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Old 2nd March 2011, 03:50 PM   #1
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Default 85vDC - 12vDC

hi there

I have a power supply of + & - 85vDC, (65-0-65vAC)I would like to know if there is a way instead of using another transformer, that i can somehow use a simple circuit to get 12vDC from that supply to run a fan.
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Old 2nd March 2011, 04:23 PM   #2
jaycee is online now jaycee  United Kingdom
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The usual way is with a zener based regulator, but the transistor is going to get far too hot with that kind of voltage drop. you'd probably be best using a mains-powered fan in this case.
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Old 2nd March 2011, 05:34 PM   #3
HighTec is offline HighTec  Croatia
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Here is a fan control circuit used in Yorkville's amplifiers, AP3400 and AP6040. The Darlington MJH11018 is obsolete but you can use MJH11022 which is still manufactured by ON Semiconductor. Some minor component recalculations needs to be done in order to work with you power supply. Of course, MJH11022 needs to be mounted on a heatsink. If you wanna complete schematic, give me the details via PM.

Click the image to open in full size.

Edit: Here is the link for AP3400 service manual.

Last edited by HighTec; 2nd March 2011 at 05:43 PM.
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Old 2nd March 2011, 05:42 PM   #4
Mooly is offline Mooly  United Kingdom
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You could investigate using a "wattless" dropper where a capacitor is used in place of a resistor.

Rectify the supply and scale the cap/s to give around 12 volt or aim a little higher and use a simple reg.

You would have to elaborate on it but this is the idea,
UK Vintage Radio Repair and Restoration - Dropper Calculations
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Old 3rd March 2011, 01:34 AM   #5
djk is offline djk
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If its a toroid, wrap a few turns on it for the fan.
Candidates for the Darwin Award should not read this author.
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Old 3rd March 2011, 01:41 AM   #6
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Use a wall transformer , most fans produce electrical noise anyway. Keep fans off the audio circuit. I've got my fan outside the steel box of my ST120.
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300
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