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What happens when I try to use a 12V rated LED bulb in a 3.7V light fixture?

andonym

Jan 31, 2022
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Hi all,

Newest of the new here. Can't say I know much about electronics other than superficial stuff.

I inherited a portable battery powered lamp. It can run off ONE 18650 3.7V battery (2600mAH, 9.62Wh). In the same battery compartment you can install a second one, to increase the length of time you use the lamp without having to recharge it.

The spec on the lamp's bulb suggests an LED bulb (G4) that is rated MAX 1.5W. Searching forever on the internet - has shown that a 1.5W G4 LED bulb (2700K, 3.7V, dimmable, 360 angle) is hard to find.

My question: What happens when I try to use a 12V rated LED bulb (also G4, 1.5W, 2700K) in the same lamp run by the one or two 18650 3.7V batteries? I've seen some info about convertors and boosting a lower voltage power source up to the spec'd value of the LED - but wasn't sure if that's the be all end all of that. The fixture can't be modified to install a "booster" - which leads me to be asking this! Am I hooped? Do I absolutely need a 3.7V LED BULB to match the 3.7V fixture's power source?

Thanks for any help and insight you can provide!

Andrew
 

Bluejets

Oct 5, 2014
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Adding the extra cell for more run time would mean adding the cell in parallel to the existing cell.
Still have 3.7v .....trying to run anything 12v from a 3.7v source would vary, depending on exactly what the 12v load consisted of, any result from dimwit to simply will not work.
 

Harald Kapp

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Have a closer look at the existing lamp, measure the voltage across the pins, if you have a multimeter, that is. Is it truly a 3.7 V lamp or isn't it more likely a 12 V lamp?
The lamp should contain a charger circuit for the battery. There could also be a step-up converter that supplies the lamp with 12 V.
 

andonym

Jan 31, 2022
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Have a closer look at the existing lamp, measure the voltage across the pins, if you have a multimeter, that is. Is it truly a 3.7 V lamp or isn't it more likely a 12 V lamp?
The lamp should contain a charger circuit for the battery. There could also be a step-up converter that supplies the lamp with 12 V.

Thanks Harald - don't have a multimeter unfortunately. The charging apparatus for the lamp is simply a USB cord that plugs into any standard USB hub charger. Which in this case is a 5V DC spec. Are you suggesting that a step-up convertor is built in the lamp that brings it from 3.7V to 12V for the LED?

All the G4 capsule LEDs that I can find are 12V.
Here is one example

Martin

Thanks Martin - yes! That's why I'm a bit frustrated. I'm wondering if a 3.6/3.7V LED even exists - or is it that the markings/specs on the current LED are actually false. Hence my leaning to trying out a 12V LED in this 3.7V system - to see if anything happens. Just was wondering if I did so - should I expect something I would regret....
 

Martaine2005

May 12, 2015
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3.6/3.7V LED even exists - or is it that the markings/specs on the current LED are actually false.
Are you saying the G4 LED in your lamp is labelled 3.7V?.
The G4 I looked up was 12V.
The circuitry will almost definitely have a boost converter.

Martin
 

andonym

Jan 31, 2022
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Are you saying the G4 LED in your lamp is labelled 3.7V?.
The G4 I looked up was 12V.
The circuitry will almost definitely have a boost converter.

Martin

Hey Harald, Martin,

I got my hands on a multimeter - checked the battery voltage first (was measuring approximately 3.1V), then checked the voltage across the pins of the LED bulb while on max setting - the reading was also approximately 3.1V.

So that's pretty much confirmation that that bulb is a 3.7V bulb (and not a 12V) correct? Unless you're going to tell me that a converter could be present in the LED? :\

Thanks for your help!
 

Harald Kapp

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Unless you're going to tell me that a converter could be present in the LED? :\
No I'm not. Most unlikely.

Are you sure this is a G4 lamp? Not a bare LED? May look similar to a G4 in a layman's eye. Post a picture (<300 kB) of the present lamp and socket.
 

andonym

Jan 31, 2022
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No I'm not. Most unlikely.

Are you sure this is a G4 lamp? Not a bare LED? May look similar to a G4 in a layman's eye. Post a picture (<300 kB) of the present lamp and socket.

Pic 1: fixture with pin socket exposed (looking down onto fixture)
Pic 2: G4 bulb
 

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Bluejets

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I think it about time you showed what the actual complete fitting is.
Might be helpful.
 

Bluejets

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Pretty much self explanatory I'd have thought.
Is what you show the complete light unit..?
Trying to get a handle on exactly what you have there, a link to where you bought it would possibly help also.
Any LED with multiple chips would be, as mentioned earlier by a few answers, 12v.
So it would seem there may be a boost converter internally that may have died.
 

Harald Kapp

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I agree. Definitely looks like a 12 V lamp.
 

andonym

Jan 31, 2022
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Pretty much self explanatory I'd have thought.
Is what you show the complete light unit..?
Trying to get a handle on exactly what you have there, a link to where you bought it would possibly help also.
Any LED with multiple chips would be, as mentioned earlier by a few answers, 12v.
So it would seem there may be a boost converter internally that may have died.
I agree. Definitely looks like a 12 V lamp.

Not much to it!
 

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Martaine2005

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For the purpose of complete understanding, can you tell us the markings, letters and numbers on three components. U1 U2 and U3?.

also, on the back of the PCB in the centre, there looks to be three solder joints that appear cracked. The photos are clear enough to be sure.

Martin
 

Harald Kapp

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Hmmh, no inductor there (at least not obviously). Not a step-up at all? Knowing what the ICs are as rquested by Martin will help to clear up matters.
 

andonym

Jan 31, 2022
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U1:TC056A; CM8U1S1
U2: no surface markings
U3: L7136A; 2024

The solder joints on the opposite side are not cracked, those look associated with the magnetic charging connecter on the main side of the board
 

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