parallel LEDs and Drivers

[johnrobholmes]

I took apart a cheap lamp I bought, and the LED board has 20 something parallel leds on it. Makes me wonder if this construction would put the light at risk for premature death. I remember reading that each LED should either have a resistor or a CC driver, but maybe if the forward voltage is low enough this isn't needed?

Basically the insides of this cheap lamp were crap and I need to either wire in a resistor or buy a CC power supply to get my lamp running again. Thoughts? I plan to run it on one cell lipo OR off 6s a123 (main pack).

 

[biohazardman]

Lots of info on the lights around. Here are a couple of links I used to decide on what to build and get parts to build it.

http://www.kayakaccessri.info/krabachwebsite/bike_light/bike_light_project/proto2.html

http://www.taskled.com/bflex.html

 

[fechter]

I've seen a few setups where LEDs are in parallel and they seemed to work. If they are well matched and at the same temperature, they seem to share fairly evenly.
In general, I wouldn't recommend it though. It's better to have them in series so the current is exactly the same. Each parallel branch really should have a resistor or constant current source.

 

[johnrobholmes]

I've seen a few setups where LEDs are in parallel and they seemed to work. If they are well matched and at the same temperature, they seem to share fairly evenly.
In general, I wouldn't recommend it though. It's better to have them in series so the current is exactly the same. Each parallel branch really should have a resistor or constant current source.

This is my concern, because for how cheap the lamp was I KNOW the variation between leds will be great.

Time to make a guess at the forward voltage & amperage and put a resistor in there, gonna stab at 3.5v voltage & 30mA each LED. Since they are in one bank a one watt 1.2ohm LEd should suffice to draw the fresh lipo down to 3.5v.


21 5mm LEDs in parallel. Score one for China!

 

[johnrobholmes]

Lots of info on the lights around. Here are a couple of links I used to decide on what to build and get parts to build it.

http://www.kayakaccessri.info/krabachwebsite/bike_light/bike_light_project/proto2.html

http://www.taskled.com/bflex.html

Good sites there, I have browsed the krabach site quite a bit looking at the neat projects. I will be making something similar in the future, as I just can't stand paying a few hundred bucks for a light that can be made from less than $50 of materials. Time to fire my mill back up!

 

[ejonesss]

i dont see how you can blow leds by paralleling them.

 

[fechter]

The current to voltage relationship takes a sudden turn when they start conducting. They blow up when you exceed the current rating (20ma for most small ones). If they aren't well matched, it's real easy for one to take way more than its share of the current and self destruct.

 

[Tiberius]

I've used LEDs in parallel a lot, without any attempt at matching, and it has always worked.

It sounds like it shouldn't work, but the knee in the curve where they turn on isn't perfectly sharp so they do share the current well enough. I've never been able to see any difference in brightness.

If one goes open circuit (more likely to be a wiring failure than the LED burning out) then the others get a bit more current. That risk can be balanced against the case of putting them in series, when a single open circuit will make them all go out.

The normal method in battery powered cycle lights seems to be run the LEDs in parallel, without any series resistors, and drive them with short pulses. Current limiting is then just a matter of battery voltage, internal resistance and pulse width.

Nick

 

[johnrobholmes]

Anybody used these AC drivers?

http://www.dealextreme.com/details.dx/sku.13552


Seems that it would work well on a higher voltage ebike. I wonder if the 85v low range is true or not.

 

[MitchJi]

Hi,

Some good DIY Instructions here:
http://www.autospeed.com/cms/search/index.html?keywords=led&x=0&y=0

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[dequinox]

Has anyone used the little 12v brake-light replacement bulbs they sell for autos? You can get a pair for under $10, and it seems to me you could wire up a brake light switch easy enough and then set the bulbs in epoxy. I think it'd be an easy source for tail-lights ...especially for those of us who run on SLA batteries. Just tap the pack you have parallel with a switch. Here's a link to an example:

http://www.dealextreme.com/details.dx/sku.3845

And also does anyone have a good way of adapting regular brake levers for the brakelight function without buying those special ones?