Very basic: 22V to 12V AC

[Mr.Mekulic]

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?

 

[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

 

[Mr.Mekulic]

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.

 

[moamps]

Motorized pot circuits must be powered with DC. Things will be much clearer when you post a picture or the schematic.

Regards,
Milan

 

[Mr.Mekulic]

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_info.php?cPath=85&products_id=247

they say it needs 12V 1A trafo

 

[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

 

[paulb]

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

 

[moamps]

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

 

[Mr.Mekulic]

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 ?

 

[moamps]

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.

 

[paulb]

Here's the circuit for a 7812.

 

[paulb]

And the diagram for the 7812.

 

[paulb]

I took another look at the board you bought. It has a regulator on board already, and it looks like it is heat sinked. You may be able to apply your transformer voltage 22VAC directly to this board. Can you email them and ask?

 

[Mr.Mekulic]

EDIT: Yes, LM350T is also fine.

Milan thanks for your help.

Hvala na pomoæi, èovjek traži pomoæ u svijetu a naðe je u susjedstvu.

 

[Mr.Mekulic]

I took another look at the board you bought. It has a regulator on board already, and it looks like it is heat sinked. You may be able to apply your transformer voltage 22VAC directly to this board. Can you email them and ask?

I'll ask the guys at diyclub. But their english is very hm... funny i hope they will understand.

 

[moamps]

Sve je ovo globalno selo.
Or in English: Today's world is a global village.

Diyaudio is always a great resource if you're stumped with a problem or just need some advice.

Regards,
Milan

 

[vuki]

The "S" versions of 78xx regulators have larger current capacity. So... 78S12.

 

[Mr.Mekulic]

Ok the guys at diyclub answered, I can use my 22V trafo, just have to check the heatsink of the regulator so it doesn’t overheat.

Thanks everybody for their ideas and suggestions, now I know there is no easy way to drop AC voltage without a trafo

 

[moamps]

Can you please tell me what's written on the blue capacitor re capacity and rated voltage? What's the spec written on the regulator?

Regards,
Milan

 

[Mr.Mekulic]

Can you please tell me what's written on the blue capacitor re capacity and rated voltage? What's the spec written on the regulator?

As the kit hasn’t arrived yet, all the info I have are those pics. I asked what is the regulator onboard, but they answered only, it's ok to use 22V check the heatsink. I will upgrade the cap to 50V kind, and check the regulator can handle the 22*1,41=31V voltage. Do I need to check anything else ?