| Mr.Mekulic |
Excuse the noob question :cannotbe:
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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| 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 |
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| Mr.Mekulic |
| 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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| moamps |
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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| Mr.Mekulic |
| 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 :D
This what I ordered:
http://eshop.diyclub.biz/product_in...products_id=247
they say it needs 12V 1A trafo |
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| 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 |
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| 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 |
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| Mr.Mekulic |
| 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 :D
Is LM350T ok ? |
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| 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. |
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| paulb |
| Here's the circuit for a 7812. |
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| paulb |
| And the diagram for the 7812. |
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| 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? |
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| Mr.Mekulic |
| quote: | Originally posted by moamps
EDIT: Yes, LM350T is also fine. |
Milan thanks for your help.
Hvala na pomoći, čovjek traži pomoć u svijetu a nađe je u susjedstvu.
:D |
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| Mr.Mekulic |
| quote: | Originally posted by 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? |
I'll ask the guys at diyclub. But their english is very hm... funny :D i hope they will understand. |
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| 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.:up:
Regards,
Milan |
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| vuki |
| The "S" versions of 78xx regulators have larger current capacity. So... 78S12. |
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| 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 ;) |
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| 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 |
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| Mr.Mekulic |
| quote: | Originally posted by 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?
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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 ? |
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| moamps |
| quote: | Originally posted by Mr.Mekulic
Do I need to check anything else ? |
Make sure you also check the temperature of the regulator. Keep us posted.
Regards,
Milan |
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| gmphadte |
Can't u use half wave rectified 22V. U will keep the heat low this way.
Gajanan Phadte |
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| DJ Exprice |
| quote: | Originally posted by moamps
Make sure you also check the temperature of the regulator. Keep us posted.
Regards,
Milan |
Exactly. The LM7812 gets very, VERY hot :hot: if enough current is being drawn and/or if the input voltage is anything above 18v! Put a heatsink on that hummer and it'll SHINE! :D
It'll run very well with even a tiny heatsink. The variable regulators can get hot, but not as hot as the standard ones. The variable regulators have a 5A output and the LM78/s/xx/LM9/s/sxx (the standard ones) only have about 1-1.5A output! How do you figure that? :D
I would still go with either no regulator and just do what the kit says, or run it through a POSITIVE variable regulator (and a NEGATIVE variable regulator if you need to have the +/- balance perfected ;) ) |
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