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Old 7th September 2008, 10:52 AM   #1
Buzzy is offline Buzzy  Singapore
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Default Powering a fan for amp

I have built a diy amplifier. It is running quite hot so i got myself a 12Vdc,0.13A fan. I am running the fan from an auxilary 9-0-9 transformer. I put 2 diodes and a 2200uF capacitor to make a rectifier.The fan is running but the voltage i measure across the fan is 7.69V dc. I don't understand why. Does anyone know y this is so ?
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Old 7th September 2008, 05:36 PM   #2
Minion is offline Minion  Canada
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Well maybe itis just Voltage drop....It the transformer is a low current type then maybe the fan is pulling enough current to cause the voltage to sag??

Just a thought....
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Old 8th September 2008, 07:13 AM   #3
Buzzy is offline Buzzy  Singapore
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Its a 6VA transformer. It is used to power a soft start circuit in my amp. I paralleled the secondaries, so one goes to soft start and the other to the fan. Both have their own rectification.
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Old 8th September 2008, 07:36 AM   #4
AndrewT is offline AndrewT  Scotland
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two diodes to rectify, sounds like a full wave rectifier.
But, full wave requires a dual secondary connected as a centre tapped secondary.
How can you parallel a 9-0-9Vac centre tapped transformer and yet use two diodes?

Please post a schematic and pic.
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Old 8th September 2008, 11:08 AM   #5
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Quote:
Originally posted by Buzzy
Its a 6VA transformer.....I paralleled the secondaries....
6VA probably has a secondary winding resistance really, really high.

I agree a schematic will help. you can't parallel a CT transformer unless you physically break the CT and parallel the two windings (one of my favorite tricks with cheap trannies )

Cheers!
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Old 8th September 2008, 01:00 PM   #6
Buzzy is offline Buzzy  Singapore
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Here is the schematic Click the image to open in full size.
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Old 8th September 2008, 03:57 PM   #7
AndrewT is offline AndrewT  Scotland
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could the diodes or the capacitor be assembled back to front?

9Vac should give around 13Vdc after the full wave rectifier.
Where have 6V gone?
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Old 9th September 2008, 01:00 AM   #8
Buzzy is offline Buzzy  Singapore
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I am wondering the same thing too. Where have all the voltage gone ?
The diodes are in the right orientation. i tested the diode and they drop 0.5V. So they are working right ?
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Old 9th September 2008, 01:35 AM   #9
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Another trick!
First check your fan is it work properly or maybe not , if the fan is somehow mechanical problem, it can pull a lot more current than nominal.
If the fan is o.k, (you should try to connect other fan), than measure voltage without load, than with soft start board only, and with the fan and you'll see where disappear the voltage.
I think that your trafo. is too low rated power 6VA only, but try with this :
Use full wave rectifier bridge connected to the both ends of the transformer and you'll get around 24 volts unstable voltage, and than just use a 7812 volt. regulator, In my case mounted on the metal chassis for eventual cooling, and that's it, you'll get now 12 volts for youur fan...work finished.
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