Re: Calculating DC Output Current From Unregulated AC Transformer

The Phantom wrote:
On Thu, 17 Jul 2008 21:53:47 +1000, "Phil Allison" <philallison@xxxxxxxxxx>
wrote:
(snip)
** So it is a circa 360 VA transformer.

The *correctly* rated secondary load is not 8 amps - but more like 15.

That would depend on the insulation system the manufacturer used, and the
resultant allowable temperature rise, wouldn't it?

Maybe it's only class O. Maybe they wanted to be very conservative.

The *correct* rating depends on the allowable temperature rise, which,
presumably, the manufacturer knew.

The oft quoted ratio of 1.6 for AC amps to DC amps applies ONLY at full
load and for a correctly rated transformer.

Wouldn't this ratio depend somewhat on the regulation of the transformer?

A transformer with poorer regulation would have a wider conduction angle, and a
smaller ratio of AC amps to DC amps.

For example, I have a cheap Radio Shack transformer rated at about 15 VA, and
its measured regulation is about 15%. The AC amps to DC amps ratio at rated
secondary RMS current is about 1.53.

The 192 VA transformer discussed above, with its better regulation has a ratio
of AC amps to DC amps of about 1.7, at the manufacturer's rated secondary
current.

That's not much variation, and the 1.6 figure would be a good rule of thumb,
although the actual number varies with the regulation of the transformer.
Perhaps slightly different ratios could be specified for different values of
regulation.

I varied the load current with the 192 VA transformer discussed above, and
measured slightly varying ratios at the various load levels:

DC current ratio of AC amps to DC amps

4.03 1.799
4.49 1.781
5.95 1.744
7.17 1.719
12.33 1.633

If all transformers were manufactured to a single regulation and temperature rise standard, the formula sought by the O.P. would be a lot closer to practical, but your two examples span only a part of the considerable range of such specifications. This range is what makes a universal formula either inaccurate if simple, or so detailed that it is impractical to fill in all the variables if complete.

--
Regards,

John Popelish
.

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