85vDC - 12vDC

[rookie amp builder]

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.

[jaycee]

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.

[HighTec]

Hi,

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 The Darlington MJH11018 is obsolete but you can use MJH11022 which is still manufactured by ON Semiconductor.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.



Edit: Here is the link for AP3400 service manual.
http://www.yorkville.com/downloads/other/smap3400.pdf

[Mooly]

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

[djk]

If its a toroid, wrap a few turns on it for the fan.

[indianajo]

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.