Can a Variac handle the current output of a 2300V microwave oven transformer?

[vk6kro]

These transformers were intended for use before solid state rectifiers became available. So, they used full wave (but not bridge) rectifiers.

Each diode conducts for half a cycle of the input and it gets only half the total secondary voltage. So the peak value of the rectified voltage is 1.414 times half the total secondary voltage.
eg half of 500 volts is 250 volts. 250 volts times 1.414 = 353 volts.
This is the voltage a capacitor would charge up to.

If you used the same secondary but put a bridge rectifier across it, the output would be
500 times 1.414 or 707 volts because the entire 500 volt secondary winding is used for each half cycle.

In this case, bigger isn't better. If you have 400 volt capacitors, 700 volts would destroy them.

 

[Bob S]

Awesome find Bob S, thanks! I see the TP PWP 52. It looks very suitable. It says output is 550 vct @ 40madc. What is vct? (I've searched unsuccessfully.) Also, the current is specified at 40madc, I'm assuming that is 40 milliamperes, but what is the dc designation for? (I'm assuming it doesn't rectify its output.) It looks much more finished than a MOT! I also like that it's hermetically sealed.

This thumbnail shows a typical HV vacuum tube rectifier circuit using center-tapped transformer and a 5U4 rectifier tube, used in hi-fi circuits in the 1950's. Note that the tube has a common filament and two plates, thus suitable for use with a center-tapped transformer.

 

[axi0m]

Ok, I believe I understand it now.

So, the beauty about going with the transformer below would be that I wouldn't need a Variac, being that it's final output voltage would be about 388VDC. At this voltage, the current would be 40mA, right?

(TP) PWP-52
Berkshire Transformer power transformer. Hermetically sealed. Terminals & posts on bottom. NSN: 5950-809-9164.

• Primary: 105 / 115 / 125 vac @ 50/60 Hz
• Secondary: 550 vct @ 40 madc,
32v @ 80 ma, 32v @ 80 ma
• Dimensions: 3" x 3" x 3-3/4"H​

Now, examing this one, doing the full-wave rectification with the center-tap, would give 238VDC at 400mA -- not enough voltage. But, if I did a diode bridge (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=330341169683&ssPageName=STRK:MEWAX:IT" ) directly on the two "outer" terminals, I should get (340 * 1.4), or 476VDC at half the current which would be 200mA, right? Then, I'd have to use a Variac to decrease the input voltage, but the advantage is that it would be capable of much higher current.

(TP) T46309
Thordarson transformer. Sealed. Top terminals. New!

• Primary: 115 / 230v
• Secondary: 340 vct @ 400 ma
• Dimensions: 3" x 4-5/8" x 4-7/8"H​

 

[vk6kro]

Yes, that's right.

The current ratings are really for average use over a period of time. For briefly charging a capacitor, you could probably exceed those ratings by 50 to 100 %. The transformers that have other secondary windings are rated assuming those windings are fully loaded as well.

If you had a series resistor of 4000 ohms to the capacitor from a 400 volt supply, you could limit the current at turn-on to 100mA but this would drop rapidly and give a charging time constant of about 40 seconds. So the capacitor would have a charging time of something over a minute.

So, the transformer would be stressed for only the first 10 seconds or so and probably wouldn't heat up at all.

There must be lots of those transformers around. TV sets used to have large heavy transformers in them before they got switch mode power supplies. So, I wouldn't rush into buying a new one off Internet. Ask around at your local junk store.

Also, you can use voltage doublers which would give you about 280 volts from a 100 volt source.