I need to know how find the correct wires on a transformer with multi pri taps. I have a transformer that can work on PRI 120vac, 220vac and 250vac. There was a switch to select the proper input voltage but the switch was removed and now I am stuck. I would like to know what to do to get the transformer working on 220/250 vac. The colour of the wires that goes in on pri side from main is:


I hope this could help in a way. The transformer is in an audio amp. The transformer is a TAMRADIO. I don't find any data about it. I'll appreciate help. Thanks


1 Answer 1


Normally this would only need 4 wires: the common, and taps for the three voltages. Having 6 wires complicates things a bit.
One way to find out is to measure resistance between wires. If there is indeed an common and three taps you should measure \$\dfrac{220}{120}\$ the resistance between 220V and common, compared to the resistance between 120V and common. So look for ratios \$\dfrac{220}{120}\$, \$\dfrac{250}{120}\$ and \$\dfrac{250}{220}\$. If you measure a relatively small resistance between pins, you're probably measuring between 220V and 250V.

Alternatively you can apply a low AC voltage (from the secondary from another transformer) to the secondary and measure voltages between pins, and find out the ratios. Even if you know the secondary's voltage and apply that, don't expect to measure 120V, 220V and 250V, since an unloaded transformer outputs higher than nominal voltage.


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