I've got some LED spotlights and I'd like to know why each one of the transformers/power supplies (Infitronic ELP6X3LSD - seems like Sunrise now makes them?) has a big resistor (Arcol HS255K6J with heatsink) across the mains input?

If this matters, the LEDs and transformers were powered by a Lutron Rania RNDU-252 dimmer with no neutral so it could be simply to pass enough current to power the dimmer itself, but then why would it be at all 4 spotlights instead of just one? It appears that the last spotlight had 2 of these resistors - could it be that they realized at the end that they still didn't put enough of them to power the dimmer (with no neutral) and decided to add one more?

The 4 transformers (with their respective resistors - one even had 2) are connected in parallel to the dimmer's output. I've now removed them all and things work fine with another dimmer (which has a neutral so no bypass resistors required) and the power consumption dropped in half (from ~90W to ~40W) across the entire set of lights.

  • Don't know but I suspect that the arrangement might be so that the power supply has a way to work with Pre-LED dimmers and "read" the intended dimming level off the resistor voltage (and allow those types of dimmer to work.)
    – Ecnerwal
    Commented Aug 2, 2021 at 15:28
  • That resistor draws about 20 mA (assuming 120VAC mains), so not much. But if each LED is a standalone unit, then each one needs its own power control. Can you post a circuit diagram? It's not clear where the power supply fits in, since the dimmer switch is in the AC-voltage path. Commented Aug 2, 2021 at 17:58
  • @CarlWitthoft this is in the UK so a 240V system. Yes each low-voltage LED is connected to its own power supply (linked above) with the resistor (and big heatsink) in parallel across its mains input. I've now removed them all with no ill effects besides a significant drop in power consumption. Commented Aug 2, 2021 at 19:16

1 Answer 1


A coupling resistor in a mains-powered LED fixture has two major purposes.

  1. Dimmer compatibility. TRIAC-based leading edge dimmers (used with older components like an RC network feeding a DIAC to control phase triggering) need a certain amount of current for the gate to stay latched. Older TRIACs needed a lot more load and trigger current than todays so-called "sensitive gate" generation, up to 50ma. While an incandescent chews up 50ma (5-12W) without any side-effect, an LED can illuminate at that low current. This would severally limit the range of dimming, rob the TRIAC's gate of enough current to latch (neutral-less dimmers), and cause strange side-effects that consumers would associate with the fixture: eg. 1 unit doesn't work, but using 2 of them does, or using with 1 incandescent in the run works fine.

  2. Off-state flicker elimination. Owing to the relative low current needs of LEDs, the tiny leakage current downstream of a neutral-less dimmer is enough to produce illumination. The amount of light depends on a few factors, mainly the topology of the LED's internal mains-to-DC converter, the voltage of the LED array, and the amount of stray current. This can be very noticeable in certain configurations, and ultimately the consumer/installer will likely blame the fixture. By placing a resistor with the load, such low level stray currents get absorbed by the resistor, instead of finding their way through the driver and into the LEDs. This solves the problem of "never going 100% off", flickering when off under transients like ACs tripping on.

All in all, it eliminates two minor yet common issues associated with LED fixtures, for a couple bucks added onto the BOM. I would expect something using these to cost a bit more than an entry-level fixture, but not as much as a fully professional integrated system where the dimmers and fixtures are professionally matched. This is for a middle path of high apparent performance with little fore-knowledge of implementation details; aka high-quality DIY gear.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.