Why are some power supplies rated in VA and some in Watts and some in amperes?

[meirman]

Why are some transformers and power supplies, rated in VA and some in
amperes, and some, I think, in watts?

For example,
1) I have a doorbell transformer whose secondary is rated AC 10V 5VA.

2) And I have a plug-in little-black-box transformer about the same
physical size that is rated AC 9.5V 750mA.

I guess for the first one, 5VA is the same as 5 watts?,
and the second one is .75 x 9.5 = 7.125 watts.

Is it that simple, or am I missing something?

It surprises me a little because the first one is heavier, but what
really confuses is me is their rating one as VA and the other as mA.

Meirman

 

[James Sweet]

Why are some transformers and power supplies, rated in VA and some in
amperes, and some, I think, in watts?

For example,
1) I have a doorbell transformer whose secondary is rated AC 10V 5VA.

2) And I have a plug-in little-black-box transformer about the same
physical size that is rated AC 9.5V 750mA.

I guess for the first one, 5VA is the same as 5 watts?,
and the second one is .75 x 9.5 = 7.125 watts.

Is it that simple, or am I missing something?

It surprises me a little because the first one is heavier, but what
really confuses is me is their rating one as VA and the other as mA.

VA is only equal to watts if the power factor is 1 (resistive load) but if
you have an inductive load (such as a doorbell) or a capacitive load, then
the watts will be lower than you would expect by measuring the current.

 

[Terry]

VA is only equal to watts if the power factor is 1 (resistive load) but if
you have an inductive load (such as a doorbell) or a capacitive load, then
the watts will be lower than you would expect by measuring the current.

Agreed. But for doorbell usage the rating no matter how stated does not
necessarily seem to be for continuous power.
Domestic 'bell transformers' in my experience rarely fail.
But we had a case where front door located 'push' button stuck 'in', in the
permanently 'on' position.
The homeowner complained that door bell didn't work and that the chime unit
had 'hummed' (i.e. it was permanently energised) for a few months before
that. The 'bell transformer' had burnt out.

On other hand I've abused those bell transformers, putting the low volt
secondaries in series with the primary to step up or step down the 115 volts
supply (the insulation held OK), testing for TVs that might shut down etc.
with low mains voltage. One and/or two such transformer arranged in a
'voltage bucking' manner with switches would step down the nominal 115 volt
supply by some 9% or by some 18%. Useful for testing radios and TVs for
rural low voltage conditions.
Many years later I finally acquired (could afford!) a variac.