With just 600mA peak current, an LM317 on a heat sink should be fine for this application. It should be positioned between the TIP32 and the fans, so that it turns on and off just as the fans do in your drawing in #16. It will be configured as a standard voltage regulator - see the data sheet. The variable resistor can be a 1K with a fixed resistor of 470Ω in series with it. This will prevent the set voltage from dropping below 4V. Use 680Ω for 5V, or 820Ω for 6V minimums. Do some testing to determine which minimum voltage works for your fans.

 

So one point on the fan-equivalent-resistance curve is 60 ohms at 12 V. If you have a variable DC supply and a VOM, connect the fan to 6.0 V and measure the current. This will give a 2nd point, and a linear approximation between them will work well for determining the details of the design.

ak

Thanks. I got 0.05A at 6V and plotted the line. 9V works out to 0.1A, or 9/.1=90 ohms. The difference in resistance between 12V and 9V is 30ohms. P=V^2/R = 9^2/90 = 81/90 = 0.9W. So I really need to try some resistors, maybe 10, 20, 30, with much higher power ratings (say 2 or 3 W) than what I have. Off to Sayal......
Thanks for your help.

 

With just 600mA peak current, an LM317 on a heat sink should be fine for this application. It should be positioned between the TIP32 and the fans, so that it turns on and off just as the fans do in your drawing in #16. It will be configured as a standard voltage regulator - see the data sheet. The variable resistor can be a 1K with a fixed resistor of 470Ω in series with it. This will prevent the set voltage from dropping below 4V. Use 680Ω for 5V, or 820Ω for 6V minimums. Do some testing to determine which minimum voltage works for your fans.

Thanks. This makes sense. I'm going to get a LM317 and heat sink and give it a go. I'm also going to try higher power resistors (as per AnalogKid) and try that as well.
Thanks for your help.

 

With just 600mA peak current, an LM317 on a heat sink should be fine for this application.

1. Actually it's 600 mA steady state current, and up to 2 A peak starting current.
2. An LM317 has almost 2 V of dropout voltage (minimum input-output differential), a lot for this kind of circuit.
3. Since the 317 can handle all of the fan power, there is no reason for the TIP32 if it is in the circuit.

I see this as either the comparator controls a low dropout regulator based on the other opamp and the TIP32 for Vout max = 11.9 V, or the comparator controls an LM317 for Vout max of 10 V. The 317 is a more simple approach, but it costs you about 15% of your fan power.

ak

 

This works and I'm happy with it. Thanks for all your help.