Variable HT supply

[mark_in_manc]

Lifted from a current 'wanted' thread about a variable HT supply:

Originally Posted by See_Mos

Nowadays I would probably do it with a variac into a transformer with 24 volt secondary feeding two 12 volt transformers in series and in reverse to step the voltage back up again and to provide isolation from the mains

How far can one push the insulation on a transformer winding designed for (say) 240v? I get that using two 20:1 units in series in this suggestion will mean no over-voltage stress. If one used say just one 12v wall wart backwards, with a variable 24v ac supply applied to the LT side (I have such a thing, allegedly capable of up to 8A at 25v), would the winding insulation stand up to it? I understand magnetic saturation is a function of current drawn, not voltage, so I guess this is not something I need to think about.

[TrevorG3VLF]

"I understand magnetic saturation is a function of current drawn, not voltage, so I guess this is not something I need to think about. "

No. Saturation occurs when the voltage is too high or the frequency too low. Current makes very little difference to the magnetic strength.

Current produces heat and voltage drop due to wire resistance.

Stuffing 24V into a 12V winding seems like trouble.

[Herald1360]

I wouldn't say that current makes little difference to magnetic strength literally, though for load current on a transformer it's true. Other things being equal the magnetic field is proportional to the current in an inductor, at any rate until the core, if there is one, saturates.

The magnetizing current is controlled by the applied voltage/frequency though, since it depends on the impedance offered by the primary inductance which is frequency dependent.

[trsomian]

Quite the reverse. The magnetising current, which is what will saturate the core of a transformer, (not the load current at all), is dependant on the applied voltage and the frequency; hence the term "volts seconds". If you are going to apply 24V to a 12V transformer winding designed for 50Hz you will need to apply it at 100Hz.

[Radio Wrangler]

Back in the days when designing power supplies with normal iron lamination-cored transformers, one of the probable events (when, not if events) was someone connecting to 240v with the thing set to 110v. There were two saviours: The overvoltage on the primary was planned to saturate the transformer and to quickly take out the fuse so the rectifiers, smoothers and regulators (and everything downstream) got protected. The second one was the use of mains driven fans. Their cores too saturated and helped significantly.

David