Subject: please help with controller problem
By: tim (IP: 192.250.34.*)
Written on: 20-03-2006 15:11

I thought I had it working but lost ability to control via trim pot.

I had built a 2 zone system using a Radio Shack 1500mA dc converter for 12v supply

Common + side connection to: fan+, two thermistors, and a 12v led indicator assembly.

Common - side to: Source side of 2 MOSFETs and mid lead of trim pot.

Seperate connections from each NTC, Back lead of 1 pot, and gate of 1 MOSFET.

Seperate connections from each MOSFET drain to fan - and led.

Seemed to work well at first, each fan (actually a set of 3 fans on each circuit), and led reacted as expected to temp changes on the respective ntc in the circuit, although I did notice a bit of cross effect, ie; turning one pot seemed to have some effect on the other circuit.

I had each ntc wired to a 3/32 plug and jack, and after plugging and unplugging a couple of times, I could no longer adjust via the pot. I checked out the pots and both showed infinite resistance with no effect on turning the adjustment. This happened twice, replacing the pots between each attempt.

I obviously have no knowledge of electronic components, so I am wondering if there is an issue with the connections the way I am attempting to wire. Is there some isolation needed between the circuits?

By: tim (IP: 24.63.20.*)
Written on: 22-03-2006 02:39

Ok, Now I am totally baffled.

I checked the voltage with no fans connected. Works completely as expected. I can vary the voltage adjusting the pot, and reacts perfectly when warmed or cooled, varying between 0 to 12V

I connect a fan and measure the voltage from the other side of the y connector. As soon as the probe is warmed, the voltage spikes to "off the meter" and does not come down even when probe is cooled off.

What is going on here? I'm about to give up.
By: Tillmann (IP: 84.58.136.*)
Written on: 05-06-2006 13:39


I think this is a problem related to the power supply. Previously, similar problems have been reported with a Radio Shack power supply (but I don't know if it was the same model as your's).

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