Anyway, phase two was a success. Using the big mosfet and the CPU heatsink.
Gotta love AMD OEM Coolers. It's like a small block laptop cooler. Using my "Lighter Putter Outer 100" from my other tongue in cheek post to run the heatsink fan and everything worked the first time I plugged a load into it.
I even trusted it enough to connect my biggest LiPo to it. 5Ah 25C 12.6V (capable of an alleged 125A). SAFETY TIP!: Along with a LiPo analyser which gave me a loud alarm if the pack unbalanced or a cell dropped below 3V.
With the ExTech monitoring true current I trusted the Chinese battery analyser to watch the voltage and cell volts. It's current reading disagreed with the Extech by 400mA, so I ignored it.
Very impressed by the AMD cooler. With my kitchen temperature probe stuck in the heat grease beside the mosfet the highest temp I recorded was 37.4*C with the load pulling 3.0A It was able to consistently hold that temperature for half an hour at 3.0A while the lipo discharged and I could see the temp fall by about 0.1*C for each few 100mV the LiPo dropped by. Even at 3 amps I could comfortably keep my finger on the mosfet case.
The little thermistor lighter putter outer worked perfectly. The thermistor all insulated up and stuff into the heatsink core. When I put the load on the fan started slow after a few seconds, got to full speed fairly quickly, drawing 100mA. When the load was shut off the fan continued to run for 10 or 20 seconds before shutting off. When the load was running at only 0.5A the fan ran at about 50%. I have no idea how I managed to randomly set the thing perfectly without trying, but it pretty much is perfectly tuned. If anything the fan could be backed off a bit more.
I have days when things always fail and I get frustrated. Today, everything worked. Pretty much the first time I plugged it in and tried it, it worked.
There are bugs however. The PNP running the fan is sinking 4-5mA for no good reason when it should be off and the fan shouldn't be that variably when it's being driven from a LM393 comparator, it should be a hard on/off. I'm not sure I want to fix the latter, but I will probe the thing to see what's going on. Also when I touch the breadboard or even the heatsink I get another 200mA current, so I have open noise being introduced from my hand somehow. (I really should probe it!)
I couldn't test it to 5A as I'm still using just a voltage divider with a trimmer and 3A was all I could get it to go to. 3A actually required I power it from 15V which kinda upset the PNP driver for the fan. It was merely toasty.
Next plan is to implement the better control circuitry with the opamp and voltage reference.
Other things I learnt today.
1. Using overrated cables (12AWG) makes soldering connections difficult. My 25 watt Weller struggled and I have several burns on my finger tips. 14 or even 16AWG probably would have done!
2. Soldering single core wire to mosfet pins is a nightmare. They don't flex when the solder takes so any movement of your hand at all when you release the heat and the joint is bad. Later I found it is better to strip back much more single core and wind it around the pin first, then just fill the winds with solder. I might redo the gate joint this way.
Busy day:
The circuit:
The high current side:
* I'm using the CPU cooler's spring bracket and a high strength carbon fibre strut bar (from an RC heli) to hold the mosfet in place. It is putting sizable (enough) force on the mosfet.
The Fan: