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I'm gradually replacing halogen lights at home with LEDs.

We have a couple of 12V rails with a 5x35W rating when using halogens. When using the same transformer with LED's, can I use the theoretical maximum power? eg. If I'm using 5W LEDs instead of 35W halogens, can I mount up to 5 x 5x7 = 175 replacement LEDs? (If I could afford that of course).

Given that LEDs are not resistive loads, does this simple math apply? Or are there other factors that need to be taken into consideration?

At the moment, I'm leaving some halogens on each rail, as I've read that the transformer needs a minimum of resistive load (not sure what that minimum would be, nor I've found comprehensive guidance on this topic). The transformers are coil based (not electronic).

Any advice? Thanks.

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2 Answers

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While most 110-240V LEDs are nasty complex loads due to their AC/DC power supply, many 12V LEDs are probably no more complex than the LED and a resistor, or possibly the LED, another diode or 4, and a resistor for 12VAC service. That sort should run fine of a 12V supply at full ratings and with no need for a regular halogen bulb in circuit.

If your 12V LEDs are straightforward "bulb replacements" or devoid of a power supply circuit between the 12V supply and the LED, they will probably behave as above - after all, when the halogens you put in circuit burn out, that's where you'll be anyway. If they have a more complex power supply, the manufacturer would be the best bet to determine the correct deployment on your power supply. In all likelihood, they will be fine.

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In general usually LEDs are sold with some sort of a current limiting mechanism (usually a resistor) so there should be no difference between plugging a regular, halogen or an LED load. With that being said, it is generally recommended not to go above 80% or so of maximum capacity.

However, most consumer devices are designed with that (or usually much greater) margin in mind so you can safely go up to that stated capacity. The stated capacity is usually a fraction of the actual capacity. One way to tell would be to find a manufacturer's datasheet and look up 'absolute maximum ratings' table.

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