Society of Robots - Robot Forum
Electronics => Electronics => Topic started by: nixter on April 18, 2010, 08:41:48 PM
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Hi!... I have a what i think is a fairly simple situation where i needed help with how to wire something up. I want to wire 2 computer fans to individual potentiometers with each having on/off indicator light. I am not sure how all the different ratings and all fit in together, if i need resisters, or anything else. Also tried straight wiring one of the fans through the pot and the fan would only spin when pot was in 85-100% range. Is my power source too weak? The LED's, how do i wire them if i will be having variable voltage controlling fans, or do i not? Confused :/
I have:
* power source: 12v 1.25A adapter (but if needed to, can find another one).
*2 pots: mini volume control pots from radioshack, 10k with audio taper, SPDT switch, rated for 3amps 30V, rated power
.05 watts, sliding noise <47mV).
*2 leds (w/ built-in 1/4W, 680-ohm resister for 12VDC operation; max ratings: power dissipation 180mW, forward voltage
12V, forward current 15mA)
*fan #1: labeled 12V and .12A
*fan #2: labeled as 12V and 1.14W
Any help at all would be greatly appreciated. Thanks!!!!!!!!!!!!!!!
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How did you try to wire everything up? That might make it a little easier to diagnose the problem. Also, did you check the voltage going to the fans when you were changing the pot value?
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A 10k pot is way large a value. It is possible to control the fan speed with a pot but lets do some basic math with assumptions.
fan #1: labeled 12V and .12A
is one of your fans. If the fan is considered to be a resistor then from Ohms Law (E = IR) the equivalence resistance of the fan is: 12V/ 0.12A = 100 Ohm. If a resistor is placed in series (PS -> resistor -> fan) the voltage across the fan would be half if the resistor is the same value as the fan (100 Ohm).
The typical DC computer fans do run at lower voltage but not too low (each fan is slightly different). So a pot of about 500 Ohm would be the largest practical value.
Do you have a selection of resistors? If so then try different values and measure the voltage on the fan. Then see if the resistor/voltage follows Ohm Law and what speed the fan turns at.
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Hi,
Then see if the resistor/voltage follows Ohm Law and what speed the fan turns at.
Everything follows Ohms law, of course, but fans are inductive and mechanical, so won't follow with a linear relationship with voltage, if that's what you meant.
nixter <-
I wonder why everybody and his uncle (at least if said uncle is very young) have this strange idea, that they can control a fan better with manual control than with a thermal sensor and my guess is, that it must be a lust for buttons making them feel in control or something, but let me assure you - electronics is far better at regulating temperature than us humans that, over a year may claim it cold at so diverse temperatures as eg. -20°C in the winter and +20° in the summer.
That aside, even with manual control, you need something more than a high value potentiometer.
A rheostat (which is in reality just a high power potentiometer) is the low tech and quite poor solution - Go get rheostats of around 60 Ohm and at least 2W. Mount them, not as potentiometers, but as variable resistor in series with the fans.
60 Ohm makes sure you can cover their entire speed range (one should be around 40 Ohm and the other around 50 Ohm, but rheostats doesn't come in every size, so anything available to you which is between roughly 55 Ohm and 65 Ohm will have to do.
The sensible way (if that word could be used here), would be to use regular pots and a simple PWM circuit for each fan, as that would be easier to control and won't waste power in heating up a rheostat. With a bit of savvy, each could be tailored to cover the speed range from 0 to max. over the range of the potentiometer.
A good fan circuit speeds up the fans to full power for a few seconds at start, to make sure the bearings/bushings won't set. With manual control, do this at least every tenth time you use the fans by cranking it up all the way for a short moment.