Wire wound 12V transformers

[roukel01]

Morning all,

I've just received an email from a technical department regarding a tradtional "wire wound" 12V transformer.

He tried to explain that a 300W wire wound transformer needs to have 300W worth of lamps connected to it, as the transformer cannot regulate the power it outputs. Therefore, if 200W of lamps were connected, 300W would be passing through them which would therefore shorten the life of the lamps.

Is this true because I can't see how it is? (perhaps I'm being daft!! lol)

Any explination would be apperciated.

 

[Marvo]

If you take his theory to the extreme then if there's zero load connected there's still 300W energy consumption....I'm not buying it. To maintain a constant power output with varying loads a plain wire wound transformer would need to vary the secondary voltage accordingly, which it can't do......or it would need to rewrite certain electrical laws that up until now have been set in stone. Unless it's a complex auto transformer the secondary voltage will always be determined by the pri-sec winding ratio, I doubt that's the case though because auto transformers have no isolation properties so they can't be used in SELV lighting systems. Just out of interest whats the make and model number of the Tx? Can you copy/paste the explanation he gave you word for word? I think there may be confusion somewhere.

 

[HandySparks]

There's an element of truth there, but it's not explained correctly. The output of a wirewound transformer will be at nominal voltage when it's connected to its rated load (assuming nominal input voltage). If you reduce the load, the voltage will rise and this will shorten the life of any connected lamps as they'll be running at a higher voltage than specified.

 

[telectrix]

as marvo says. if you've quoted him correctly , he's talking cobblers. say you had a 300w lamp, drawing 300w. if the lamp blew ( open circuit) then there would be no current flow therefore no power consumed except for that drawn by the primary winding under no load conditions.

 

[Engineer54]

I take it this so-called technical department, was insisting that you use their transformer/driver, rather than your existing 12v transformer(s). ...,lol!!

 

[roukel01]

ok chaps.... I've copied in the email..........

The Livewire Transformers are the old ‘wire wound’ type, therefore they do not regulate the output power so they will always put out 200W and 300W respectively. If you use a 300W Transformer to power 10x 20W lamps then you will be putting 300W into 200W of lamps, the result being you will shorten the lamp life, you should really use a 200W Transformer for this application.

 

[spark 68]

HS has it about right in post #3

As it is unregulated the output voltage is determined to an extent load dependent.

A SMPS type is usually voltage regulated subject to minimum loading.

 

[Engineer54]

lol!! using a 200W transformer to a supply 200W load will shorten the transformers life!! Never try running any transformer at it's max output for any length of time!! ...Dear oh dear, manufacturers these day's, ...and everybody is always going on about following manufactures instructions!! ...Not this one i hope!!!

Ask them how a 200W load is going to absorb 300W?? lol!!

 

[spark 68]

If the transformer is a much larger VA type, and it is grossly under loaded it's output voltage will rise, and could shorten the life of the lamps, but as the lamps run hotter the filament will have a higher resistance and reduce the current somewhat, so the net wattage may not be as high as first thought.

Filament lamps are not a linear ohmic load, they exhibit negative resistance.

But we are talking extremes here, if you are using a transformer suited to these lamps then the manufacturer will give recommendations to how many lamps or total wattage can be safely run, both min and max.

 

[Toonlad]

Ive read these posts twice now, and its just crackers. "Engineer54" said in laymens terms....Ask them how a 200W load is going to absorb 300W?? lol!! ..... Now this is how i see it.

Whats there view on dimmers??

 

[spark 68]

lol!! using a 200W transformer to a supply 200W load will shorten the transformers life!! Never try running any transformer at it's max output for any length of time!! ...Dear oh dear, manufacturers these day's, ...and everybody is always going on about following manufactures instructions!! ...Not this one i hope!!!

Ask them how a 200W load is going to absorb 300W?? lol!!

Hi E54,
Talking in extremes,
If you double the voltage through a lamp assuming the filament resistance stays the same (it doesn't), you would be doubling the current, with the result that the lamp would draw 4X the wattage (2*V) * (2*I) = 4* W, but as I said earlier a filament is not a linear load, the filament resistance changes depending on temperature, IOW the higher the temperature of the filament then the higher resistance.

This is of course taking things to extremes, in reality a transformer designed for these lamps would not push things to the extreme example above, and in reality probably would not make much difference.

 

[Knobhead]

Unless someone changed the laws of physics and not told me.

Power in = power out plus internal losses. The 300W is the transformers maximum loading.


E54, are you going to take all the inefficient transformers you’ve installed out? I know I’m not.

 

[spark 68]

That's partly what I was getting at Tony,

The only time you would have a problem with unregulated supplies being underloaded and overvoltage would be if the transformer was massively mis-matched to the load.

These small ELV lighting transformers are correctly designed for the expected loads/characteristics of the lamps, and so are no problem.

As another poster pointed out, it sounds like marketing BS to me.

 

[Engineer54]

Unless someone changed the laws of physics and not told me.

Power in = power out plus internal losses. The 300W is the transformers maximum loading.


E54, are you going to take all the inefficient transformers you’ve installed out? I know I’m not.

Nope!! ...lol!!

 

[Engineer54]

I've just converted for a friend, some old 4 X 50W halo fittings with individual transformers, to 4 X 7W (or 10W) LED lamps, and used 2 of the original four 60VA transformers to supply them!! These fittings are for a shop that's open 9am to 10pm 6 days a week. ...No complaints to date!! lol!!