Transformer Question

[AndrewT]

Hi,
do you mean;- take a guess at the output voltage, find a good transformer that matches this voltage, connect the low voltages in series, now back feed low voltage into the unknown transformer, then measure the two primary windings? seems like a lot of guessing and time consuming.
BUT it prompts another thought. What if you connect 110Vac into 1 primary & then measure the voltage on the other primary (carefully) with one end of each primary connected together. Would this give 220Vac out? if connected correctly?
regards Andrew T.

[jacco vermeulen]

Hi Andrew,

with a variac it need not be that time consuming.
Locate two secondary endings, attach the variac and raise the voltage.
Your second idea sounds logical.
As the flux is constant two primaries, without a load on the secondary windings, should respond as primary and secondary windings.
With dual 110V primary windings they should respond as 1st and 2nd winding on a separation transformer.

In the transformer example you mentioned, with several split primary ends, to me it seems that applying a voltage on a secondary winding with a variac, measuring the voltage outputs on the primaries gives the fastest result.

[hifileslie]

Back to the original question. As the transformer powers a power amp and the mains current draw will vary, would a thermistor do the job? Are there better alternatives to reduce the inrush current?

[jacco vermeulen]

An NTC can be used, some use a thermistor for Pass amps.

I find a soft start circuit with relays much better, one that switches timer controlled from a high power Dale to a relay i find most attractive.
An NTC needs to be heated to keep a low resistance.
A relay needs less power, and with a good one the continuous current resistance is very low. There are plenty of schematics and PCB layouts on the web. Per-Anders Sjöström has a nice design of a softstart circuit on his homepage.

[jacco vermeulen]

In case you are interested, in-rush current behaviour graphs of toroidal transformers can be found at some toroid manufacturers.
Toroidal transformers do not have an airgap in the core as traditional transformers have.
At switch-on the toroid does not have an induction but resistance only.
Measure the resistance of your transformer and with your local voltage it will give you an idea of the inrush current of the transformer.

As i use very large toroids a soft start circuit with a high power resistor and a relay is common practice for transformers of 1000VA and larger.
I use it for smaller toroids too, even a 300VA can blow the fuse, it will certainly make the lights flicker at turn-on.

Somewhere i have read that European DIY-ers have a 15 year headstart over the US. Overhere it has been common practice to use toroids for well over 20 years, in the USA up till a number of years ago Toroid usage was mainly in the professional sector.