Transformer failure modes

[Zodiac]

I have a question - I have a 50VA frame transformer whose secondaries are 15V. After rectification this should be 21V (15*1.414) when the secondaries are loaded. Problem is, this transformer drops to 19.5V with a load that draws ~150mA from the secondary.

It's not my wiring, as I swapped in a similar transformer and that worked fine. However, when I was 'testing' the circuit to which the transformer was attached to, I did manage to fry a 3A diode in the discrete bridge rectifier (obviously I didn't have any fuse in the secondary. Yes I know, smack my hand ) The transformer is rated at ~2A.

Could I have fried the transformer? I thought these things either worked or they didn't!

Any explanations?

[peranders]

This value isn't very extreme especially if the smoothing caps are small.

I'll guess that you have fuses, suited for the purpose? You won't fry a transformer that fast or easy if you have fuses....connected.

[Zodiac]

There were no fuses on the secondary when I burnt out a diode. My guess is output was 3A for 30seconds. Smoothing caps are 12000uf...not that small...

[moamps]

Originally posted by Zodiac
[B]I have a question - I have a 50VA frame transformer whose secondaries are 15V. After rectification this should be 21V (15*1.414) when the secondaries are loaded. Problem is, this transformer drops to 19.5V with a load that draws ~150mA from the secondary.

Hi,

After rectification this voltage is theoretically 15*1,41 (perfect sinusoidal shape and no voltage loss on diodes) WITHOUT any load.
Voltage drop depends of current, rectification type, diode type, caps value and transformer losses.
19,5V at 150mA is just OK.IMHO

Regards

[peranders]

Zodiac, your transformer must have a fuse of 315 mA F (230VAC) or 630 mA F (115 VAC) at the primary side. Still the transformer seems OK.

[AudioFreak]

For voltages as low as that, and assuming that the load is within the capabilities of the transformer, the loaded voltage will most likely be somewhere in the vacinity of Vac x 1.1 and Vac x 1.2 ...

[Zodiac]

With the transformer rated at 10% regulation, 15V no load, 13.5V full load, you would expect 2V DC drop due to loading effects. I guess you will also lose 0.6V or so from forward diode drop, so theoretically it could go down to 18.4V or so! Hmmm, the other transformer I swapped in was a torodial and rated at 300VA, which might explain the lack of loading effects....

[Christer]

Originally posted by Zodiac
Hmmm, the other transformer I swapped in was a torodial and rated at 300VA, which might explain the lack of loading effects....

Actually, I think you can safely skip the "might" above.

[Zodiac]

Thanks all, I will remember the 1.1 to 1.2 rule. It is these things that are useful rather than 'the voltage after full wave rectification will be 1.414 * the output voltage, in AC RMS, of the transformer in question' stuff that you find in textbooks.

I will get an 18V transformer, 50VA to get my 15V DC out

[djk]

If I have a load that uses 1A average current, and have a 1000µF filter cap, the average current and the peak current through the diode bridge will be about 1A.

If I increase the filter cap to 100,000µF with a 1A average load current the peak diode current could be close to 100A. The limit is only the DC resistance of the transformer winding.

If I was using 12,000µF with a 300VA transformer I would use a 25A bridge.

Another trap is fast, or Schottky diodes. While quiet, they have lower surge current ratings than regular diodes. A series resistor may be needed to prevent failure from repetitive surges as in the above example.