PSU help needed

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Hi hope somebody can help as I'm pretty new to this.

I have my supply to main power amp board sorted out with a 25-0-25 vac toroidal into a regulated dc converter giving me + _ 35vdc to the board.

The problem is I also need 12 dc to power speaker protection relay board (straight supply 12 and grnd)

Do I need to use separate traffo or some sort of power reg to feed this.

any suggestions welcome.
 
Pulling extra current from one rail only would cause some imbalance and might make some difference. So, the "proper" way to do it is have a small separate transformer.

IMHO, the few mA won't matter. Assuming that you really are using a center tapped transformer, make a separate supply using a general purpose diode (1N4007 or similar) and a small electrolytic cap. That will give you roughly 16V. Like this except the transformer - connection is the center tap.
An externally hosted image should be here but it was not working when we last tested it.

You can use a 12V three pin regulator to reduce that. Or, since the protection circuit doesn't require super clean power, you can use a zener shunt regulator. Calculate the current draw of the protection board and relay and calculate the resistor that would drop 2-3V at that current (R=V/I so at 100mA R=20-30). Place that resistor in series with the power supply and put a 12V 1/2 or 1W zener from the resistor to ground. like this:
An externally hosted image should be here but it was not working when we last tested it.
 
If you just take a piece of regular wire and wrap say 10 turns around the toroid, then measure the AC volts on your new secondary. From that you can work out how many turns you need for 9V. 9V rectified will give you the 12V that you need for your aux circuit. It doesn't need to be regulated, its only for a relay drive.
 
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