newbie question-testing transformer

[Mbira]

Hey guys,
I have a power transformer here. I know it's good and I tested it a while back and got voltages. I don't have a variac, is it OK to just hook up the primaries and switch it on without ramping up the voltages? It's for a tube amp and in the 300v-0-300v range but I don't exactly remember...

 

[Mbira]

To clarify, I'm just wanting to find out again what the voltages are

 

[12afael]

use a small value transformer (12V for example) conected to the primary , then read what you get at the sec. some math and you will know your trafo.

I did a mistake with a power transformer it was for 110V primary and it use a power resistor to disipate the other 110V(we have 220V here) I bypased the power resistor so connecting to 220V primary I had 1600V on the secondary which burned my tester

 

[bcarso]

[quote author="12afael"]use a small value transformer (12V for example) conected to the primary , then read what you get at the sec. some math and you will know your trafo.

I did a mistake with a power transformer it was for 110V primary and it use a power resistor to disipate the other 110V(we have 220V here) I bypased the power resistor so connecting to 220V primary I had 1600V on the secondary which burned my tester [/quote]

A resistor would only produce a voltage drop based on the loading of the transformer, so with light loading (your meter for example) it would only reduce the overall primary voltage by the magnetization current (determined by the inductance of the primary winding and the frequency) times the resistor value, which might not be much. So even had you not shorted it, it could still overload a meter.

I like your suggestion of the low voltage excitation though, if Joel is not certain of the primary voltage rating.

 

[Mbira]

well, I'm sure it delivers less than 400v per side. My meter can handle what it puts out. I just didn't want to damage the transformer. Is it bad for a transformer to just turn it on full without a load?

 

[bcarso]

No, there should be no problem turning it on and off without a load. Except, when you break the circuit you will get an arc wherever the switch contacts are, and potentially (depending on the precise timing relative to the current) a brief overvoltage. That should not be enough to blow up a meter, but your mileage may vary.

12afael has a good approach. But in your case just disconnect the meter before you disconnect the mains power.

 

[Mbira]

Cool. I got me a 336-0-336 with heater taps. Hmmm-gonna build me a deluxe

 

[12afael]

A resistor would only produce a voltage drop based on the loading of the transformer, so with light loading (your meter for example) it would only reduce the overall primary voltage by the magnetization current (determined by the inductance of the primary winding and the frequency) times the resistor value, which might not be much. So even had you not shorted it, it could still overload a meter.

Thanks for the explanation Brad. :sam: :guinness: