insideGadgets

A while ago I built a simple constant current dummy load for testing my SMPS however the maximum load was about 1 amp and 2 amps with a fan on it and I always had to use a multimeter to measure the current. I’m looking at having a load of up to 4 amps or more, adding an ATtiny to control a 4 digit LED display to show the current, use a proper potentiometer and use a bigger heatsink.

https://www.youtube.com/watch?v=tWYfkr0ul5w

Originally I was planning to have it all powered by input voltage like before however at low voltages the STP22NF03L N mosfet that I had lying around didn’t switch on enough with the MCP6242 op-amp so after thinking about how I could do it – use 2x coin cell batteries, 9V battery, etc, I went with a Lipo battery. An upside of using a battery is that this allows us to use any input voltage between 0V to 30V to test our load with.

Before I had 1 ohm resistors which were almost getting as hot as the mosfet so I’ve gone down to 0.1 ohm 3W resistors – I added the option of another resistor in parallel if need be. I used a 1.5K resistor on the op-amp on the non-inverting side of the op-amp for a bit more fine tuning of the pot.

I had some previous code for controlling the 4 digit LED display so it was the easy part and then we’ll just use the ATtiny’s ADC to measure the voltage drop on the resistor. Download ADC_to_4_Digit_LED_Display_v1.0 and CCDL_with_4_Digit_LED_Display_v1.0

There used to be a 0.1uF cap on the drain of the mosfet but I found it made things worst – there was oscillation and the current load would randomly change anywhere between 2 to 5 amps. One problem with our design is that when no load is connected the op-amp output will be at the maximum until a load is applied, so there will be a small current spike at the start.

After a little bit of calibration, I was able to get it with-in 20-50mA of the real current, the maximum current was 8.5 amps with the multimeter in series but once it was removed it got up to 9.9 amps.

For the heatsink, I went with what I thought would be a good size but it wasn’t good enough in terms of temperature, so I grabbed an old CPU heatsink drilled a hole and cut/moved some fins out of the way for the screw – it took a while but it’s very nice now.

When testing with the new heatsink at 5 volts at 3 amps and 29C ambient – the mosfet got to 50C, 0.1 ohm 3 watt resistor was 82C resistor. I changed the resistor to a 5 watt ceramic and now I can go up to 4 amps with 55C on the mosfet and 82C on the resistor. I might change the resistor to a 0.05 ohm 5 watt so it’s still high enough voltage drop for the ADC to get a good reading and may add another resistor in parallel to see how much I can draw before the mosfet gets to 70C or so but apart from that everything works great!