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Yeah, sorry, changing subjects...I am referring to the fan in he PSU that I am going to be selling, not the one in the dummy load. Also, I am going back to using ()blivions circuit on this one with just the MOSFET, thermistor, resistor, & variable resistor.
That circuit is kind of temperature sensitive but.....
The tab on the fet is also the drain. Is it somehow isolated from the case ground? Is the fan in the drain or source side?
Well, maybe not a problem, some FETs don't have the tab connected to the drain. The circuit may need a "tweek" but I'm not sure what to tell you to change. Here is what I think is happening. In a FET the threshold voltage (the voltage where it starts to turn on) gets lower as the FET temperature goes up.
The power in the FET is maximum somewhere around 12 volts across the fet and 12 volts across the motor. So you can see how it might want to turn back on as it slows down, but you should be able to adjust it to where this does't happen if the reistance in the themistor eventually goes high enough. I have to leave now but maybe () will know what to change since he has built the actual circuit.
Just checked and even though the MOSFET is glued to the case it is still isolated, the epoxy is not conductive. I checked for continuity between the drain and the case and there is none.
Just did some trouble shooting on my own and realized that my problem is not related to the thermistor, there is something wrong with the way I have things hooked up because I took the thermistor completely off of the PSU, so it is just sitting on the table beside me, not being affected in any way by the change in temperature in the PSU, yet the circuit is operating exactly the same as when I had it hooked up to the heat sink, so the increase in the current at the dummy load is somehow making the fan go faster, not the temperature. I have the fan being powered by the same DC power that I am running through the dummy load.
...as I go from 47 down to about 20 amps, the fan slows down as it should, but then from about 20 amps down to 0 amps the fan speeds back up again, going full on at 0 amps.
At 0 amps it was 12.44
6 was 12.39
18 was 12.38
28 was 12.42
38 was 12.38
47 was 12.40
On a side note, there are current sense pins on this particular PSU that I have read if you hook them up to the load will automatically adjust voltage up or down to keep it constant. I wonder if that would help. Although by my readings voltage doesn't seem to be varrying too much anyway.
Yep just tried that, behaves normally with a wall adapter, i.e. it goes up and down with temperature but changing the current load has no effect directly on it (which would make sense because the fans are not connected to the PSU in anyway when powering them off a wall adapter). Now I am worried, I cannot power the temperature controlled circuit off a wall adapter on the PSUs that I sell. How can we get this to work?
Fear not, there is more than one way to skin a cat.
It is almost certain that the PSU is increasing voltage to the fan circuit, but is seems odd that your multimeter readings don't agree with this theory. With this info I can't help but think that you have measured the wrong points, as there is little other explanation that fits. It's a text book case of power supply drift, which should show up on a multimeter test. Are you measuring directly at where the fan circuit gets it's power from right?
Another thought, is it a two, three, or four wire fan? And are the other wires hooked up to anything in the PSU? The fourth wire on four wire fans is a speed control wire, so the PSU could be increasing the fan speed for some odd reason through that.
Finally, it could be that the circuit is shorted somewhere and thus getting it's power thorough a different path that you didn't measure.
Test
For a more or less concrete test, I would remove the circuit from the PSU entirely and just hook up the power wires, heatsink the thermistor to something like a large coin, then measure the power into the circuit, right at the connections with a meter as you run it through the full range. If it still does what it's doing, and the meter doesn't change it's voltage by more than a few volts, then the only other possible explanation is that the power to the fan is getting current limited... some how.
I just watched your video and I have a new theory of whats going on. You tap your power for your fan circuit in to the main power leads at what looks like about 6 inches up a good length of power wire. This is a bad place to get power as the wires themselves will start to act like the current sense resistors and the voltage on them will change widely with current change. You should tap into power somewhere inside the PSU case DIRECTLY on the circuit board. There should be more than one place to get 12V for powering fans.
Just be careful not to electrocute yourself when hunting for it.
I can get power right at the posts inside the PSU, there are mounting screws I can unscrew and put connectors on, but I took the voltage readings right at that point where I had the bullet connectors connected to the end of the wire and it was consistent, results posted above, if your theory is right, shouldn't the voltage have varied here?...anyway, I will try connecting to the posts tonight and see if it makes a difference.
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