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What watt resistor do I need for use on the pins of a poweredge PSU?

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balloonshark

balloonshark

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I bought a Dell Poweredge 2850 PSU to power my hobby chargers. I found a mod to control the fan speed based on amperage draw. It needs two 1k ohm resistors or one 1k resistor and one 1k potentiometer. It doesn't mention the resistor's watts. Here is a link to the brief mod with a pic. http://www.rcgroups.com/forums/showthread.php?t=1005309&page=31#post17672520

Here a pic of the pinout if it's needed. http://static.rcgroups.net/forums/attachments/5/2/1/1/6/8/a6557375-76-Dell NPS-700AB pinout.jpg

Also, when I ground the circuit is it ok to ground it one of the larger ground blades?
 
If you used 1k potentiometer and dialed it to offer no resistance then the worst case scenario is the resister would see 6v of potential

Resister power = V^2 / R = 6^2 / 1000 = 36 / 1000 = 0.036 watts.

Most standard resistors are higher wattage than this.

You should be able to ground it to any ground on the supply. I would probably ground to one of the pins.
 
Thank you serpretetsky for your reply and explanation. After seeing your reply I placed an order for 1/4 watt resistors. A little overkill but they were a bit cheaper.
 
Just put the resistor in series with the power_in and the positive terminal of the fan, and forget connecting the resistor directly to ground. Fans have very low impedance, (rated voltage)/(current rating), but the current rating is worst case, and running current will be approximately half. Don't ignore the worst case power dissipation of the variable resistor.
 
So going by my pic I want to eliminate "to fan pin" and move it "to ground pin.

I should also mention that at idle the Vin green (current share) pin is 120.5mV. I then connected two chargers. One charging a 3 cell lipo (11.1V +) at 4 amps and the other charging a 2 cell lipo pack (7.4V +) at 4amps and the current share pin was 1.? volts.
 
I received the resistors the other day and was able to solder up the mod according to the schematics (or at least I soldered them up according to my drawing). The fan ramped up when I started charging a battery. It also ramped up again after I started charging a 2nd battery. It seems to work as advertised although I forgot to check the temp of the resistors. I didn't come close to maxing the PSU's output since I was only charging a 3s 4500mah lipo and a 2s 4500mah lipo.

If what I did was wrong please let me know and explain it to me in layman's terms.
 
Yeah that looks good. Make sure there is no chance that something will short with your wiring. Heat shrink tubing is good for this.
 
bryanl said:
Just put the resistor in series with the power_in and the positive terminal of the fan, and forget connecting the resistor directly to ground. Fans have very low impedance, (rated voltage)/(current rating), but the current rating is worst case, and running current will be approximately half. Don't ignore the worst case power dissipation of the variable resistor.
Click to expand...

I think one of the points of this mod is he doesn't have to open up the power supply to get access to the fan power.
 
serpretetsky said:
Yeah that looks good. Make sure there is no chance that something will short with your wiring. Heat shrink tubing is good for this.
Click to expand...
Great and thanks! I did heatshrink them along with the other pins I had to connect. I used the wires and pins that I pulled from a servo pigtail. They fit perfectly on the PSU's pins.
 
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