Quick Electrical Question

[dave carver]

I need to drop 6 volts down to 3 volts for a bike LED tail light setup. Power is coming from a 6 volt 16 amp hr SLA battery recharged from the motor 6 volt line out through a solid state charger. The tail light assembly is set up to run on 2 AA batteries but I don't want to have to deal with carrying batteries when I'm on tour.

I'm pretty sure it's a resistor, right? If it takes a circuit of some kind I can still do that but I'm thinking not if I remember right. Just feed me the numbers I need to give to the guy behind the counter at the electronics parts store

Thanks.

 

[bytesiz]

Yup, a resistor is what you need. We need to know what current the LED requires. From that, we can figure out the resistance and wattage rating of the resistor.

 

[SCE to AUX]

You could also use 5 silicon diodes in series, which will have a more or less constant voltage drop independent of current. Each diode will drop ~0.6V, so 5 in series will give you your 3V reduction. This way, you don't need to know the actual current draw of the existing LED light.

Dirt cheap 1N4001s should work fine here. They will handle up to 1A forward current, far more than a AA powered headlamp is going to need. They cost pennies apiece from any electronics parts place.

 

[dlb]

You could also use 5 silicon diodes in series, which will have a more or less constant voltage drop independent of current. Each diode will drop ~0.6V, so 5 in series will give you your 3V reduction. This way, you don't need to know the actual current draw of the existing LED light.

Dirt cheap 1N4001s should work fine here. They will handle up to 1A forward current, far more than a AA powered headlamp is going to need. They cost pennies apiece from any electronics parts place.

That's the trick, since we don't have the ma. info required to drive the LEDs, otherwise a resistor would work too!

 

[bobkrech]

The 5 diodes in series is a good, cheap method, but does not limit current.

Another option is to use a 3 volt 3 terminal device voltage regulator, and choose the 3 terminal device to limit maximum current flow.

Bob

 

[rockets4kids]

Ideally LEDs should be driven via constant current. While a dropping resistor can be used, it is extremely inefficient. Likewise with a simple voltage regulator.

By far the best way to drive LEDs is with a constant current switching regulator. These regulators are better than 80% efficient and often better than 90% efficient these days. The best part is that most designs allow for a huge range of input voltages, and a 5 - 24 volt input range is not uncommon.

 

[SCE to AUX]

Current limiting and constant current drive are certainly important considerations when designing the entire system.

Here, the OP already HAS a 3V powered taillight (presumably with proper current limiting for the LEDs already inside), and is just looking for a way to run the thing from 6V. Not knowing the exact current draw, the diode string seems the easiest way to go.

If the taillight manufacturer did something REALLY cheesy, like depending on the internal resistance of the AA cells to limit the LED current, then there will be a problem connecting it to a 16AH SLA battery that can source LOTS more current.

Whenever dealing with a battery that can source lots of current, I would suggest putting a fuse in the positive lead, right at the battery. A 1/8A fuse will protect the wiring from short circuits, while allowing plenty of current to run the load.

 

[davel]

Could always buy a second identical tail light and run the two in series. If it uses a 'frame ground' you would need to isolate the upstream frame....

 

[plano-doug]

By far the best way to drive LEDs is with a constant current switching regulator. These regulators are better than 80% efficient and often better than 90% efficient these days.

It may be the most effiicient way, but for someone who's not an EE, the series diodes sound like a winner. Most folks aren't up to rolling their own linears let alone switchers. (Heck, most double E's can't do a switcher without a cookbook.)

The user is already getting a huge increase in effiiciency with LEDs (versus incandescent bulbs), so the IR losses in the diodes are tolerable.

Doug

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[dave carver]

THAT'S why I love this place, you get all answers from the simple diode chain to an atomic pile:cyclops:

The tail light is Chinese so you can pretty much guess theres not much more than the mininum needed to get it to work.

I got a 6 volt headlight ment for the charging system. I've seen beer cans made of thicker metal than they made the mount from

This is a Chinese motorized bicycle kit I got off of eBay. It's an add-on motor for a bike and it's been lots of fun and very useful for short trips. Lucky I live in a state where the cops are more curious than anything. Stay off the sidewalks and wear a helmet and they just wave:wink: