# Dumb question ...

Discussion in 'General Electronics' started by Jeff Strickland, May 30, 2007.

1. ### Jeff StricklandGuest

.... mostly because I'm dumb.

What's the difference in a volt-amp and a "regular" amp?

I have a transformer that makes 24vAC, and is rated at 40 VA, but the amp
rating is only 1.67.

I have a lamp (fiber optic system) that takes 24v and 250W, which works out
to 10.4A. The VA rating does not give me the current I need, and when I plug
the light in, the voltage drops because the draw exceeds the power by a wide
margin.

Thanks,

2. ### Rich WebbGuest

http://en.wikipedia.org/wiki/Volt-amp is a starting point. To avoid
recapitulating what's explained better and in more detail elsewhere,
volt-amps and WATTS (not "'regular' amps") are the related
measurements.

3. ### Jeff StricklandGuest

Okay, so to distill it down, 40VA is not near enough to drive a 250w light?

Actually, this is a bit moot because another multi meter in my fleet has
determined that the lamp is open. I can't explain why I couldn't figure this
out before I bought the new power supply ...

4. ### DanielGuest

Gee, that Wikipedia explanation is a bit hard to understand, so here is
my attempt.

In D.C. circuits, the voltage peaks occur at the same time as the
current peaks, so the power in the circuit is found by multiplying the
voltage applied, i.e. if applying 6 volts causes 2.5 Amps to flow, the
power dissipated by the circuit is (6 x 2.5) 15 watts.

However, in A.C. circuits, the effects of the components in the circuits
can cause the peak current to not flow at the same time as the peak
voltage occurs. So you may apply 6 volts a.c. to a circuit, and at some
times 2.5 Amps a.c. may flow at some time, but because the 6 volts and
2.5 amps do not occur at the same time, it is, technically, incorrect to
to same the power in the circuit is 15 watts, it is correct to say that
the circuit draws 15 VA.

Daniel

5. ### JasenGuest

It's similar to the difference between a foot-pound and a regular pound.

it's mreasuring a different quantity.

Bye.
Jasen

6. ### GeorgeGuest

....In other words, VA is the power rating, similar to saying 40 Watts. So a
40VA transformer cannot drive a 250W lamp.

7. ### DanielGuest

Watts is Power measure in DC circuits where as VA is power measurement
(sort of) in AC circuits because of possible phase shifts between the
applied voltage and the Current.

If, and that's a very big IF, the circuit is PURELY resistive, the the
AC Power will be the same as the VA rating.

40 VA transformer can, usually, very easily drive a 250 W lamp, but the
lamp will, most likely, be fairly dim.

Daniel

8. ### DanielGuest

Good explanation, Jasen. Like it.

Daniel

9. ### GeorgeGuest

Sounds to me that the lamp is going to try to get its 10 Amps and will blow
up the transformer.
George

10. ### DanielGuest

<snip>

The lamp will try to draw its 10.4 amps, but the supply cannot deliver
that much, so the voltage out of the supply will drop.

The 40 VA does NOT mean that it can supply 24 V at 1 A. It can provide
40 VA, which could be 0.5 A at 80 V, 2 A at 20 volts, whatever the
output of the Supply is set to provide, so it may be that you are
connecting your 250 V lamp to a supply that only gives 40 volts, so the
lamp will only be one sixth as bright a it could be, if it were
connected to the right supply, something that can deliver 250 V at upto
1 amp.

Daniel

11. ### neon

1,325
0
Oct 21, 2006
40va means that @40v you expect to get 1amp with 20 v you can expect 2 amps that is the rating 40v x 1a =40 watts 20v x 2a=40w nothing has changed. to get 250 w this device will not do it it is only 40watts.get the idea