# If current is fixed, is it possible to increase more voltage to a led?



## Rimbaldo (Jan 28, 2013)

Hi! 

I have an old dental headlight.. I don´t know which led it uses, but it´s reaaaly bright. I plan to build my own headlight, but I came to this doubt...

This headlight has 8.4V in its source, and it´s current is fixed in 0,48A. It must have a led driver inside the source. And it goes directly this way to the Led. And it never burned the led. So, 8.4V * 0,48A gives 4.03W. My guess is that the Led is 5W then. But... Can a LED, Cree, Luxeon, stand 8 volts, instead of the usual 3.2, 3.4, without failing? 

If I limit the current, can I increase a led´s voltage way more that it´s designed?

Thanks!


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## bshanahan14rulz (Jan 28, 2013)

Voltage will have to be dropped somewhere. Try dropping it via a resistor instead. If you limit the current, whatever regulator you use will probably be what ends up taking the brunt of the extra voltage, and turning it into heat. It's like the LED takes the power, says "Oh, this is going by at 0.5A? I'll just drop 3.3V." The regulator gets the leftovers and says, "Oh, you missed this 5.1V, I'll just burn that up for you" and turns that last bit of voltage into heat, so that the battery doesn't have to take the heat, so to speak.

The part of the equation you are missing is this: your light most likely has a DC-DC converter, with current limiting already built into the output side.

Imagine the old water bucket clock, a bucket with a hole in it. Lets say for our own thinking that water always flows out of that bucket at the same rate. You could say that the bucket outputs a constant current of water.

Now, let's say you want to "power" that bucket with a different water source, a pressure washer. If you just point the pressure washer into the bucket and turn it on, the bucket will fill up and overflow. So, a switch turns on and off this pressure washer in such a way as to make sure the bucket always has enough water in it to flow water out at the same rate. So, even though you are feeding it with a pressure washer that you are turning off and on, you have a steady, slow stream of water to power your water-powered LED ;-)


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## MikeAusC (Jan 28, 2013)

Rimbaldo said:


> . . . . This headlight has 8.4V in its source, and it´s current is fixed in 0,48A. . . . . .



I can't imagine a Dental Headlight using 5 watts - it would be way too bright and too hot.

It's more likely it's 0.48a at the LED so it's a 1.5 watt LED.

All white LEDs operate with about 3 volts across them. You need a driver that drops the battery voltage and regulates the current. If you increase the LED voltage by 10%, you may increase the current it draws by 100%.

Just what is it you want to do ?


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## Rimbaldo (Jan 28, 2013)

Ok... Let's go in parts...

The board inside the power source (I opened it) for my headlight (old) gives me on the output 8.4 v (from a 7.4V 2400mh Lithium battery - possibly 2 18650s). This output goes directly to the led, with no resistors in the way. Just the cable to the Led (I also opened it)

By turning the potentiometer in the power source, I can vary the current from very little to 0,48A.

So The power source has a DC-DC converter (7.4V -> 8.4V) and these 8.4V are constant untill the battery looses power. I tried different LEDs (the one from the headlight and one I had lying around) and both used at max 0,48A. So I guess there's a limit in the current in the power source's circuit also, because different leds at the same voltage (8.4V) used the same current (0,48A).

The Led from the headlight never burned and I used it a lot, for a year. The other one I had lying around I just tried it for some 30 seconds on it's own star heatsink.

So... you're saying that these 5 extra volts are not in the led, they're being dealt in the dc-dc inside the power source, and if I measure the voltage, when the circuit is on, on both poles of the led, it'll give me 3 Volts? I didn't try it though yet....

I want to build my own headlight...My loupes are 4.5x, and I wish I had more light in them. I will buy a 8.0x loupes, and the more magnification you have, the more light the optics use. Everything gets more dark the higher the magnification is. So I need a more powerful headlight.

****EDIT****

Ok, I tested here the voltage between the led´s terminals with the led turned on, plugged into the 8.4v battery... and... as you both said... it draws 2,94 volts... so what happens to the other 5.4V ? The driver circuit inside the battery deals with that? Is this the right design to power a led? I´m planning to build my own, but with the led´s voltage fixed to the led´s specifications.


So


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## SemiMan (Jan 29, 2013)

The dental light has a switch mode power supply. These are able to convert from one voltage to another with normally minimal losses. It is tough to explain exactly how they work without some basic understanding of electronics. 

Semiman


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## MikeAusC (Jan 29, 2013)

Rimbaldo said:


> . . . The driver circuit inside the battery deals with that? Is this the right design to power a led? I´m planning to build my own, but with the led´s voltage fixed to the led´s specifications.



If you want to drive an LED from 8.4 volts, a switchmode buck regulator is the ONLY efficient way to do it.

LED drivers to do this are readily available and cost less than $10 even for driving dozens of watts.

Don't worry about the LED's voltage specification - they very very little. Voltage regulation worked well for filament lights - forget that for LEDs.

The Driver controls the current through the LED and that's what determines power to control brightness and prevent LED-death.


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## Rimbaldo (Jan 29, 2013)

Thanks! I've seen a couple of this buck drivers, regulators on ebay. There's one which one pot deals with the voltage desired, and another one with the current.... I've even thought of desoldering the pot for setting the current (which is attached to the pcb, and is tiny) and wire another pot to this one in series. So I could set the maximum current on the pcb's pot, and regulate the amount of current from 0 to the max in the other pot (that I could attach to the power source's case, externally). So I could dim the led from 0 to max current allowed, keeping the voltage set.


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## VegasF6 (Feb 3, 2013)

The way to do this would be a potentiometer in series with a fixed resistor.


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## MikeAusC (Feb 4, 2013)

Rimbaldo said:


> . . . . So I could dim the led from 0 to max current allowed, keeping the voltage set.



These controls set the MAXIMUM voltage and MAXIMUM current from the driver.

If you apply a fixed voltage, the LED will determine what current flows through it.

If you apply a fixed current, the LED will decide what voltage drops across it.


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## Rimbaldo (Feb 28, 2013)

MikeAusC said:


> These controls set the MAXIMUM voltage and MAXIMUM current from the driver.
> 
> If you apply a fixed voltage, the LED will determine what current flows through it.
> 
> If you apply a fixed current, the LED will decide what voltage drops across it.



Hum.. ok... So... if I use a potentiometer (in the led driver circuit) to set from the highest current that I think it´s desired to zero (the current will always be fixed in this case, due to the regulation set in the potentiometer), the led will automatically ... allow... a voltage across it?

Thanks!


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## MikeAusC (Mar 1, 2013)

The Maximum Voltage setting must be higher than the normal operating voltage of the LED at the current you have set - keeping in mind that voltage will vary with LED temperature.

Set the Voltage about 0.5 volt higher than the LED operating voltage - any higher and you risk damaging the LED if there is a short break in the connection to the LED. The power supply output capacitors cause a current surge through the LED.


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## CaptainBrock (Mar 1, 2013)

Rimbaldo said:


> If I limit the current, can I increase a led´s voltage way more that it´s designed?


LED's are Light-Emitting DIODES. They have a fixed relationship of voltage to current. This relationship is not linear; it is a curve with a "knee". Past the knee, an increase in voltage means the current has increased drastically. The efficiency goes way down, and you get more heat, but not much more light.
Good luck, be safe, and have fun!


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## Rimbaldo (Mar 11, 2013)

Ok... I received the led driver from ebay, in which I can set the desired voltage and desired max current using pots. soldered to it´s board. It´s a circuit that can be used alternatively as a LIPO battery charger. So, I fixed the voltage and the max current allowed and managed to light my led for my loupe accordingly. I did some tests and I eventually wired a 10k potentiometer in paralel with the 10k pot. that sets the voltage, (letting the max current. pot fixed in the desired allowed current). I can then dim the led from no light to the max light allowed (max current allowed). 

I have one concern then. When dimming the light (adjusting the pot in paralel with the voltage set pot. in the board) there´s some kind of ... flickering... It´s not a ... smooth... change of brightness, not a smooth transition... Eventually it stands still in the desired brightness I want, but when adjusting, dimming the light, there´s this .... flicker..... Why would this happen? It´s something related to the driver circuit itself? 

Thanks for all the answers!


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