Hi all,
Hoping ye folks can point me in the right direction. This is my first post and I'm pretty much a beginner in electronics as a hobby, aside from a years worth of dabbling, watching videos, reading, and buying a boat load of kits and components. As a decent sized project to start on, I decided to attempt a programmable DC load. After purchasing the main components I made a start but have quickly discovered that I'm struggling on the analog side.
Attached is a schematic of what I'm working to. Originally everything was on a breadboard (arduino nano, 2.048 precision voltage reference, DAC, op-amp, MOSFET, load resistor (1Ohm 10 watt), and heatsink. All testing is being done against a Tenma power supply set to 5V. Without any calibration at the DAC, I was seeing expected current (+/- 1-3mA) up to about 800mA. Every 100 mA after this, I started to see significant inaccuracy and fluctuations. Wiggling wires also seemed to cause issues and I measured varying voltage drops between different points on the ground buses. I moved everything to a single ground bus and played around with where I connected the load ground with only minor success.
Thinking it might be something to do with the cheapo breadboards, I (probably badly) moved everything after the DAC onto a through hole prototype board to see if that helped matters (swapping the 1ohm 10 watt for 10 x 10ohm .6 watt resistors in parallel). Maybe it's my soldering or I've somehow mucked up my connections but it seems to be even worse. At lower currents, I'm getting 4-5mA more than expected and only 920mA for expected 1A.
As a last resort, I put a 10uF electrolytic capacitor directly across the load connections and am now seeing the power supply track to within 3-6mA for currents up to about 1.4A with zero fluctuations on power supply current readout.
This has me really muddled as I haven't seen this done in any schematic on the web and kinda seems like cheating
. I'm more looking for direction here in terms of things to try. Could it be my terrible wiring or soldering, could it just be the power supply itself, is my schematic dodge. Any pointers or guidance in terms of how to diagnose the problem would be very welcome. I have very limited test equipment - just a few Aneng multimeters, a cheap DSO-138 scope that I soldered myself, and a few other bits and bobs.