Wire wound 12V transformers

[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..