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

Originally Posted by jwhazel

Why do you say that?

A resistor divider is great...as long as anything attached to it is high-impedance. As soon as you attach something with a low impedance, the voltage divider no longer works as designed. The reason is this: whatever load you attach to the voltage divider will draw additional current through the top resistor. That additional current changes the voltage (V=IR and all that). If the load is high-impedance (like an op-amp), the additional current is small enough that the change in voltage is insignificant. If the load is low-impedance, like a fan, the additional current will very strongly affect the voltage.

Another way to look at it is this: attaching a fan is like putting a low-value (<50 ohms) resistor in parallel with the lower resistor.

Originally Posted by jwhazel

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)

3.3V is too low to spin up big part of fans. Without knowing behavior of particular fan already 5V can be very risky if it's low speed fan variant.

And like said resistor drops voltage further during spin up surge while diode faithfully cuts away only its forward voltage so from technical perspective diode is way superior because of its guaranteed/known behavior.

5V is often too low a voltage and the fan wont start, so 3.3V is very unlikely to work.

Tap onto +5V as gnd/earth and +12V as V+ to get 7V.
This has worked great for me, quietening down some pretty loud fans in the past.

Originally Posted by Nenu

5V is often too low a voltage and the fan wont start, so 3.3V is very unlikely to work.

Tap onto +5V as gnd/earth and +12V as V+ to get 7V.
This has worked great for me, quietening down some pretty loud fans in the past.

not a wise choice.... for several reasons.... it is much wiser to take +12V to a resistor to fan to ground.....

Originally Posted by thaltek

not a wise choice.... for several reasons.... it is much wiser to take +12V to a resistor to fan to ground.....

pls explain why its not wise

Originally Posted by Nenu

pls explain why its not wise

Several years ago, there were isolated cases of unloaded power supplies reacting badly to having a fan across the 12V and 5V lines. In essence, there was current entering the 5V rail when the power supply was only expecting current to go out, and the typical result was that the power supply shut down.

If there are other things drawing off the 5V power supply (like, say, the rest of a computer), there's no issue.

Originally Posted by Mohonri

Several years ago, there were isolated cases of unloaded power supplies reacting badly to having a fan across the 12V and 5V lines. In essence, there was current entering the 5V rail when the power supply was only expecting current to go out, and the typical result was that the power supply shut down.

If there are other things drawing off the 5V power supply (like, say, the rest of a computer), there's no issue.

I agree, its not an issue

Originally Posted by Nenu

pls explain why its not wise

i really should not need too....

Originally Posted by thaltek

i really should not need too....

You made the statement, pls elaborate.

i said it was not wise, i however did not say that doing so would not work.... so i will share why i would not do so....

1. noise bridging: any induced noise from things like fans now has a direct bridge from the 12V rail to the 5V rail......
2. uneven loading: as mentioned by moho.... some of the regulator circuits on the market are quite sensitive to load jumping from one rail to another......

Originally Posted by thaltek

i said it was not wise, i however did not say that doing so would not work.... so i will share why i would not do so....

1. noise bridging: any induced noise from things like fans now has a direct bridge from the 12V rail to the 5V rail......
2. uneven loading: as mentioned by moho.... some of the regulator circuits on the market are quite sensitive to load jumping from one rail to another......

1) PC fans have circuits that deal with ripple current very effectively.
3 components are needed for the RC filter (2 caps, 1 resistor), extremely cheap to implement.
The RC circuit not only vastly reduces electrical noise the fan generates but provides an effective filter/smoothing for cross rail noise when bridging 12V and 5V rails with such a fan.
A very very old PC fan might not have an RC filter but even then its unlikely to cause an issue with modern high current PSUs.


2) This has been demonstrated to be a none issue.
As moho pointed out, it was an isolated incident on an unloaded PSU, a very different environment to a running PC.
It is not a general problem with PC PSUs nor one I hear about in practise.
Decent PSUs will shut down on "high enough" reverse current detection so no damage could occur.
Even without a PSU shut down safety circuit, it would take a lot of reduced voltage fans to cause enough reverse current to raise PSU component temperatures to anywhere near critical...
that is unless there is a PSU fault, like in the isolated case mentioned


I have run only a single 120mm fan on a very old 230W PC PSU for about 1.5 years, cooling a projector bulb.
That PSU is now running a smoothwall firewall PC.

I have also run multiple fans at 7V on the same voltage rail for more than 3 years in an older (highly overclocked) PC without any issue.
fyi

i need a hug now......

Squeeze

*eye pops out and rolls across the floor*

it is not my intent to be an ass... i just like to do things my way....