Re: basics of transformer

On Mon, 30 Jan 2006 14:42:07 -0600, "Abstract Dissonance"
<Abstract.Dissonance.hotmail.com> wrote:

>I have a 40VCT .25A transformer and I'm wondering what those numbers mean..
>
>does the 40VCT mean 40VAC from the center tap? If so across its full
>secondary its 80VAC? and the .25A means that it can handle at most .25A
>across its secondary coil(since its a step down from 120VAC the max current
>in its primary should be smaller... ~80/120*.25?).

---
40VCT means that with the rated voltage into the primary and the
rated load on the secondary, the secondary voltage will be 40V
across the entire secondary. (20V from the center tap to either
end.)
---

>Also should I assume these are all RMS values?

---
Yes.
---

> When I put my variac up to
>100VAC and hook it up to my transformer I get about 40 volts across the hole
>secondary(hence 20V's across from the center tap). But this means that at
>the mains(117VAC?) I'll be getting more than 40VAC across the full
>secondary? Is this ok to have the transformer have a little more than its
>voltage rating?

---
Yes. It's supposed to because of the fact that since there will be
resistive losses in the primary and secondary windings when the
transformer's secondary is fully loaded, there will be an inevitable
rise in voltage across the primary when it's less than fully loaded.
The difference between the no-load and full load voltage is known as
the regulation of the transformer, and for small transformers like
you have is typically 30%


--
John Fields
Professional Circuit Designer
.

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