# Help Modifying Fan Control Circuit

Discussion in 'The Projects Forum' started by GRNDPNDR, Feb 4, 2013.

1. ### GRNDPNDR Thread Starter Member

Mar 1, 2012
435
7
I have a fan control circuit that currently works to infinitely adjust a 12V fans speed by using a reference voltage set by a pot (22.4K Max).

When running at low speed there is 0V across the pot and the fans receive 9.3V

As you adjust the pot to maximum the voltage increases until at maximum the pot has 2.8V across it, and the fans run at 11.6V

I want to turn this into an automatic temperature based control.

I have these two circuits (not sure which would be best to use) the first one is reverse engineered from a physical \$2 Chinese fan controller that works pretty well.

The other diagram I found on the internet.

Both output 0-12V, but what I would like to do is modify one of them to output 0-3V, with a temperature range of 20*C to 50*C or 65*F to 120*F

Can this be done?

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2. ### GRNDPNDR Thread Starter Member

Mar 1, 2012
435
7
I realized this circuit came with a portion with an LED that gets brighter as the speed increases.

I made some changes and it's kind of doing what I need.

When the RTD is at 5% the 3V output is right where it needs to be, until it gets to the 50% point.

@ 50% the voltage output is 2.09V, @ 55% the output jumps to 2.3V @60% output is 2.5 and at 65% output is 2.8V and at 70% output is 2.9V

So it's basically maxing out at 70%.

How do I modify this to output 2.9V at 95-100% without affecting the bottom end (The RTD @ 5% gives 900mV which is perfect)

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3. ### GRNDPNDR Thread Starter Member

Mar 1, 2012
435
7
Anyone? I can't get the output just right and I'm not sure which part of the circuit I need to change to get the output to be what I want it to be.

4. ### GRNDPNDR Thread Starter Member

Mar 1, 2012
435
7
Well, I've come to realize how stupid (smart) I am.

I had an epiphany last night while I was asleep...... I'm trying to make a fan controller, or some device to turn temperature into a voltage.....

ALL I NEEDED TO DO WAS REMOVE THE POT AND ADD A THERMISTOR. DONE.

The pot that's on there now will control voltage between 0-2.8V, which gives the fans about 9.3V-12.3V

BUT, if I remove the pot I can measure 16V across the reference lines, and 16V across the fans....they move crazy air.

So I need to figure out the curve because I can't quite piece together how the pot only allows 2.8V max, and how that reference applies to the voltage going to the fan.
All I need to do is find the right thermistor and I have what I want.

@ 22.4K Ohm the voltage is 2.78 across the pot
@0 Ohm the voltage is 0

I can speed up the fans all I want, but I can't make them go slower, or reduce the minimum voltage to the fans to be less than 8.8V, but I would like to get the low end even lower.

Last edited: Feb 5, 2013
5. ### GRNDPNDR Thread Starter Member

Mar 1, 2012
435
7
So I'm thinking, If I add a a 12.5 Ohm resistor in series with the fan, it should drop the 8.8 min voltage down to 7V.

This will obviously affect the top end speed, but the question is could I compensate by using a higher value pot, that without a resistor on the fan, would run the fan at 1.8V higher than the nominal top end voltage?

would that work?

6. ### GRNDPNDR Thread Starter Member

Mar 1, 2012
435
7
Well I pretty well have this all worked out now I'm just lacking the cash to buy the replacement fans and an RTD.

I have 5 fans I'm working with, and an unimpeded reference voltage of 16V maximum, although I still don't understand what kind of voltage controller is used in this device.

at 0V reference, the fans receive 8.8V.
at 16.6V reference, the fans receive 16.6V .... and man do they move air lol.

the 22.4K pot adjust the reference voltage by controlling how much voltage is dumped to ground, at full resistance the pot drops 2.8V and the fans receive 11.6V, I'm guessing the "missing" 2.2V is being put to ground through the pot.

at no resistance the pot drops no voltage, and the fans receive 8.8V.

Fan 1: 12V @ 0.2A
Fan 2/3: 12V @ 0.12A each
Fan 4/5: 12V @ 0.08A each

My replacement fans are

Fan 1: 12V @ 0.35A
Fan 2/3: 12V @ 0.22A each
Fan 4/5: 12V @ 0.1A each

By using a 9.1Ω 2x15Ω and 2x30Ω resistors I can drop the bottom end voltage from 8.8V to 7V, keeping within the fans 6V-13.2V operating range.

I haven't figured out which value to use for the RTD, but I think if I use a 33.5KΩ PTC RTD that will allow the full 16.6V reference range, which will provide the fans roughly 13.2V at maximum AFTER adding the resistor to change to the bottom end to 7V.

So basically it works out perfectly. the hard part will be putting the resistors on the TINY fans.

Also the fans pull more current at maximum speed, since they move more air than the stock fans they should never really run at top speed but I'm assuming the voltage regulator circuit is rated for 1A since the stock fans run at 0.8A at maximum speed and my fans will pull 1.08A at full speed.