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

wrote in message
oups.com...
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



You will need a 65VA transformer for each downlight.
and yes, it probably melt.
--
Ellis Greensitt, Site Admin, UK Electricians Forum
http://supplychain.org.uk/phpBB2/index.php

In article .com,
wrote:
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?

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.

--
*The fact that no one understands you doesn't mean you're an artist

Dave Plowman London SW
To e-mail, change noise into sound.

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?

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.

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

In article .com,
writes
Thanks. I didn't know that VA was watts. Writing watts on the
transformer would have be easier.

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

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

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.

In article .com,
wrote:
Thanks. I didn't know that VA was watts. Writing watts on the
transformer would have be easier.

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.

--
*On the other hand, you have different fingers.

Dave Plowman London SW
To e-mail, change noise into sound.

wrote:

Just a quick one. I have 3 50W 12 v downlighters and a 230v 65VA
transformer. Will the transformer be big enough?

For one lamp. yes.

If it is not would
it catch fire?

Possibly.

thanks

In message .com,
writes
Thanks. I didn't know that VA was watts. Writing watts on the
transformer would have be easier.

VA is a reactive term, Watts are resistive (in noddy speak)
--
geoff

In message .com,
writes
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?

Well 3 x 50 makes 150 watts (=150VA not splitting hairs)

So you prolly want a 200VA transformer to give you a bit of leeway

--
geoff

On Mon, 13 Jun 2005 20:49:54 GMT, raden wrote:

Well 3 x 50 makes 150 watts (=150VA not splitting hairs)

So you prolly want a 200VA transformer to give you a bit of leeway

Leeway is bad - for transformers at least, it's not so bad for
electronic PSUs.

If you over-rate the transformer then (because it's poorly regulated)
the output voltage will be higher than it ought to be under the design
load. This reduces lamp life.

Andy Dingley wrote:

On Mon, 13 Jun 2005 20:49:54 GMT, raden wrote:


Well 3 x 50 makes 150 watts (=150VA not splitting hairs)

So you prolly want a 200VA transformer to give you a bit of leeway


Leeway is bad - for transformers at least, it's not so bad for
electronic PSUs.


? lamps are almost always a totally resistive load.


The VA rating is almost 100% the watts rating when driving them.

If you over-rate the transformer then (because it's poorly regulated)
the output voltage will be higher than it ought to be under the design
load. This reduces lamp life.


Not on a toroid, and not on a properly designed and regulated SM supply.

Cheap crap will always catch you out, because its cheap, and decent
regulation costs a little bit. But its not a necessary feature of either
type of 'transformer'

In message , Andy Dingley
writes
On Mon, 13 Jun 2005 20:49:54 GMT, raden wrote:

Well 3 x 50 makes 150 watts (=150VA not splitting hairs)

So you prolly want a 200VA transformer to give you a bit of leeway

Leeway is bad - for transformers at least, it's not so bad for
electronic PSUs.

If you over-rate the transformer then (because it's poorly regulated)
the output voltage will be higher than it ought to be under the design
load. This reduces lamp life.

So you're talking about using transformer saturation to limit the
voltage ?

That's the only effect I can think of which would have an effect

A 200W transformer should run cooler and therefore be less of a risk

--
geoff

raden wrote:

In message , Andy Dingley
writes

On Mon, 13 Jun 2005 20:49:54 GMT, raden wrote:

Well 3 x 50 makes 150 watts (=150VA not splitting hairs)

So you prolly want a 200VA transformer to give you a bit of leeway


Leeway is bad - for transformers at least, it's not so bad for
electronic PSUs.

If you over-rate the transformer then (because it's poorly regulated)
the output voltage will be higher than it ought to be under the design
load. This reduces lamp life.

So you're talking about using transformer saturation to limit the voltage ?

That's the only effect I can think of which would have an effect

Shows how little you understand transformers.

Thge output voltage of a transformer is the input voltage divided by the
turns ratio, less the loss factor.

By using highly UNSATURATED iron, the losses can be very very small
indeed. So the regulation becomes as good as the mains voltage feeding
it. That's how the whole national grid distributes electricity - using
transformers.

In the case of switched mode regulators, (el;ectronic transformers) teh
same applies - a decent big one will not sag under load, and most of the
piddly little ones have some form of feedback to stabilise the voltage
anyway.


the only transfornmers that will sag badly under load are overaretted
torioids that have reached saturation, or cheaply designed electroinic
ones that have very poor regulation designed (or not designed) in.



A 200W transformer should run cooler and therefore be less of a risk


Well you got that bit right...almost. Its not necessarily true of
swtitch mode ones, where the actual 'idle' losses may be quite large and
where alos they may require a decent load to work properly at all.

"T N Nurse" wrote in message
...
In article .com,
wrote:

Just a quick one. I have 3 50W 12 v downlighters and a 230v 65VA
transformer. Will the transformer be big enough?

Nope.

If it is not would
it catch fire?

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


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

In message , The Natural
Philosopher writes
raden wrote:

In message , Andy Dingley
writes

On Mon, 13 Jun 2005 20:49:54 GMT, raden wrote:

Well 3 x 50 makes 150 watts (=150VA not splitting hairs)

So you prolly want a 200VA transformer to give you a bit of leeway


Leeway is bad - for transformers at least, it's not so bad for
electronic PSUs.

If you over-rate the transformer then (because it's poorly regulated)
the output voltage will be higher than it ought to be under the design
load. This reduces lamp life.

So you're talking about using transformer saturation to limit the voltage ?
That's the only effect I can think of which would have an effect

Shows how little you understand transformers.

Thge output voltage of a transformer is the input voltage divided by
the turns ratio, less the loss factor.

You're pulling my todger there, aren't you


By using highly UNSATURATED iron, the losses can be very very small
indeed. So the regulation becomes as good as the mains voltage feeding
it. That's how the whole national grid distributes electricity - using
transformers.

Really ?

You didn't really read what I wrote in answer to the previous statement
did you ?

"If you over-rate the transformer then (because it's poorly regulated)
the output voltage will be higher than it ought to be under the
design
load. ..."


In the case of switched mode regulators, (el;ectronic transformers) teh
same applies - a decent big one will not sag under load, and most of
the piddly little ones have some form of feedback to stabilise the
voltage anyway.

Err ...

where were we talking about SMPSs?

We weren't

OK ?



the only transfornmers that will sag badly under load are overaretted
torioids that have reached saturation, or cheaply designed electroinic
ones that have very poor regulation designed (or not designed) in.


A 200W transformer should run cooler and therefore be less of a risk


Well you got that bit right..

Ah, you managed to read it correctly then ...


.almost.

I'm trying to reply in a realistic manner for the requirements of the
post.

I'm not writing a bloody technical paper on the things

Its not necessarily true of swtitch mode ones,

See above



--
geoff

On Tue, 14 Jun 2005 10:59:48 +0100, The Natural Philosopher
wrote:

Not on a toroid,

Get a LV lighting toroid

Stick a voltmeter on it.

Vary the load.

Note the results.


Now I'll grant you that toroids don't vary by as much as non-toroids do,
but it's still a significant effect, enough to affect lamp life.
Electronic PSUs don't do it by any easily measurable amount.

In article ,
Andy Dingley wrote:
Get a LV lighting toroid

Stick a voltmeter on it.

Vary the load.

Note the results.


Now I'll grant you that toroids don't vary by as much as non-toroids do,
but it's still a significant effect, enough to affect lamp life.

All cheap transformers seem to vary their output according to load - and
one for low voltage lighting *will* be cheap.

Electronic PSUs don't do it by any easily measurable amount.

But they'll have other problems.

--
*Learn from your parents' mistakes - use birth control

Dave Plowman London SW
To e-mail, change noise into sound.

Andy Dingley wrote:

Leeway is bad - for transformers at least, it's not so bad for
electronic PSUs.

If you go the SMPS route, then take care to not overrate by too much. I
have noticed that you can get some strange behaviour using dimmers on
electronic transformers when they do not have enough load.

I have one of the TLC 250W dimmable electronic ones that will only
function correctly with at least 150W load on it.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Thu, 16 Jun 2005 01:38:46 +0100, John Rumm
wrote:

If you go the SMPS route, then take care to not overrate by too much. I
have noticed that you can get some strange behaviour using dimmers on
electronic transformers when they do not have enough load.

I've not had this problem. All the electronic PSUs I have are clearly
labelled with both maximum and minimum ratings. Go below that minimum
and they certainly do get grumpy (or shut down entirely), but you can't
say they didn't warn you.

Andy Dingley wrote:

On Tue, 14 Jun 2005 10:59:48 +0100, The Natural Philosopher
wrote:


Not on a toroid,


Get a LV lighting toroid

Stick a voltmeter on it.

Vary the load.

Note the results.


Now I'll grant you that toroids don't vary by as much as non-toroids do,
but it's still a significant effect, enough to affect lamp life.
Electronic PSUs don't do it by any easily measurable amount.

Its a function of design.

Get a 20 ton transformer and it won't budge an inch with up to 4000
light bulbs on it.

Assuming its fed from a perfect supply.

There is a compromise between size, cost and how hot it gets.

Cheap ones sag badly and get rather hot. Expensive ones sag less and run
cooler. Because essentially thay have more iron in them.

electronic ones can be designed with anything from almost no drop under
load to a huge drop under load, depending again on how many bits they
have in them.