After digging up the old topic concerning the Dummy Load, I finally decided to go for a separate thread on this one.
I built this dummy load* as well, using a 9k/9k divider for the set voltage at the second opamp, and a IRFZ44N n-MOSFET.
My load consists of 10 resistors 10R/2W in parallel, which add up to roughly 1.2R.
First, I ran into the same problem with supply voltage and thus switched to a 9V battery supply.
With approx. 8 V available, the dummy load works fine and is capable of more than 1.5 A (that's the limit of my homebrew bench PSU).
I just added a fan to the flimsy aluminium heat sink that I had in my parts box.
With that I'm quite stable at 70°C at 1.25 A. Without fan I'm quickly exceeding 120°C at the MOSFET case.
Note: With the fan in parallel, I have an operating voltage of ~7.8 V DC.
But:
I wondered if the dummy load would regulate the current with variations in the tested power supply.
It does not.
If I dial in a current of 1.0 A at 10 V DC, and crank up the PSU to 15 V DC, the current rises to about 1.2~1.25 A.
(current measured with an inline multimeter in 2A range and another multimeter measuring the voltage across the load resistor)
I couldn't yet figure out, why that's happening.
It's not like the MOSFET was just acting as an adjustable resistor, but the current is not really as stable as I would have expected.
I was thinking, that maybe the power supply is too low at 7.8 V DC, but I have no second bench PSU to take that part.
So I'm trying to figure out the theory first.
Any ideas?
Or even better, advice?
Second thing:
Does anyone from Europe/Germany have a suggestion for a heat sink that fits the TO-220 package of the MOSFET
and is suitable for mounting a 40x40 mm 12 V DC fan?
(Links to Reichelt/Conrad/HBE-Shop are very much appreciated)
Thanks!
Thilo
*:
https://www.eevblog.com/forum/beginners/dc-dummy-load-problem/