Regulating rail voltage for fans

[Matttcattt]

I am building an amplifier that will have voltage rails between +/-12 and +/-40v, using OPA541. I want to use fans to cool the heatsinks.

Te two fans i have are 12v, 0.29A. I want to load both rails equally, so i will need to regulate 24-80v down to 12-24v.

I thought of the two most obvious solutions: using a regulator, or a zener diode. The regulators I have found have an input rating of 40v max. I think it will be to much voltage to drop with a zener diode, and I have not used them much, so I dont know the best way to use them.

My question: how should i regulate the voltage? Should I use a ~33v zener diode (the highest rating i can get), and then a regulator (15v)?

Thanks in advance,

Matthew

[millwood]

have you thought about using smps regulators? I use a LM simple switcher for my fans.

[UrSv]

You will have two rails which means one regulator on each will have maximum input 40 V. You can connect the two fans in series to have a 24 V fan if they are identical. You can take any standard regulator and apply the Zener diode trick on adjust to allow higher input voltage than 40 V. Check the datasheet for e.g. LM317. Use a couple of regulators to keep dissipation down. A Zener diode regulation is most likely not very good with 0.3 A and drop of 28 to 78 V on the resistor. Probably you don't want to connect an SMPS supply on the same rails that your amp run off.

Most important question though:
Why would you need fans? Are the sinks that small?

[joensd]

If you for low voltage rails like +-18V go for a regulator but anything higher you´ll dissipate quite a lot if you intend supplying the full 12V to the fans (which you probably don´t need anyway).

Supplying the fans with a little 9V wallwart can be a solution likewise.
Depending on your heatsink size you could install a switch to turn the fans off for normal listening.

An SMPS might be quite noisy but I haven´t tried myself and the OPA chips might have enough PSRR to make it inaudible.

[millwood]

how about just putting the two fans in serial and let it run off higher voltages?

[Jennice]

Hi,

Fans aren't that critical about their operating voltage. One way to regulate your rail (repeat this for each rail) is to make a voltage reference with a 12V zener diode and a simple resistor to send a few mA through the zener (or maybe even a constant current source, if your rail voltage can vary a lot). This gives you a reference point at about 12V rel. to GND.
On the positive rail, connect this reference point to the base of a NPN driver (power) transistor (a transistor with a reasonable current gain, and capable of handling the power). The collector goes to positive rail, and the emmitter goes to the fan.

This should give you approx. 11V across the fan motor.

How about it?


Jennice

[FLZapped]

Put a zener diode in series with a regulator to bring the input voltage down to within tolerance for the regulator.

Just make sure that you are always drawing enough current to reach the avalance point of the zener.

-Bruce

[Jennice]

FLZapped:

I think one of the concerns is that the current through the zener, along with the woltage drop will cause too much heat and destroy the zener, if it is active in the "supply line" to the fans.

Jennice

[Matttcattt]

i was thinking of connecting the fans in serial, and using abou 15v to run the fans at about 7.5v each.

i probably dont NEED the fans, but i want to be extra safe.

i think it would be better to use a zener, as i can run them from any input voltage, but i dont know the best way to use them. do i need a series resistor before the zener?

if it is too difficult to regulate the rail voltage, i could always use a sperate (~12v) transformer to run them.

what do you think i should do?

[millwood]

once the motor is moving (0.29a), the power dissipation on the regulator isn't that big. For example, if you are dropping +/- 36v down to +/- 12v, that's 24v*2*0.29=15w, give or take a few. Not too bad.

The problem is that motors usually need a much larger current to get started. 2x - 3x of current in the first second or so isn't that rare. So you are talking about 30w - 45w of power dissipation. That's a lot.

In my case, I used two fans, each 12v and 0.15amp, off a LM simple switcher. Unshielded and next to a JLH1969 (poor psrr). No noise whatsoever.