LED driver delay on startup - why?

[Dr Mike Oxgreen]

Not sure if this is the best forum, or Homes Gardens & DIY - if mods think of a better place for it, please feel free to move! Isn’t there an electronics forum?

I recently installed LED tape lighting under the cupboards in our kitchen. They’re driven by drivers that supply 24V DC - these ones, to be precise. And yes, they’re probably cheap Chinese cr@p, but easily replaceable if/when they fail.

I’m pleased with the result, but curious about one thing: when switched on, there’s a delay of about a second and a half before they light up. Can anyone explain what is going on during that delay? I’m just intrigued to know.

[Mr Pointy]

I've found the same with the Varilight wall switch dimmers that feed to bedroom downlighters. I assume they ramp up the supply gently like a soft start to protect the LED drivers.

[dhutch]

I dont know, to an lesser extent you get the same with a lot of '240v' LED bulbs, and flood lights.

Particularly noticeable if you mix brand/spec on the same switch and they have different delays.

[Dr Mike Oxgreen]

Mr Pointy said:

I've found the same with the Varilight wall switch dimmers that feed to bedroom downlighters. I assume they ramp up the supply gently like a soft start to protect the LED drivers.

Yes, I've got a dimmer in another room that does something similar.

But in the case of these LEDs in the kitchen, there's a short delay and then they come on at full brightness with no ramping up. And it must be the drivers themselves that are creating that delay, because they're connected directly to the mains via a normal switch.

The lights that were there before were G4 bulbs into which I'd put LED G4 replacements and replaced the drivers with LED-friendly ones that could cope with the low load. I assume those drivers provided 12V AC and the LED bulbs had rectifying circuitry in them. Those came on instantly with no delay.

Edited by Dr Mike Oxgreen on Thursday 22 February 11:18

[Road2Ruin]

It's the driver. I believe something to do with the power output of it. We have two sets in our kitchen of different lengths, so need different size drivers. The longer one takes a second longer to come on. The electrician tells me it's the driver.

[Dr Mike Oxgreen]

Yep, I'm certain it is the driver that's introducing the delay.

It's interesting that the higher power drivers have a longer delay. Perhaps that's a clue to what's actually happening during the delay. Is there a whacking great capacitor in there (perhaps to smooth the DC output) that has to charge up before the output is stabilised? A larger power driver would presumably require a larger capacitor...?

And I'm glad I went for identical drivers for each of the three runs of LED tape. At least they all come on together, albeit a second or so after the ceiling lights have come on.

Road2Ruin said:

We have two sets in our kitchen of different lengths, so need different size drivers.

Couldn't you have done what I've done, and simply use identical drivers for both lengths? It doesn't matter that one of them would have been bigger than required.

Edited by Dr Mike Oxgreen on Friday 23 February 18:07

[OutInTheShed]

I bought some cheap no-name 12V drivers on ebay, they have no noticeable delay.

LEDs at 'chip' level have delays in the fractions of microseconds league, at most.
LED bulbs may have soft-start or just smoothing circuits to avoid flicker/strobing when fed by dirty supplies.

[Dr Mike Oxgreen]

OutInTheShed said:

I bought some cheap no-name 12V drivers on ebay, they have no noticeable delay.

Is that 12V AC, or 12V DC?

The 12V AC drivers I used to have, powering G4 LEDs, also had no delay. Essentially the “drivers” would simply have been a step-down transformer converting 230V AC to 12V AC.

The DC drivers I now have do much more: as well as reducing the voltage, they have to do an AC/DC conversion, which is a highway to hell.

So the delay appears to be unique to DC drivers. Anyone know why?

Edited by Dr Mike Oxgreen on Friday 23 February 18:11