LED series resistor question

Undervalued

I wonder if anybody can help me?

I have a number of LED "workmens' torches" which have an encapsulated LED strip (about 8 LEDs I think) and take 3 AAA batteries (4.5 volts). There is no separate series resistor so I assume something must be built in to the encapsulated strip? They draw 680 mA so the batteries don't last very long! Roughly 3 watts. I would like to convert them to run of a car cigarette lighter socket so 12 volts.  So my questions are....

How do I work out what series resistor to add if I just use the LED strip from one torch? 

alternatively

If I join three of these strips in series can I run it directly off 12 volts?

Any other ideas?

Thanks

GDB2222

This is just ohm’s law. One strip has an effective resistance of 6 ohms. If you run it off 12v instead of 4.5v, you’ll need around a 12 ohm resistor, but one capable of dealing with 12w of heat generated within it.

As you say, join 3 together, and at least you can use that wasted 12w to create light.

grumbler

Undervalued said:

If I join three of these strips in series can I run it directly off 12 volts?

I don't see why not.

And if you use one strip with a resistor it has to be a pretty big one as it has to dissipate  0.68*(12-4.5)=5.1W

One strip resistance is 4.5/0.68=6.6 ohm

To get 0.68A from 12V you need 12/0.68=17.6 ohm

The resistor has to be about 11 ohm.

Undervalued

Thanks both.

Is this why I have read that higher power LEDs generally have a more elaborate regulating circuit and not simply a resistor in series?

Would that be a less wasteful option and is that maybe what these encapsulated strips are doing internally?

Heedtheadvice

I would think it more likely they are internally connected in a series parallel combination to forward bias each one.

However that is really irrelevant. Just connect 3 units together in series.

EDIT [Thanks tallmansix I had totally forgotten avout car voltage fluctuation that can be as high or higher than you suggest]

[Deleted User]

Undervalued said:

Thanks both.

Is this why I have read that higher power LEDs generally have a more elaborate regulating circuit and not simply a resistor in series?

Would that be a less wasteful option and is that maybe what these encapsulated strips are doing internally?

Yes, using a resistor to regulate an LED is inefficient because you just waste power in the resistor. Using a car cig lighter is not going to give 12v either, it could be anything from 12-15v so potentially you could end up overdriving the LEDs

Proper regulation of LEDs is done using a current limiter rather than a resistor, hopefully, this graph helps explain why:



LEDs don't have a fixed resistance, so although they obey the ohms law, the calculation will need to be adjusted depending on the voltage because the resistance reduces exponentially with increasing voltage.

In this typical example, a 0.4v difference (20%) means 10 times more current, and potentially the magic smoke escaping from the LED.

For torches that run from a battery, the max voltage is pretty certain because batteries will generally not exceed the 1.5v specification so manufacturers don't need to worry about that as long as they can cope with the max battery voltage.

But running it on a car circuit can provide around 25% more voltage than 12v once the engine is running which can totally exceed the LED spec.

Undervalued

Many thanks.

Are you able to point me towards a simple current regulating circuit? At the moment these things draw 680mA from three new AAA batteries (4.5 volts). To be honest they are plenty bright enough for my application so I could maybe aim for regulating to say 600 mA?

I have quite a few of the things but using but using three at a time in series would be overkill. Also, they would only be used when the ignition is off so I assume the maximum voltage would be the highest the battery every gets to when not on charge (13.6??).

grumbler

I think this all is a big overkill. Even with the engine running the voltage is unlikely to rise a lot unless you disconnect the battery. Just take a multimeter and check.

Or just use one strip and a cigarette lighter USB adaptor. I think, 5V are fine instead of 4.5.

Heedtheadvice

My previous post edited.

Car voltage can fluctuate even more than suggested by tallmansix. At times it can be down to 11volts ( or even less under starting load for combustion engines) and as high as 14.8v by design under charging.

That can result in too low a voltage to drive the leds or too high risking failure. That depends upon the particular led specs, which in this case are unknown. They work well on the 4.5v of the battery but we do not know where on the forward voltage/current curve they are so operating!

If they are low down on the curve they possibly will not fail but not light or be very dim when the car voltage falls. Conversly if they are already driven hard (and this could be the case to make them bright especially given the current quoted by the OP) then failure risk is high.

Using two in series with voltage regulation would seem wise. Such as voltage limited to 9.1v which  can be achieved by several methods such as the zener diode and resister combination, that indeed relies on resustive voltage drop and is not very efficient......but in the case of a car battery, ok if it follows a cars inbuilt switching. You would not want a permanent if small drain on a battery where not necessary.

If as posted above only to be used when ignition is off, if  just an indicator of a single led might be best considered ( with associated circuitry) Drawing a continuous half an amp (600mA estimated) would drain a battery too fast in many circumstances. Fine when running but otherwise too high by design. One led and something like 25mA (especially if flashing on and off) would be much better!