Zener diode

[CECAA850]

Maybe not the right section, but I thought I'd try here. I'm trying to slow down a 24v fan. The fan specs say that it will run with a minimum of 17 volts. Let's say I want to run 20v through it. Would I find a zener diode with a value of 4v (voltage drop) or 20v (desired voltage)?

Next question, let's say I've found my desired voltage and want to slow the fan even more. Can you run a resistor in series with the diode?

Last and final question, what are the advantages/disadvantages of slowing a motor with a resistor vs a diode?

Thanks.

[Guest " "]

The diode vs resistor question really depends on the current draw of the fan.

The best bet would be to use a mini adjustable power supply circut board like the ones sold on ebay. I bought a few that can adjust to 0 - 37 volts at 1.5 amps with out a heat sink and more if you use a heat sink. I think I paid 10 bucks for the few I have.

[CECAA850]

Any links?

[CECAA850]

http://cgi.ebay.com/Adjustable-Voltage-Power-Supply-Project-Kit_W0QQitemZ7619481590QQihZ012QQcategoryZ50912QQrdZ1QQssPageNameZWD1VQQ_trksidZp1638.m118.l1247QQcmdZViewItem

Is that what you're referring to?

[Marvel]

The zener might introduce some noise into the system. A voltage divider made with resistors would work, but you will dissipate some heat trhough the resistors. The is what a cheap (or not so cheap) volume control does.

If it is a DC fan, you can usually slow them way down using a PWM supply. This is what model trains use. The full 24 volts would be applied to the fan, but in pulses. For it to recieve the equiv. of 12 volts, it is only on 50% of the time. This could also introduce noise. Computer power supplies operate this way, but for set voltages.

[Guest " "]

http://cgi.ebay.com/Adjustable-Voltage-Power-Supply-Project-Kit_W0QQitemZ7619481590QQihZ012QQcategoryZ50912QQrdZ1QQssPageNameZWD1VQQ_trksidZp1638.m118.l1247QQcmdZViewItem

Is that what you're referring to?

yes, but I bought 2 built for 10 bucks each. The circut board is about half the size of a credit card and has a handful of parts on it.

[CECAA850]

The zener might introduce some noise into the system. A voltage divider made with resistors would work, but you will dissipate some heat trhough the resistors. The is what a cheap (or not so cheap) volume control does.

If it is a DC fan, you can usually slow them way down using a PWM supply. This is what model trains use. The full 24 volts would be applied to the fan, but in pulses. For it to recieve the equiv. of 12 volts, it is only on 50% of the time. This could also introduce noise. Computer power supplies operate this way, but for set voltages.

The application would be for an amplifier, so I definately would not want noise. I'm not familiar with a voltage divider, unless if you're referring to installing resistors in series in the power line. That's what I had in mind.

Isn't a zener diode like a conventional diode except it has the ability to limit voltage? If so, how would it introduce noise?

[CECAA850]

http://cgi.ebay.com/Adjustable-Voltage-Power-Supply-Project-Kit_W0QQitemZ7619481590QQihZ012QQcategoryZ50912QQrdZ1QQssPageNameZWD1VQQ_trksidZp1638.m118.l1247QQcmdZViewItem

Is that what you're referring to?

yes, but I bought 2 built for 10 bucks each. The circut board is about half the size of a credit card and has a handful of parts on it.

How hard are they to assemble? It looks like it has a pretty long list of parts. I'd be afraid that I'd install something backwards and burn something up. That is of course unless they came with a "printed circuit for dummies" instruction manual.

[Guest " "]

this one is assembled. I use them to convert AC tube filiment circuts to DC. current price is 9.50 or buy it now for 12.00.

http://cgi.ebay.com/Universal-DC-AC-Power-supply-regulator-for-DVR-camera_W0QQitemZ250216585177QQihZ015QQcategoryZ48636QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

[CECAA850]

Thanks, I bid on both of them.

[DrWho]

A Zener diode is usually used in parallel with the power (not in series) to shunt the voltage lower (and requires a resistor in series with the leg before the zener to prevent shorting out the device providing the power).

If you're looking for a 4V drop, the power dissipated by the zener diode is going to be the same as the power dissipated by the resistor. It really is as simple as P=V x I

In fact, that fancy power regulating device you're looking at it is probably just an LM317 (http://www.national.com/mpf/LM/LM317.html). Talk about overkill for your application []

I bet your fan is drawing about 100mA, which is going to require a device that can handle 400mW (piece of cake). You could just put a variable resistor (like a pot) wired in series with your motor. A 100ohm pot would probably work just fine. Even at half an amp (which would be large for a cooling fan), you're only talking about 2W of power dissipation - you could easily get a 5W resistor for like $1 at radio shack that would work just fine. You could probably find a pot for like $3.

Btw, why are you trying to slow a fan down? Usually the fan is there to keep the amplifier cool.... []

[CECAA850]

In fact, that fancy power regulating device you're looking at it is probably just an LM317 (http://www.national.com/mpf/LM/LM317.html). Talk about overkill for your application

It may very well be overkill. It will however let me try different voltages and find one that is desireable for my application. It's also assembled and for those of us who are schematically challenged, it seemed right up my alley.

I bet your fan is drawing about 100mA, which is going to require a device that can handle 400mW (piece of cake). You could just put a variable resistor (like a pot) wired in series with your motor. A 100ohm pot would probably work just fine. Even at half an amp (which would be large for a cooling fan), you're only talking about 2W of power dissipation - you could easily get a 5W resistor for like $1 at radio shack that would work just fine. You could probably find a pot for like $3.

I don't know a pot from a pan, that's why I'm asking those who know more than I.

Btw, why are you trying to slow a fan down? Usually the fan is there to keep the amplifier cool....

Fan noise is more of an issue to me than cooling efficiency at this point.

[Guest " "]

In fact, that fancy power regulating device you're looking at it is probably just an LM317 (http://www.national.com/mpf/LM/LM317.html). Talk about overkill for your application

It may very well be overkill. It will however let me try different voltages and find one that is desireable for my application. It's also assembled and for those of us who are schematically challenged, it seemed right up my alley.

I bet your fan is drawing about 100mA, which is going to require a device that can handle 400mW (piece of cake). You could just put a variable resistor (like a pot) wired in series with your motor. A 100ohm pot would probably work just fine. Even at half an amp (which would be large for a cooling fan), you're only talking about 2W of power dissipation - you could easily get a 5W resistor for like $1 at radio shack that would work just fine. You could probably find a pot for like $3.

I don't know a pot from a pan, that's why I'm asking those who know more than I.

Btw, why are you trying to slow a fan down? Usually the fan is there to keep the amplifier cool....

Fan noise is more of an issue to me than cooling efficiency at this point.

DrWho

$1+$3 for some maybe's vs 12 bucks for something that works. 8 buck's of a difference in price is not worth sqwalking about. Remember a variable soultion was requested.