## Sunday, October 30, 2011

### I am confused...

So, I have a transformer that has two primary windings that can be typically be hooked up in parallel for 115 VAC operation or in series for 230 VAC operation as well as a couple of identical secondary windings.  It looks something like this:

Let's take the scenario where we hook up only one primary to the mains.  My thinking is that I can treat the second primary winding as another "secondary" and should see the mains voltage on this winding and it would therefore act like an isolation transformer as well as a step down transformer for the secondary voltages.

Where I am confused is the following scenario.  Let's say we hook the mains to 1 and 2 above and use 3 and 4 as the input voltage to a full wave bridge rectifier, filter and regulator for my B+ supply.  The B+ only needs about 100 ma current capability.

So, what if the transformer is rated at 50 VA and we are drawing sufficient current from the combined secondaries to equal 48 VA including the current drawn from the second primary winding.  This is obviously less than the 50 VA rating of the transformer.  But, in this case does the manufacturer expect that the primaries will be always wired in parallel and actually used as primaries?  Do I risk drawing too much current through the single primary wired to the mains in this scenario?  Anyone have any ideas?  Is it likely that each winding can handle the maximum rated VA?

If I understand this correctly, if the transformer is rated at 50VA, then each winding must handle the equivalent current (voltages chosed to make the math easy):

50VA
------ = .5 A primary current total or .25 A each winding
100V

50VA
------ = 2 A secondary current total or 1 A each winding.
25V

So, if I have this correct, I risk over-heating the single primary hooked to mains if I am running at close to the rated VA of the transformer unless the manufacturer over-engineered the windings wire size.  It seems I should limit the total secondary current to 1/2 the VA rating of the transformer if only a single primary is hooked to mains.