Low Volt LED Question

If the transformer is a traditional 'iron-core' type, then it will be running at much less than its rated power and will therefore run much cooler since the internal losses (which appear as heat) will be much less. I am assuming the transformer is a traditional iron-core type. If it is a more modern "electronic" type then these have a 'minimum load' requirement and if run at or below this they may give problems with an LED load, flickering, strobing, shortened lamp life, radiated interference to TV signals, etc. A 600 Watt iron core transformer is going to be quite a heavy item, so this may give you a clue about what type you have.

Of course Dr that should be 48 watts of lights but then why would you need an 88-w transformer

Yes there will be a difference using the old transformer; depending on their internal resistances and the like, the lights will probably be brighter than s/b. Being lazy however I'd probably use 'em anyway

When you day "(300w + 300w)" I presume there are two secondary windings each supporting 5 lights. So if the array proved too dazzling I might connect the two 5-led banks in parallel and run 'em off one winding

If you have already changed the lights and they are running steady with good brightness then you are clearly OK.

The reason for the 88w trans is I had it lying around and figured it would be more economical to run it rather than the 600w one. The 600w trans is heavy but not to heavy so I'm guessing it's an iron core type as is the 88w one. I still am not sure if I'm better off (total power consumption) running the 10 - 4w led's off one side of the 600w or using the 88w. Regardless of which one I use is the power consumption just the 40 total watts of the lights or is there an additional loss within the transformer, if so which trans would be greater loss?

The 600 Watt rating of the larger transformer means "this transformer can supply up to 600 Watts". It does not mean it is necessarily more expensive to run than a transformer that can only supply up to 88 Watts.

Transformers are not 100% efficient, they have internal losses which appear as heat. Practically, these losses are a small percentage of the load being supplied at the time.

We do not know which transformer is the more efficient of the two, although generally speaking, all things being equal, larger transformers are more efficient (lower percentage loss) than small ones. However I feel we are chasing a red herring here.

We are talking about losses of maybe 2% to 5% of load. 40 Watts worth of lamps is going to demand 40 Watts from the low voltage side of the transformer which will result in the high voltage side pulling probably less than 41 Watts from the house AC, the difference appearing as heat in the transformer. These are ballpark figures and the point is that the heat loss is [EMPHASIS:] tiny, very small, negligible, can be disregarded.

Whether you use the smaller or larger transformer does not matter from the power consumption point of view. The difference in power consumption is going to be tiny, not worth worrying about.

Personally I would use the large transformer because it will be running at less than 10% of its rated load and will therefore be running cool.

The people who need worry about transformer losses are power companies and industrial users installing transformers rated at thousands or millions of Watts. Often a more expensive but more efficient design (less losses) will be a better buy than a cheaper (more lossy) one because the more efficient design will need less cooling equipment to be bought (and kept running during its life).

However, I repeat!!! Transformer losses are negligible at the domestic level! Use the one you prefer, it isn't going to make any difference to your power bill.

I fully endorse Con above, possibly excepting

Cool of course but the LED's will probably be running slightly hotter and so suggest you find someone with a voltmeter, to confirm the overload isn't excessive

I don't understand what you mean here. A step down transformer with a secondary output of 12 volts, rated at 600 Watts, compared to one rated at 88 Watts, is simply one with a bigger core, thicker wire in the windings, etc. Thus it can cope with a greater amount of heat arising in the windings and core (the copper and iron losses). Like the smaller transformer, it only presents 12 volts to the load, and is not going to over run the LEDs or make them "run hotter". Please clarify.

Thank you all for the info. I guess I will continue to run the 600w trans (one side at 300w) at 14% capacity rather than the 88w at 46% capacity. This way if the need arises for more power it will be there without having to change it out again.

I'd suppose, depending on factors like winding resistance, the bigger transformer at a reduced load might put out a slightly higher voltage

To utter such nonsense, Dale probably never connected a LED.

As usual, he is just pretending he knows something about electricity..

Good point. I kind of imagine it would not overdrive the LED lamps much though. The amount by which a transformer's output voltage varies with load is called its "regulation". A good power transformer should exhibit a regulation percentage of less than 3%.

One thing I have thought of is the issue of fusing. The 600W transformer will happily supply 50 amps into a short on the 12 volt side without breaking sweat and plenty more before any fuse in the primary circuit lets go so maybe if there is a fuse on the secondary side its rating should be checked. For a 40 Watt load a 5 amp fuse would be plenty. I know 12 volts won't electrocute anyone but there could be quite a burn-up if a short happened. If it does not have one you can get an in-line automotive fuse holder.

Yea Con, I was supposing the remote chance of an unusually high internal resistance so lousy reg. However doesn't the load presented by a semiconductor like the LED increase disproportionally as voltage rises --compared to, say, a common resistance like the incandescent bulb--so that mightn't a slight increase over its rated voltage push its ratings more than we might otherwise guess

Agreed though, unlikely

A wise move

I have to retract what I said before. I spoke to a friend who installs lighting yesterday, and he said "When going from halogen to LED lighting, you should get a smaller transformer because the old, large one will over-run the LEDs and shorten their life". So there you have it. LEDs are not a plain old resistive load. Dahileman, you were right. I apologise for misleading the OP.

Why, thank you Con. It's not everyday…..