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!!

 

[spark 68]

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!!

That is not a very good example to use E54,
The LED fittings/lamps have drivers built in, either constant current, or constant voltage types, and so have some secondary regulation applied to the low wattage fittings/lamps.

Again so long as you don't have massive discrepancies between the supply and demand then it is not a problem.:icon7:

 

[Engineer54]

Maybe not an overly good example, but a good enough example for me, to demonstrate the point being made!! lol!!

 

[Marvo]

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.

Under no-load and normal load conditions a standard inductive (wire wound) transformer will have a secondary voltage pretty close to it's stated voltage. Only if you overload it then the iron and copper losses start running away and the secondary voltage could drop.

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.

The OP stated it's a wire wound transformer. A switch mode supply requires a minimum load to ensure a stable output voltage, an inductive transformer doesn't.

 

[spark 68]

Hi Marvo,
Yes I know the OP was about conventional wound transformers.

If you run a conventional wound transformer significantly under loaded the voltage does rise, I have an 18V transformer here and it's open circuit voltage (no load) measures approx. 25V rms, this falls as you load it to it's constant rated VA (circa 6VA on this one) and then it measures near 18V rms.
It does depend to a large extent on the construction of the transformer and it's size (VA rating).

The point I was making was that they are unregulated.

We were agreeing if the ELV lighting transformer was used to the manufacturers recommendations, min-max number of lamps then there should be no problems, as it was designed for this purpose at the outset, and the manufacturer would have already done all of the calculations necessary to ensure it fell within acceptable limits.

It may reduce lamp life by a small amount with a lower number of lamps than the max allowed for (if one failed for example) due to a slight rise in output voltage (less loading), and I was pointing out that a filament lamp is not a linear load and so the situation may not be as bad as if it was calculated as such. :icon10:

 

[GLENNSPARK]

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!!

this sounds a bit suspicious eng...

 

[Outspoken]

Interesting comments...has no-one actually considered that the rated output of the Tx will be maximum design load of the Tx for constant use and not the absolute maximum demand that can be placed against it. If it is to meet BS or EN rating then the Tx should not see a rise in output voltage of ~+/-10% when operated at +/-50% rated output. I'll have a hunt about and try to find the exact wording and the technical data to back that up so you can pass it back to "Mr Technical department".

The relevant paper is BS EN 61347-2-2, Performance requirements for lamp control gear are the subject of IEC 60921, IEC 60923, IEC60925, IEC 60927, IEC 60929 and IEC 61047 as appropriate for the type of lamp control gear.

Clearly the LED's you used E54 are AC rated and thus have built in voltage/current control because connecting a standard LED to an AC output, even at 12V would blow the LED to hell and back..