Sizing transformer

[timegoesby]

Just a quick one. I have 3 50W 12 v downlighters and a 230v 65VA transformer. Will the transformer be big enough? If it is not would it catch fire?

thanks

[Ellis Greensitt]

You will need a 65VA transformer for each downlight. and yes, it probably melt.

[Dave Plowman (News)]

Very roughly and in this case VA = watts. So three 50 watt bulbs need a minimum 150 watt, or 150VA transformer.

Your 65 VA transformer is designed to run *one* 50 watt bulb.

It has probably got an internal fuse which would blow with such a gross overload. If not, a fire is quite possible.

[Christian McArdle]

No it is not remotely big enough. You'd need one of these transformers for each light, assuming they are genuinely 12V transformers intended for the purpose.

The effect would depend on the individual transformer. It would probably be one of the following:

(a) Set on fire (b) Blow internal fuse (c) Overheat, possibly leading to (a) or (b) above after some time. (d) Automatically shutdown (e) Run at reduced voltage indefinitely (unlikely this one)

Christian.

[timegoesby]

Thanks. I didn't know that VA was watts. Writing watts on the transformer would have be easier.

[Tim Mitchell]

In article , [e-mail] writes

It isn't the same thing, but for the purposes of low voltage lighting it's a good approximation.

[Christian McArdle]

But less accurate. When you have a poor power factor but the same useful power (in watts), the VA goes up, requiring a larger transformer. Halogen lamps have a unity (good) power factor.

Christian.

[Dave Plowman (News)]

Transformers have to be de-rated for inductive loads. So it's usual for them to be specified in VA so they should cope with near anything.

But a low voltage lamp is a near perfect resistive load.

However, it will have a larger start up current when the filament is cold

- hence specifying a 65 rather than 50 watt transformer.

[The Natural Philosopher]

For one lamp. yes.

If it is not would

Possibly.

[T N Nurse]

Nope.

Maybe. Depends on type. Some switch mode types shut down on overload.

[BigWallop]

And some exploded in a spectacular fashion. :-) Or is that just me?