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Bama Slamma 24th November 2006 09:29 PM

Rail voltages from 15-0-15 volt transformer
I salvaged an old battery charger from work the other day that a customer had returned. I took it home, opened it up, and powered it up. The transformer was still good but there was no output on the charger cables. Bad rectifier I guess. The circuit was basically a center-tap half wave rectifier and simple charge regulator. I measured 30 VAC on the two hot wires and 15 VAC hot to ground (center tap). When rectified, what would that be equivalent to? It was a 10 amp charger so I guess the tranny is a 300 VA.

N-Channel 25th November 2006 05:36 AM

After rectification

15-0-15v X1.414 = 42.42V(pk). Since you will probably center-tap full-wave rectify this, your voltages after rectification and filtration will be ~ (+/-) 20V, taking into account each diode drop. Your curtrent will be 10A/1.414, or 7.07A, peak. For a low- to medium-powered chip amp or something like that, this should do nicely.

For 60Hz applications, a good rule-of-thumb is at least 2000uF per ampere RMS. So, for +/-10A, I would recommend 20,000uF per rail.

Of course, to increase the efficiency a bit, you could always use Schottky diodes of at least 100PIV, because their forward voltage drop is more like 0.7V instead of the 1.0 - 1.2V for standard-recovery diodes.


Bama Slamma 26th November 2006 03:38 AM

I measured the input voltage. It read 129 volts, 9 volts over nominal value. I guess we have pretty good service in our area. All the primary lines in our neighborhood were recently upgraded to 7200 volts and there are two other houses on our brand new 25kva pole pig. I live in an old part of town where there are still a lot of the old 2400 volt primary lines. I measured one of the outlets at work and it's 120 volts right on the nose. I guess I'll measure it there and see what the correct readings are. This transformer apparently has two primary windings since the charger also has a 2 amp trickle charge setting.

AndrewT 26th November 2006 10:27 AM

a 10Amp battery charger will almost certainly not be a 300VA.

It is more likely to be (10A*14V) =140VA.

This should be big enough for a pair of chipamps (or are you planning a different end use?).

But is the voltage suitable for a pair of chipamps?

The transformer rating MUST be derated to 70% or so when feeding a capacitor input filter. That leaves about 100W.
If you are getting +-18 to +-20Vdc after the rectifier and smoothing then the continuous output current will be 140VA *0.7/36Vdc=2.7Adc continuous. But this is based on a guess at 140VA.
Peak currents into the chipamps are met by slightly discharging the smoothing caps not from the transformer. So the amplifiers ability to drive the load is determined by the stored charge in the smoothing caps. Don't skimp here.

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