Re: Calculating DC Output Current From Unregulated AC Transformer

On Thu, 17 Jul 2008 21:53:47 +1000, "Phil Allison" <philallison@xxxxxxxxxx>
wrote:


"The Phantom"
"Phil Allison"


I have a transformer with a nominal 24 volt secondary, rated at 8 amps.
It has a measured series resistance (secondary plus reflected primary)
of about .125 ohms.


** This is a fabricated falsehood - the numbers simply do not add up.

A 192VA rated transformer does not have 4% regulation - correctly
rated
it has 8%.

This one does.


** Cos it is not correctly VA rated.

The manufacturer rated it, and printed the rating on the outer paper.



Look up makers data if you doubt this.

This transformer has no maker's identification, so I can't look it up.


** So you also have no idea what its VA rating is.

I am taking the manufacturer's word for it. Printed on the transformer is the
designation 24 volts, 8 amps.



However, the primary (it's a 60 Hz transformer) says 120 VAC and it
measures .961 ohms, cold.


** 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




..... Phil



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