PSU mod using Pot and thermistor...help...

PSUs: The source of DC power for all components in the PC & often a big noise source.

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madd02
Posts: 114
Joined: Tue May 31, 2005 12:15 am
Location: New Zealand

PSU mod using Pot and thermistor...help...

Post by madd02 » Mon Jun 20, 2005 1:07 am

I have been trying to mod a PSU Like this below for about 2 weeks now...on and off. i have some elctronics/electrocial knowledge...but its very limited. So if i mention something that is completely wrong please advise ASAP...so i can get this going...

I have tried the following (note all testing performed outside PC using the green - black jumper trick)

1) adding a 10k trim pot in series with the thermistor...worked but not quiet enough.

2) so i then went to add a 25k wirewound pot 16mm lin/sg. So i could control speed from outside the PSU. BUT this is where it gets strange... the fan just stops going now.... so i removed the thermistor...leaving just the pot. And if i leave the PSU off for an extended period..eg 10min then retart the fan will go...when i adjust the pot from 0-ohms to say 2-3k the fan jsut stops... and will not start unless i leave the PSU off for a while.

I have tried this from the 12v header on the board(pot added in series with the fan) and from the temp control mini board (pot added where thermistor was).

I just tested it and ..YEP...it did it again....

I have been live testing without the cover on the PSU...so please for my safety...(i am starting to get complacent)..please any suggestions.




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madd02
Posts: 114
Joined: Tue May 31, 2005 12:15 am
Location: New Zealand

Post by madd02 » Mon Jun 20, 2005 1:26 am

As an update i hooked up my fanmate 2 from my Zalman CPU fan.

AND

If i decrease the voltage too quick the fan shuts off...and will not work again untill i rest the PSU. (eg turn it of for about 5 minutes)

i am almost out of ideas....

I tried earlier this evening, the fan with 2 silicon rect diodes in series...1n4004 dropping fan to about 9.5v.

Ok so ive gone from having a temp sensitive pot controlled PSU...to one with a few diodes in series... NOT HAPPY !!!

any suggestions would be appreciated


(on the plus side i am now the grand master of putting together a test circuit)
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larrymoencurly
Posts: 170
Joined: Tue Dec 03, 2002 11:29 pm

Post by larrymoencurly » Sat Jun 25, 2005 3:02 pm

There's probably a fixed resistor in the 10,000s of ohms range that the fan control circuit uses to set the voltage at one of the thermistor's leads, and it may even be connected to that lead. Changing its value should make the fan speed change.
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madd02
Posts: 114
Joined: Tue May 31, 2005 12:15 am
Location: New Zealand

Post by madd02 » Sat Jun 25, 2005 11:25 pm

thanks for the reply, i spent most of saturday trying to fix the controller circuit. with no luck.

So i removed it.Yes very satisfying...after all the hours looking at it..thinking..why aint you working.

I now am running the fan of the 12header, bypassing the 'seems to be faulty' controller circuit.

To control speed externally, I added a 100ohm wirewound lin/sg pot and a 5w 20ohm resister. Supplying a range of 3.3 - 9v to the 0.23 amp fan.

It will start on 3.3v and runs very quiet at this speed.

So hopefully problem solved....

This PSU will jsut be a temporary measure till those thermaltake fanless PSU's come down in price...and im sure about reliability.

Thanks to all for your help with this little (ha ha) PSU project of mine.
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