• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Power transformer selection madness

I am very confused about my choice of power transformer. I would like to get around 330V. 250V RMS will get me this with a bridge rectifier configuration and a regulator (using the Morgan Jones set up).
Here is where my confusion starts. If I look at the Hammond web site, all there transformers are center taped. I.E. 250-0-250. That’s fine, I know I just have to use the two diode configuration. However, when I set this up in my modeling software
(Linear Technology switchercad III
http://www.linear.com/designtools/index.jsp
)
I end up with 0.5V of ripple at the output that I did not have with the bridge rectifier.
My inclination is to order a 125-0-125 transformer and leave the center tap un-connected. Presumably, this will get me 250V RMS and a lower ripple

Is my thinking correct?
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> I end up with 0.5V of ripple at the output that I did not have with the bridge rectifier.

There's always ripple. How much is the bridge showing?

For practical values of load and capacitance, the raw-supply's ripple will be 1% to 10%, probably 5 Volts to 10 Volts. 0.5V ripple suggests very small load or huge capacitors.

> switchercad

Getting power supply results out of a SPICE engine is hard. Use Duncan's power supply calculator.

> I would like to get around 330V. 250V RMS will get me this

250VAC into FWB (or 500VAC CT into 2 diodes), sand rectifiers, gives 353V peak. Diode, copper, and ripple losses usually yield 80%-90% of this, though if the transformer is oversized you may get more. Still, 300VDC is a good bet. Normal wall-power variations can be +/-10%, so realistically you could get 270VDC to 330VDC.

> and a regulator

Allowing 10V minimum drop across the (sand-state) regulator, 260VDCreg is about the most you can be sure of.
 

amperex

Member
2005-02-20 1:42 pm
MI
Hammond transformers

Remember, Hammond current ratings are for a full-wave center tap rectifier & a choke input filter. Using a bridge rectifier, one has to drop the current rating in half with choke input design.

Using a capacitor input, multiply that current rating by .65 for full-wave CT & approximately .35 for a bridge.