Help with PSU (Temp control fan, load bank, & PWM circuit)

[jocanon]

Now we're talking! This works like a beauty! I'll have to play around with it a little to get it fine tuned...may have more questions, but so far the sensitivity of the thermistor seems about right and I am getting very very slight increase in fan speed with load increase due to the switching noise problem, hardly even noticible at all. The temp control circuit seems very stable now. Seems we are burning off some current in the 24 ohm resistor as it gets a little warm, nothing too crazy just warm. Curiosity has me wondering...when I turn the current up quickly I hear a quiet hissing noise, is that the "switching" you were talking about where it is converting AC to DC? The noise is only there at certain levels and only there for the moment after I turn up the current and as I am adjusting it, once it settles in after a second or so the hissing goes away, and it is very quiet, I am not concerned about it (should I be) more just curious.

 

[jocanon]

If it goes backwards reverse the outside ones.

PSS. It won't be as sensitive now so you may not see much change by touching it.

Yes it did go backward but I just compensated by adjusting the pot the other way. Also, yes it is way less sensitive, but on this particular PSU it was way too sensitive before. I think it is just right now, still need to test it out inside the PSU, but I need to get ready to take my wife out to lunch right now , will be back at it later!

 

[()blivion]

Now we're talking!

Good to see you got it figured out. So just a wiring problem?

 

[jocanon]

More or less, we also added that 24 ohm resistor between source on the FET and ground.

I am wondering now, what are my adjustment points, i.e I know the pot adjusts the tempurature at which the fans kick in and R2 adjusts the sensitivity of the pot (I took out R2 on this one by the way), I don't think those have changed, but from what I can tell it seems my max speed on the fan is slightly lower, which actually may be OK, but what part of the circuit could I tweek if I want to increase that, the 24 ohm resistor?

 

[ronv]

very slight increase in fan speed with load increase due to the switching noise problem, hardly even noticible at all.

Yes, it would seem so. The noise will still be there but the circuit is about 10X less sensitive to it now. Also moving the cap will reduce the amount getting to the gate.

The temp control circuit seems very stable now. Seems we are burning off some current in the 24 ohm resistor as it gets a little warm, nothing too crazy just warm.

The resistor is running at about 30% of it's rating so it will be a little warm.

Curiosity has me wondering...when I turn the current up quickly I hear a quiet hissing noise, is that the "switching" you were talking about where it is converting AC to DC? The noise is only there at certain levels and only there for the moment after I turn up the current and as I am adjusting it, once it settles in after a second or so the hissing goes away, and it is very quiet, I am not concerned about it (should I be) more just curious.

Could be. Sometimes inductors or transformers will make a little noise.

()blivion.

Seems like several little things. Evidently the big supply has a little bit more noise on the 12 volt line. This was made a little worse because a start cap was added fron +12 to the gate to make the fan start on power up. The circuit has a small linear region (.1 .4 volts depending on the FET) so the noise was enough to turn it on and off. We added a small resistance in the source to make the circuit less sensitive (~1.2 volts from off to on). This made it less sensitive to the noise but also had the nice effect of making the temperature band wider. Now instead of 3 or 4 degrees from off to on it is more like 25 or 35 degrees. I can send you the spice if you want to play with it.

 

[jocanon]

Once I fine tune the trim pot on the PSU fan and figure out exactly what amount of resistance I want, can I replace the pot with two fixed resistors of the same resistance I adjusted the pot to, one from the thermistor to the MOSFET gate and another one from the MOSFET gate to ground? I want to do this to make it easier to mass produce these, so I can just solder everything onto a small circuit board and not have to adjust the pot everytime I make one.

 

[ronv]

Right now you loose about 1.25 volts from the 24 ohm resistor. You can make it a little smaller and reduce the it, but at the same time the temperature spread between off and full on will be reduced and the sensitivity to the noise increased.
The pot adjusts for a lot of tolerances but you might get away with 2 fixed resistors. It's a long shot to get elecronic parts at the extreams of their specs.

The op amp version might work better for ajustment free and it gets almost all of the 12 volts to the fan.

 

[jocanon]

It seems there is something else resulting in loss of speed on the fan because I don't think 1.2v would slow it down that much, right now if I connect a jumper across the thermistor to bypass it completely, the fan only goes less than half it's full speed. That might be OK because I am not sure it needs to go much faster, but in case I do want to increase the top speed I was just wondering how. I think, correct me if I am wrong, but I think that lowering the overall value of the voltage divider between the thermistor, gate, and ground from 10k down to something lower might accomplish this.

 

[ronv]

Maybe you could measure the voltage on the gate source and drain with the thermistor jumpered?

This is the little fan. Right?

FETs are voltage controlled devices. So all you need to do is get the gate more positive (NFET) than the source by a few volts (Threshold voltage) and they start to turn on.

 

[jocanon]

With thermistor jumpered:

Gate=1.51v
Drain=6.54v
Source=6.85v

 

[ronv]

Sounds like a darlington transistor not a mos fet. What is the part number on the transistor?

Are you sure you have 24 ohms in the source and not 240?

 

[jocanon]

I am sure, I double checked with my volt meter, it's 24.4 ohms. I had to put two resistors in series because I didn't have a 24 ohm, it's a 22 ohm and 2.2 ohm, colors red-red-black-gold and red-red-gold-gold. I am using the LM-358 MOSFET left over from the dummy load.

 

[ronv]

I think the LM358 is the op amp.

 

[jocanon]

Ha, right you are , I am using the STP80NF12.

 

[ronv]

OK>

Lets double check cause nothing seems right.

The black lead of the voltmeter on ground from the supply.

Lets measure the +12 this time as well and ground at the 24 ohm resistor.

 

[jocanon]

Hm, I am not sure what happened before but the reading at the gate is now 12.42v. I disconnected the MOSFET and reconnected it in between readings, maybe there was a bad connection or maybe I didn't take the reading correctly the first time??? The readings on the drain and the source were 5.81v and 5.74v, +12 was 12.42v. I can't tell but I think the fans are going faster now, I think the top speed is fast enough, still wonder if the readings are out of whack what I might be doing wrong though. I say I think the fans are fast enough because people have been using these PSUs to power RC chargers for years with slowing the fans by shorting certain jumpers on the pins and by doing that mod the fans go much slower than my top speed as it is right now and they aren't temp controlled with the shorting the jumpers mod yet no problems have been reported so I am sure my fan speed is more than enough as is.

 

[ronv]

Hmm. Here is the deal.

From the picture of your little fans I see the current is .06 amps. at 12 volts. From your measurements there is almost as much voltage drop across the 24 ohm resistor as there is across the motor. The voltage at the source should be about 1.25 volts. Might check the solder on the 24 ohm resistor.

 

[jocanon]

No solder, I have everything on a breadboard. Does it make a difference that I am running two of those fans off the one circuit?

 

[jocanon]

The one fan did speed up a couple volts worth when I unplugged the second one.

 

[ronv]

Yes it does.
Sorry, I thought you were selling 1 or 2 supplies tied together so I assumed each would have its own temperature control.

Lets measure again.