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4.5 volt to 12volt

jerryb

Feb 18, 2014
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Hi, working on a project, I've got a wire string of 20 red led's running off 3 x 1.5 v aa batteries and I need to be able to run them off a 12volt LED driver. Where can I find some info on calculating the resistance? Or am I looking to the wrong solution. Thanks for any help.
 

Audioguru

Sep 24, 2016
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An LED driver is usually constant current then it adjusts its own resistance.
Set the LED driver's current for the brightness you want and use as many LEDs in series in each string as their max forward voltage and the minimum output voltage of the LED driver allow.
 

jerryb

Feb 18, 2014
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I think they are either 0803 or 0603 red. Look like these only red and with 3 AA batteriesled bat.jpg
 

Martaine2005

May 12, 2015
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May I ask why you “need” to run them on 12V?.
Is it a car or you have a 12V adapter?.


Martin
 

Bluejets

Oct 5, 2014
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I was thinking specs.....anyhow, what is it you want to do exactly, run off a 12v supply..?
I cannot see any control board in the battery casing.
If so, just run off a dc dc down converter set to 4.5v.
 

jerryb

Feb 18, 2014
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dc converter.jpg I'm trying to create a special effect of dragon fire. I've got two pc fans creating the wind and I need red LED's to light it. The fans run off 12v so I'm trying to run the lights of the same driver as it is on public display and I don't want to keep having to go and change the batteries. Do you mean one of these?
 
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jerryb

Feb 18, 2014
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Ok, so tried the buck converter but no joy, draws too much power from the 12v supply and the leds are flashing.
 

bertus

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Nov 8, 2019
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Hello,

What is the current draw of the leds?
It can be the buck converter has gone in current limiting to protect itself.

Bertus
 

jerryb

Feb 18, 2014
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HI, afraid I don't know the current draw, not sure how to find out.
 

Audioguru

Sep 24, 2016
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I think the buck converter does not detect enough output current so it goes into burst mode with the output turning on and off.
 

jerryb

Feb 18, 2014
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Curious thing, when I disconnected the lights from the battery box there was a resistor on the negative side, I think it was 570 ohms. How does that work?
 

Audioguru

Sep 24, 2016
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570 ohms is not a standard resistance.
LEDs must have a resistor or circuit in series to limit the current. Without the resistor then an LED will burn out immediately when a voltage too high is connected to it.
The white LEDs use about 3V and three new batteries produce 4.5V. Then the voltage is 1.5V too high and is high so that the LEDs still light when the used battery voltage drops to 3V. Ohm's Law calculates the current. 1.5V across the resistor divided by the max current you want is the resistance needed.
 

jerryb

Feb 18, 2014
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OK, I'm confused, how come when I connect the 20 leds to a 3v cell with no resistor they light up? I don't understand why I cannot connect to a 12v dc supply with a resistor on the positive side. Basically I haven't a clue.
 

Audioguru

Sep 24, 2016
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A cheap battery cannot produce a high current. The "Super Heavy Duty" AA cells from ebay are mostly full of rice.
An expensive Name-Brand battery produces enough current to explode LEDs that do not have something to limit the current.

Oh, maybe your 3V battery is a little CR2032 coin cell that is too small to produce a high current.

If course you can use 12V and a series resistor to limit the current so that the LEDs do not burn out.
But then the resistor will get VERY hot wasting a lot of the battery power.
 
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jerryb

Feb 18, 2014
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Admittedly the cr2032 does not give enough power to make the leds very bright but it lights them so I would have thought that there was a way of lighting the 20 (0603 I think) leds with a 12v dc supply.
 

Martaine2005

May 12, 2015
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The CR2032 doesn’t supply much current at all, hence no resistor required.
The 12V OTOH can. Series current limiting resistor will be required.
It if worked on CR2032 but doesn’t now on 12V, you probably killed the LEDs.

Martin
 
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