Op Amp would need to let more voltage through again.
From what I can tell, this happens so rapidly you can’t really tell just by using a voltmeter. I have yet to put it on my oscilloscope to see if I can visualize what is going on.
But, this cycle of the op amp allowing voltage through, the feed back voltage increaseing to meet the input voltage to the op amp and then the op amp decreasing voltage, the feed back voltage decreasing until the op amp increases voltage, just keeps going in a loop….
Until of course, I change the input voltage via the potentiometer. This process lets me control the amount of current flowing through the FET by controlling the amount of voltage passing through the op amp. In effect, I can use low voltage ( like a digital signal ) to control a very high amperage.
In my test, I was only testing from 0 to 135mah. But, what mattered was that I could see the potentiometer that controlled the op amp input voltage control the FET output amperage.
Breadboard mess with small perfboard for my .1 ohm resistor
Supply voltage
Milliamps passing through FET at near maximum voltage supplied to op amp v+
Voltage making it to v+ ( I have a voltage divider that decreases the voltage from the supply before going into the potentiometer – that is part of another divider).
Video of my hand adjusting the potentiometer to see the current going through the FET decreasing and increasing.