Lower PT primary voltage == greater current capacity?

[gmoon]

Kind of a simple question:

Given the transf. power equation: Np/Ns = Ep/Es = Is/Ip. If the primary voltage is halved, can I expect the current capacity of the secondary to double (approx) ?

For instance, A 500-0-500 PT, rated for 100 mA, with both US and Euro primaries. If the 240 primary is instead attached to US 120V, the output is ~ 250-0-250, will the secondary supply 200mA?

I'm fairly sure the answer is yes (conservation of energy ,etc.), but it never hurts to check....

[Rob Mercure]

No, because the 120 V would only push 1/2 of the expected current through the 240 VAC winding impedance - so with 1/2 the current flow in the primary you get 1/2 the current flow in the secondary.

TANSTAAFL ("there ain't no such thing as a free lunch" - Robert Heinline - "The Moon is a Harsh Mistress").

Rob

[Ptron]

For instance, A 500-0-500 PT, rated for 100 mA, with both US and Euro primaries. If the 240 primary is instead attached to US 120V, the output is ~ 250-0-250, will the secondary supply 200mA?

I'm guessing you meant "will the secondary be able to supply 200mA?"

The answer is still no. The the secondary current rating and I(s)from the transformer ratio formula are not the same thing. The secondary current rating is mostly determined by the internal resistance of the wire in the secondary winding and how fast the transformer can dissipate the heat caused by current flow through it. Those things won't change with the lower voltage. All that formula really tells you is how much current will be on the primary based on what current is being drawn from the secondary by the load.(Is=Vs/Rload (ohms law))

[gmoon]

Thanks Rob & Ptron. Sometimes reality is so uncooperative...

I'm guessing you meant "will the secondary be able to supply 200mA?"

Yeah, I saw the phrasing was awkward, but didn't think it was worth an edit...(thought the previous para was clear enough.) Thanks!