Undervolting without resistors

Asmordean · Post by **Asmordean** » Mon Nov 11, 2002 7:14 pm

I'm no electronics guru but my father is. When I told him that I wanted to lower the voltage to my fans from 12v to between 6 and 8v his first answer was not a pot or a massive resistor or even PWM. He told me to buy some diodes and connect them in series. Each diode has a 0.7v drop but produces very little heat.

So I soldered 6 up in series and measured it with a multimeter. It read just over 7.9v.

So I made up a circuit with jumpers for 12v and 7.8v. Works fine and was pretty cheap to build. Cost me about $4.

Post by **Rusty075** » Mon Nov 11, 2002 10:11 pm

If you're looking for voltage between 6 and 8 why not just wire it for 7 volts via the power supply?

Melchior · Post by **Melchior** » Mon Nov 11, 2002 10:53 pm

Since P=IV (where I is the current flowing through the circuit and V is the voltage drop over the resistor or diodes), you'll have the same amount of waste heat with diodes as you would with a resistor-- I've calculated it out to somewhere in the neighbourhood of 0.4 Watts to undervolt a 12V 0.1A fan to 7V.

powergyoza · Post by **powergyoza** » Tue Nov 12, 2002 12:59 am

I like the diode method of controling voltage for the same reasons you do Asmordean (cheap, no need for heatsink, predictable voltage drop), but Rusty075's right, 7v is easier to get by rewiring a 4 pin power connector.

If you want voltage control in 0.7v increments, diodes are a still good way to go:
http://www.cpemma.co.uk/sdiodes.html

Renod · Post by **Renod** » Fri Nov 15, 2002 4:12 pm

You can make a 12V to 7V or 5V molex cable in less than five minutes. See http://www.dslwebserver.com/main/fr_ind ... apter.html

I use them to test fans and (being lazy) have ended up leaving them in the systems when I find the speed I want.

Gxcad · Post by **Gxcad** » Mon Nov 18, 2002 9:23 pm

What is the easiest way to run a fan under 5v? I still haven't found a good way to undervolt my fan lower than 5v. The fan in question is a panaflo 120mm L1A. At 5v it is quiet but still audible, I would like to try running it at 4v or maybe even less, so ideally a variable speed adjuster is best so I can experiment just by turning a knob, however they usually don't go low enough (such as the zalman fan speed control).

-Ken

Post by **Rusty075** » Mon Nov 18, 2002 10:34 pm

An easy way to get below 5v would be to build something like this:
http://www.heatsink-guide.com/antinoise.htm

If the thermo-control doesn't interest you the thermosistor could be replaced with a normal resistor. I built one of these units for about $3 US at the local Radio Shack.

My 120mm Panaflo starts at under 4v, and will run down to about 3 volts.

Gxcad · Post by **Gxcad** » Thu Nov 21, 2002 11:30 pm

Whooo...don't think I'm ready to build something like that yet, nonetheless thanx for the info. Also I've noticed you've tried to make sure I see your info about your panaflo starting at 4v and going as low as 3v once started, I got it just FYI, thx rusty;)

-Ken

Post by **MikeC** » Fri Nov 22, 2002 12:12 am

however they usually don't go low enough (such as the zalman fan speed control)

Ken, if you have a fanmate, adapt it so it is fed 5V from the PSU 4-pin connectors. Then the max voltage you'll get from the fanmate is ~4-4.5V. Min will probably be 2V or less.

Post by **Rusty075** » Fri Nov 22, 2002 12:22 am

Good idea Mike. You could also wire one to give you a range from 7volts down to 2

nerotek · Post by **nerotek** » Sun Jan 05, 2003 12:04 pm

thx Asmordean! i tried connecting 6 diodes in series and the voltage dropped as suggested... fyi, if u want to try this, make sure to align the diodes in the same direction (same pole position)... i accidently flip one diode and it screwed up my voltage drop...

added: i agree the fanmate would be easier, but i got extra diodes lying around the house...

loply · Post by **loply** » Sun Jan 05, 2003 2:54 pm

Gxcad, if youre running the fan at 5v and you can still hear it, the problem is in the case/coupling not the fan. Its something the fan is directly or indirectly attatched to that must be vibrating. Try some rubber grommets or fan isolators.

Take the fan out of the case and hold it in your hand with the power on, it should be silent! Its the case which makes it noisy.