Encouraging results

Some quick tests work out quite well

Had a tiny pocket of time with which to test a few things with my aforementioned dummy load I just build. In looking through my collection of junked wall wort power supplies I found a low power one, a 9V 210mA. I wanted to check and see if the voltage headroom for the opamp would be worse. As it turns out, the stated 9V is actually 11.3ishV but whatever. I knew the supply would only need to deliver a few milliamps so I ventured to see how much exactly.

Turns out it’s a mere 7.2mA :)

To the left is the current draw of the control circuitry. To the right is the 12V battery charger putting out 3A

To the left is the current draw of the control circuitry. To the right is the 12V battery charger putting out 3A

That’s peanuts really, but I wouldn’t expect it to draw much current, at least not this end of the circuit. It simply powers the measly current needed to power the opamps and voltage regulator, with a very tiny amount of current to feed into the opamaps for the set voltage. I am a little concerned that it could use a volt or two more to be sure of getting that 3A from a variety of power supply voltages so I may up it a bit more to close the very small gap between my set voltage and voltage i’m reading off the shunt (and thus hit my design maximum). Easy thing that. Fortunately, I have quite literally a pile of wall worts kicking around so one of them will suit just fine or I can even replace it later if I am unhappy with it.

I thought, well since I’m here, how about I test it’s thermal performance. My first run of this beast resulted in me melting a MOSFET and since then I have been testing it very gingerly and not leaving it on high current settings for more than a few seconds. I taped my thermocouple to the TO-220 package of the MOSFET (which of course is mounted on a large heatsink) and ran it full out.

The temperature seemed to stabilize at around 78°C after a few minutes of operation. This is great news. It means my heatsink is sufficient. Though the device could theoretically stand up to 170°C I wish to go nowhere near that. 120°C case temperature is probably the outside maximum and I would definitely try for cooler than that. Obviously, more rigorous thermal tests will have to be done to really get a good idea of how hot it could get under heavy use.

Temperature measurements are encouraging

Temperature measurements are encouraging

This also brings up the question of an enclosure. Ideally, I would very much not like a very hot heatsink exposed where my hands could touch it (yeeouch) yet enclosing it would hamper it’s heat dissipation unless I used active cooling. The circuit itself is quite small and I really only need panel space for the display, pot, a switch or two and a LED. This makes me not want to waste a large enclosure that will be mostly empty. I’ll think on it.

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