G4 bulb replacement with LEDs?

[drossall]

We've got an Ikea lamp that we like - I'm pretty sure it's this Kryssbo.

We had one or two bulbs go, and I've found that they have cooked the fittings, which are falling apart and need replacing. So LEDs (and new fittings) seem the way to go. I've read this and, daft as it seems, I'm unsure whether I need DC or AC, even though I know the difference. The replacement bulbs are just marked 12V 10W.

Of course I could replace the transformer as well, but I was hoping not to do that.

[Diver300]

Halogen G4 bulbs will run from AC or DC, so it's not possible to tell what the transformer is putting out, other than it will have an RMS voltage of around 12 V.  Many older lamps that take G4 bulbs will have traditional transformers which output 12 V at 50 Hz, so LED replacements will usually contain bridge rectifiers so that they will run from either AC or DC.

Have you got any information about the transformer?

Some electronic transformers which are designed to run halogen bulbs will only produce 12 V RMS with the load of the halogen lamp, so they not work correctly with an LED load. However, if your lamp has several lamps on one transformer, then the transformer is less likely to need a particular load, or it wouldn't work if one or two bulbs fail.

I would guess that most would LED 12 V lamps would work fine.

[drossall]

That's helpful already. I did notice though the way that that page to which I linked said that it was important to know whether the output was AC or DC?

All I know about the transformer is what is printed on it:



That hard-to-read bit in the middle says PRI 220-240V 50/60 Hz - SEK 11.4V 70-105W - ta 25 degC tc 55degC so nothing much unexpected.

There are six bulbs in all.

Thanks.

[Diver300]

On the G4 bulbs, I can't see any problem running a G4 12V bulb designed for ac on dc. There are lots of things that need ac, but I can't see any way that a 12 V bulb would only work on ac. Obviously a dc lamp on ac would only work for half of the ac cycle, so will be dim and flickery. I doubt if ac would kill a dc G4 bulb as that would imply that inserting such a bulb the wrong way round would kill it.

The fact that the power supply has a minimum load as well as a maximum is a bad sign. It implies that it might not work, or the voltage would be too large or unstable below 70 W, so that would be a big problem with LED lamps.

However, G4 LED bulbs that replace 20 W halogens are only about 2W, so you would only need 15 W or so. A new power supply that only has to supply that much power isn't expensive:- https://cpc.farnell.com/pro-elec/pel00848/ac-dc-psu-12v-2a-2-1mm-dc-plug/dp/PW04264?st=power%20supply so you could just swap for that, and you will have a stable 12 V dc for up to 12 LEDs, and it would run one halogen for testing purposes.

[drossall]

Thanks. That's not much at all. The current supply (sorry) is variable output, and my wife likes to dim the lights to whatever level is best for reading etc. Any recommendations on a variable supply? Oh, and daft question, but I've obviously got to join six or seven cables in two places in a very limited space in the top. At present they are crimped and soldered. My usual connector strip isn't going to work - it's just for pairs What's the best kind of connector for that job, preferably re-usable just in case of future failures?

This is going to be a complete re-wiring. There won't be much left of the old lamp!

[Diver300]

Variable power supplies are rare because most loads are expecting a fixed voltage. Does the existing power supply have a control on it?

LED replacements may not dim in the way that you are expecting. You definitely need all the lamps to be the same, and I have bought LED lamps that differ when supplied in one batch. I seen (well, I've even designed and built) LED lamps that have constant brightness over wide voltage ranges, so if you get ones like that, dimming isn't possible.

For the connectors, I suggest you look at the Wago range. Just note that some of those, generally the ones without a release lever, aren't suitable for stranded wires, only solid.

[drossall]

Thanks. Yes, the existing supply has a large slider that acts as a dimmer switch. I rather think that ability to dim will be a customer requirement

[grams]

The ~ on the "transformer" next to the output voltage indicates it outputs AC. It being 220-240 V input rather than 100-240 indicates it's of the unsophisticated type that's unlikely to work with LED loads.

You need a PWM DC type dimmer like this:
https://www.ebay.co.uk/itm/12V-DC-PWM-Manual-Knob-Dimmer-Switch-With-Button-for-LED-Strip-DR/353063397244

For this to work you'll need the bulbs to be as simple as possible - hopefully just a resistor and 3 LEDs (or a multiple of three). They don't need to be marked as "dimmable", as that means they have some special circuitry that translates a dimmed AC waveform into LED brightness.

[Kim]

Almost all of the G4 LED bulbs I have seen are non-dimmable. The others don't have any indication. I am sure that if they were dimmable it would be on the box (in a shop) or in the description (online).

LED replacement lamps that work with a traditional leading-edge AC dimmer will indeed make a fuss about being dimmable on the packaging, as it makes them more complicated and expensive.  I'm not sure you could actually fit the required electronics in a G4 package.

You either need a PWM LED driver that can drive the LEDs directly (and accept that anything less than 100% brightness will be strobe-o-vision), or an unobtanium current dimming driver and carefully matched LEDs.

[Diver300]

If you're going to have to rewire anyhow, how about adding a switch for each light? 0 - 7 brightness in whole number steps would be good enough for just about all circumstances.

You could just have one or two switches, giving  0 - 3 - 7  (with one switch running 4 of the lights) or various combinations that I can't be bothered to work out if you have two switches.

[drossall]

Interesting idea though, if you look at the link I gave, the lights all point in different directions and the result could be very uneven.

[Diver300]

I was under the impression that the light output from each bulb is nearly omnidirectional. I couldn't see anything in the line drawing that looked like anything other than frosted glass. A G4 lamp will need a reflector to make the light output narrow.

There is obviously shading from the the other lamps and from the stems.