Can I safely use a diode to decrease voltage to a fan?

Considering fans are typically brushless, is it safe to use a diode? I'll be putting it on the fan in my power supply to slow it down. Thoughts?

As long as the diode is rated for the current the fan will draw, yes, you can do it with no worries. Keep in mind that the diode will only drop (most likely) 0.7V, so you'll need to daisy-chain a bunch of them in order to get a significant voltage drop.

If your trying to lower the speed of the fan quite a bit then connect it up as 7v, (red wire of fan to 12v and black wire from fan to 5v).

wouldnt a zener diode placed backwards do the same trick only with more effect?

Originally Posted by krupted

wouldnt a zener diode placed backwards do the same trick only with more effect?

Why's that? It drops more voltage?

Originally Posted by skadebo

Considering fans are typically brushless, is it safe to use a diode? I'll be putting it on the fan in my power supply to slow it down. Thoughts?

whats the point? just use a resistor and keep your voltage drop range options open....

as for zener diode they resist up to a certain voltage beyond that the diode gate is open and allows current flow.... only practical if your using a thermistor in a circuit with the zener to activate the fan at a certain temp....... >_>

the other thing.... diodes are check valves for current only allowing one way flow...... they are simply not meant to be used in place of a resistor......

Originally Posted by thaltek

as for zener diode they resist up to a certain voltage beyond that the diode gate is open and allows current flow.... only practical if your using a thermistor in a circuit with the zener to activate the fan at a certain temp....... >_>

It would work fine for OPs application. Once a zener is in breakdown it still has the zener voltage dropped across it, accomplishing the goal of reducing the voltage applied to the fan. It would, however, be silly to use one since it offers no advantage over a resistor.

the other thing.... diodes are check valves for current only allowing one way flow...... they are simply not meant to be used in place of a resistor......

Diodes are not meant to be used in any particular way. They have certain properties, and you can exploit those properties in different ways depending on your needs. Since the voltage they drop is relatively constant with the current through them, they can be useful for dropping a fairly constant voltage when the load is unknown or variable.

Your conclusion I agree with though. Since the fan load is constant, there's no reason not to just use a resistor.

Originally Posted by keenan

It would work fine for OPs application. Once a zener is in breakdown it still has the zener voltage dropped across it, accomplishing the goal of reducing the voltage applied to the fan. It would, however, be silly to use one since it offers no advantage over a resistor.

Some fans (usually super-high-RPM monsters, blower fans, etc) have a big inrush current when they start up, and using too large a resistor causes the fan voltage to sag when the fan starts up. In these applications, a zener works better than a resistor.

Originally Posted by keenan

Your conclusion I agree with though. Since the fan load is constant, there's no reason not to just use a resistor.

just tryin to save the boy some money.....

Originally Posted by gee

Some fans (usually super-high-RPM monsters, blower fans, etc) have a big inrush current when they start up, and using too large a resistor causes the fan voltage to sag when the fan starts up. In these applications, a zener works better than a resistor.

logically there is a point at which one has reached called "overkill" and good god unless you are trying to vent a server room really "shouldn't need" such airflow.....

http://thegrandnarrative.files.wordp...dead-horse.gif

however i must concede that i am in a land called america and more is better so more power to you...... i would like to suggest the following for your next build...
http://www.industrialfansdirect.com/...FSUMDQodsmPa1Q

The resistor would introduce more current draw into the PSU's fan circuitry which is something I don't want to do in the off chance that the circuit can't take it. That's why I would rather lower the voltage using a diode or zener diode, which would not add more current draw.

Anyway, the operation is a go as I have the parts, it's just a matter of balls and time, haha.

Originally Posted by skadebo

The resistor would introduce more current draw into the PSU's fan circuitry which is something I don't want to do in the off chance that the circuit can't take it. That's why I would rather lower the voltage using a diode or zener diode, which would not add more current draw.

Anyway, the operation is a go as I have the parts, it's just a matter of balls and time, haha.

The choice between a resistor and a diode won't affect the current draw from the PSU, so that worry is misplaced. Here's a quick difference between the two:

Diode: drops the same amount of voltage (typically 0.6-0.7V), no matter how much current goes through it. (roughly speaking)
Resistor: drops voltage proportionate to the current flowing through it.

For your application, either a single resistor or a string of diodes will do the trick. Just make sure they can handle the power dissipation.

Originally Posted by skadebo

The resistor would introduce more current draw into the PSU's fan circuitry which is something I don't want to do in the off chance that the circuit can't take it. That's why I would rather lower the voltage using a diode or zener diode, which would not add more current draw.

Anyway, the operation is a go as I have the parts, it's just a matter of balls and time, haha.

if this is the case consider then use a capacitor to compensate the initial high current draw of the fan....... once the capacitor has discharged the current will stabilize, assuming the fan is up to speed by the time the capacitor is done discharging.

Originally Posted by thaltek

if this is the case consider then use a capacitor to compensate the initial high current draw of the fan

An ideal capacitor appears as a dead short (ie. infinite current) when power is applied to it, decreasing to zero current after a long time, assuming DC is applied. Like the fan, it will have a high initial current draw, eventually reaching ~0 once the capacitor has charged. Slowly charging a capacitor and then using it to start the fan is possible, but requires more than just the cap.

The main problem I can see with using a resistor might in fact be this increased initial draw. The fan needs more current to start than to run, but as it tries to pull more current, more voltage drops across the resistor. This might mean that the fan will work properly with a lower voltage if you use diodes, since the initially high current draw won't affect the voltage on the fan, allowing it to start successfully at a lower 'run' voltage.

Don't even worry about the PSU. They're rated for many amperes, while the fan probably draws under 0.25A even at startup. It's negligible, diodes or no.

Originally Posted by keenan

An ideal capacitor appears as a dead short (ie. infinite current) when power is applied to it, decreasing to zero current after a long time, assuming DC is applied. Like the fan, it will have a high initial current draw, eventually reaching ~0 once the capacitor has charged. Slowly charging a capacitor and then using it to start the fan is possible, but requires more than just the cap.

The main problem I can see with using a resistor might in fact be this increased initial draw. The fan needs more current to start than to run, but as it tries to pull more current, more voltage drops across the resistor. This might mean that the fan will work properly with a lower voltage if you use diodes, since the initially high current draw won't affect the voltage on the fan, allowing it to start successfully at a lower 'run' voltage.

Don't even worry about the PSU. They're rated for many amperes, while the fan probably draws under 0.25A even at startup. It's negligible, diodes or no.

*facepalm* how did i not regurgitate that right......? sorry guys uni is getting to me......

Originally Posted by skadebo

The resistor would introduce more current draw into the PSU's fan circuitry which is something I don't want to do in the off chance that the circuit can't take it. That's why I would rather lower the voltage using a diode or zener diode, which would not add more current draw.

Anyway, the operation is a go as I have the parts, it's just a matter of balls and time, haha.

Its the opposite
Resistors resist current flow.
The current flowing through the resistor causes a voltage drop across it, therefore feeding less voltage to the fan.
This reduces the power load, it doesnt increase it.

Careful that the resistor you use can "easily" carry the current otherwise it may get very hot and even burn up.
If the resistor heats up much, put it somewhere near airflow or get a higher wattage resistor.

Originally Posted by Nenu

Its the opposite
Resistors resist current flow.
The current flowing through the resistor causes a voltage drop across it, therefore feeding less voltage to the fan.
This reduces the power load, it doesnt increase it.

Careful that the resistor you use can "easily" carry the current otherwise it may get very hot and even burn up.
If the resistor heats up much, put it somewhere near airflow or get a higher wattage resistor.

i knew i wasn't completely losing it......yet...... any ways getting back to the meat of the thread.... just use a resistor ......

I'm all for the technical discussion in this thread, but I'm kind of confused as to why so much thought is being put into it. If you want to decrease the speed of the fan, then decrease the voltage being applied to it. Since it's going into your power supply, you already have the option of tapping 12v, 5v, and 3.3v (fast, medium, and slow respectively) without messing with any diodes/resisters. If you absolutely need a different speed, then build a voltage divider out of 2 resisters and use this calculator to figure out your target voltage: http://www.raltron.com/cust/tools/voltage_divider.asp

A divider like that won't work well in this application.

Why do you say that?

Originally Posted by jwhazel

Why do you say that?

The fan acts as a much lower value resistor in parallel with one of the divider resistors, reducing its value a great deal and changing the fan voltage. If you use sane resistor values, that probably means the voltage will be so low that the fan won't run. If you use the very low values necessary to make it work, you're going to be burning a ton of power in the divider.

And really there's no point in the divider, it's basically the same thing as just putting a resistor in series with the fan, except for it to work it needs to burn more power than the resistor alone.