Replacing wall wart questions?

semi932

Member
2009-11-29 9:07 pm
This is pretty much a rookie type of question but I can't seem to grasp it.

I'm using a 24v AC wall wart connected to a diy power supply using 78 and 7915 v regulators for pre amp power.
I know that the 24v is pushing the limits of the voltage regulators. I've got them heat sinked an everything seems ok for now.

If I wanted to change to the recommended input of 16volts or so and went to a toroidal, would I order 8-0-8 or 16-0-16?

My thinking is 8-0-8 but I'm probably wrong.

Thanks
 
You would need 16-0-16 IF you have +/- requirements.

Those are the RMS voltages of the windings. After being rectified and fed into a reservoir capacitor, there will be 16 * 1.4 = 22.4 Vdc, being the peak value of the 16V rms sine wave.
However there are losses: diode drops at 0.7v each and resistive losses in the transformer, so 20V DC is a reasonable expectation.

A bridge rectifier, or 4 discrete diodes, will give +/- supplies if the transformer centre tap (the "0v" connection is used as the common line.

If only postive rails required, then an 8-0-8 will be fine as a 16V winding, ignoring the 0V connection.
 

semi932

Member
2009-11-29 9:07 pm
You would need 16-0-16 IF you have +/- requirements.

Those are the RMS voltages of the windings. After being rectified and fed into a reservoir capacitor, there will be 16 * 1.4 = 22.4 Vdc, being the peak value of the 16V rms sine wave.
However there are losses: diode drops at 0.7v each and resistive losses in the transformer, so 20V DC is a reasonable expectation.

A bridge rectifier, or 4 discrete diodes, will give +/- supplies if the transformer centre tap (the "0v" connection is used as the common line.

If only postive rails required, then an 8-0-8 will be fine as a 16V winding, ignoring the 0V connection.

Great explanations. Thanks

Basically I'm using this power supply with the wal-wart
Power Supply for Preamps

+ and -15v.
I can build another power supply to rectify the ac but I'm trying to substitute toroid for wall wart.

Might not work.
 
With Rod's P05 you have two options - half wave voltage doubler (as you are using now) or split +/- supply. The split supply offers a bit better performance.

If you wish to go with a split supply then you will need the 16-0-16 transformer, but you will also need to modify your PCB with two extra diodes and also wire the transformer with one extra connection to the PCB. If you don't want to modify anything you need the 8-0-8 transformer and just use the two outer wires and leave the centre tap unconnected, just tape it up.
 
Last edited:

semi932

Member
2009-11-29 9:07 pm
With Rod's P05 you have two options - half wave voltage doubler (as you are using now) or split +/- supply. The split supply offers a bit better performance.

If you wish to go with a split supply then you will need the 16-0-16 transformer, but you will also need to modify your PCB with two extra diodes and also wire the transformer with one extra connection to the PCB. If you don't want to modify anything you need the 8-0-8 transformer and just use the two outer wires and leave the centre tap unconnected, just tape it up.


Just the stuff I was looking for.
Thanks to you and godfrey for your input.
As I said I can use a bridge rectifier and go in that direction but I like the way this power supply sounds now and thought I might try a toroid to see if it improves it any.
Could sound worse but not much of an investment for 8-0-8 so cheap experiment and education.

I'm thankful for everyones responses cause they all add to my limited knowledge.

Chow