Power transformer voltage

[mhamini]

I need to replace a power transformer in an old (1940) vintage tube radio. The schematic does not provide the A/C output voltage of the transformer but it does specify that the DC voltage after the filtering is 240 VDC and total current draw is 80.1ma. The circuit also uses the field coil (1060 ohms) for filtering which results in a 85 VDC drop. Is the correct transformer output voltage 325 volts? i.e. 325-0-325 for full wave rectification?

 

[VVV]

Actually, that is incorrect.
You need to remember that the rectifier works like a peak detector, so it will be the peak of the input sinewave that equals the output DC voltage.
Since the DC output is 240V, assuming there is 10% ripple (you should post the value of the filter cap, so we can do an accurate calculation), then the actual peak of the sine wave should be at 240*1.05=252V. Add the drop of the diode (what diodes is it using?) and let's say that results in 255V. Then the transformer needs to output:
255/√2=180VAC. This is an RMS value, the one you would measure with your meter.

So it should be 180-0-180 (under load).

All of the above is to be viewed as a guide.

 

[jonw0224]

What VVV said is correct, as long as your filter circuit does not contain a series choke (inductor/coil). If you have a series choke (as you indicate), the filter will average the rectified wave instead of being a peak detector.

If this is the case Vdc is approximately 2•√2•Vrms/pi or 0.9•Vrms

I'm not sure if this is what you should use. I'm confused about the filter coil having such a high resistance (you sure that's not 1060uH?).

Hope that helps.

 

[VVV]

I do not mean to start an argument here, but typically those radios used a Π filter. Hence, the cap right after the diodes does work as a peak rectifier.
A typical value for that first cap is about 100µF, which should result approximately in the 10% ripple I indicated.

Then the inductance and the cap after it filter out the ripple present on the first cap.
The 85VDC drop across the inductor means it has about 1060Ω of DC resistance.

 

[jonw0224]

VVV,

No offense taken. There is no doubt that anyone reading this forum knows that you have very good information to contribute. I was not trying to undermine your post or start an argument.

We were talking about two different cases. I am not familiar with power supplies of old radios, but I do know that different power supplies have different types of filtering circuits and was merely pointing out that the answer depends on the type of filter used.

I don't know what people think of my posts, but I hope they're somewhat valuable. I admit, I do not know everything. Perhaps I had no business posting a reply here. But I'm still reading with interest...