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Old 2nd December 2006, 09:10 PM   #1
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Default Very basic: 22V to 12V AC

Excuse the noob question

I have a 350VA 22-0-22 trafo for my amp, but I need a 12V 1A supply for the input select and motorized pot circuit.

What is the easiest way to get 12V from 22V AC?
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Old 2nd December 2006, 10:00 PM   #2
moamps is offline moamps  Croatia
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Hi:
You can use an LM338 voltage regulator to reduce the voltage from the rectifier capacitors of the output amplifier to 12V DC. You may need to add a heatsink to the regulator. The schematic is here:
http://www.national.com/pf/LM/LM338.html

Regards,
Milan
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Old 2nd December 2006, 10:07 PM   #3
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Quote:
Originally posted by moamps
Hi:
You can use an LM338 voltage regulator to reduce the voltage from the rectifier capacitors of the output amplifier to 12V DC. You may need to add a heatsink to the regulator. The schematic is here:
http://www.national.com/pf/LM/LM338.html

Regards,
Milan
Thanks for the reply but I need to get 12V AC from 22V AC.
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Old 2nd December 2006, 10:12 PM   #4
moamps is offline moamps  Croatia
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Motorized pot circuits must be powered with DC. Things will be much clearer when you post a picture or the schematic.

Regards,
Milan
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Old 2nd December 2006, 10:26 PM   #5
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Quote:
Originally posted by moamps
Motorized pot circuits must be powered with DC. Things will be much clearer when you post a picture or the schematic.

Regards,
Milan

This one has PS on board, I just need 12V AC to run it

This what I ordered:

http://eshop.diyclub.biz/product_inf...roducts_id=247

they say it needs 12V 1A trafo
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Old 2nd December 2006, 10:46 PM   #6
moamps is offline moamps  Croatia
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The picture in the link you have provided shows an onboard power supply, made of the rectifier, capacitor (the big blue thing) and regulator (mounted on a heatsink). As I said earlier, you should use an LM338 to build a voltage drop regulator. The +output of the LM338 regulator must be connected to the +pin (plus) of the big blue capacitor whereas the -output of the LM338 (ground) must be connected to the -pin (minus) of the blue capacitor. This is the simplest way to do it. Otherwise, you would have to use a non-standard transformer to reduce 22V to 12V AC, which complicates things considerably.

Regards,
Milan
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Old 2nd December 2006, 11:06 PM   #7
paulb is offline paulb  Canada
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I would use an LM7812 (or LM340T12) as it is simpler to use and much cheaper than the LM338. The motorized pot does not draw very much current.
http://www.national.com/pf/LM/LM140.html
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Old 2nd December 2006, 11:13 PM   #8
moamps is offline moamps  Croatia
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Yes, the question is how much current is drawn by the entire circuit (including the pot and relays). If it's not too much, paulb has a point.

Regards,
Milan
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Old 2nd December 2006, 11:39 PM   #9
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Quote:
Originally posted by moamps
The picture in the link you have provided shows an onboard power supply, made of the rectifier, capacitor (the big blue thing) and regulator (mounted on a heatsink). As I said earlier, you should use an LM338 to build a voltage drop regulator. The +output of the LM338 regulator must be connected to the +pin (plus) of the big blue capacitor whereas the -output of the LM338 (ground) must be connected to the -pin (minus) of the blue capacitor. This is the simplest way to do it. Otherwise, you would have to use a non-standard transformer to reduce 22V to 12V AC, which complicates things considerably.

Regards,
Milan
Ok I think I got it. Please check if i got the resistor values right.
As I see you are from Croatia, LM338 is expensive could you suggest a replacement from Chipoteka

Is LM350T ok ?
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Old 2nd December 2006, 11:51 PM   #10
moamps is offline moamps  Croatia
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The resistors are OK.
You may replace the LM338 with 7812CT TO3 or 7815CT TO3 (2 EUR in Chipoteka) if you wish BUT then you would have to use a different schematic!

Regards,
Milan

EDIT: Yes, LM350T is also fine.
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