Lighting a large number of LED's at the same time.

Martin_H

This is more of a thought experiment than anything I have planned. But I started thinking about this when I saw a laser diode array and found out how much current it draws. It occurred to that the same problem applies to LED traffic lights as well. Naturally when I realized I didn't understand how a traffic light worked, it started bugging me.

I normally think of LED's and their laser diode cousins as trivial current consumers. Powering an LED is easy with a battery and a resistor, and eight isn't much harder. Just wire an LED in series with resistor, and the LED/resistor units in parallel with each other. A battery could still delivery the required amount of current.

But suppose you wanted to build a stop light with 100 to 200 LED's and light them at the same time. I understand you could put them in a grid and light the LED's in sequence while relying on persistence of vision. But I don't think that would be bright enough. I would imagine they must all be one at the same time to be as bright as an incandescent bulb they are replacing.

The problem is that as the number of LED's increases, so does the number of resistors. Obviously costs would dictate wanting to use the least number of parts. So this raises some questions:

* Do LED's in parallel act like resistors and create a current divider?

* Can you eliminate all but one of the resistors as long as the current divider created by the parallel wired LED's is within spec for a single LED?

* When an LED fails does it act like an open circuit or a short circuit? I imagine that this would slowly change the characteristics of the current divider and might lead to a cascading failure.

In the limiting case you would need a low voltage power supply that delivered several amps of current to many LED's all in parallel. Most power sources I've seen are low voltage low current, or high voltage and high current.

So how on Earth does the power supply to the LED stop lights work?

PJAllen

They have step-up switchers. The LEDs are in strings and banks.

JasonDorie

With higher voltages you can wire more LEDs in series, and a resistor is only required for a series of LEDs. With a voltage drop of 2v, you could light 12 LEDs in series with 24v. Then put 12 of those in parallel and you have 144 LEDs. Flexible and rigid LED bars work this way - 3 LEDs in series with a resistor, and then many of those in parallel for the length of the strip.

Heater.

Martin_H,

In short they use switched mode power supplies.

If your SMPS is controlling current you don't need that resistor.

This all sounds like a lot of expensive circuity but now a days the chips and components to do this are smaller than the power LEDs they are driving.

I had an older LED signal lamp once. There were 60 bright red LEDs on the front plate, at the back was a screw fitting so that the whole thing could be screwed into a normal lamp socket. In between was a traditional looking power suply complete with big transformer. Bolted on the side was a huge high wattage resistor! I had this thing because the resitor had heated enough to melt the plastic case. There were no energy gains from using LEDs with that thing.

Modern traffic signal head lamps are much nicer.

* Do LED's in parallel act like resistors and create a current divider?

Can do. As long as the applied voltage is over the forward voltage drop for that type of LED it will start to take a lot of current. Hence the need for the resistor or switcher current limiter. Put two in parallel and they will both light up BUT if there forward voltage drops are mismatched due to manufacturing tolerences one will hog more currrent that the other. Not a good solution.

* Can you eliminate all but one of the resistors as long as the current divider created by the parallel wired LED's is within spec for a single LED?

Yes, see above. Of course with this arrangement pulling one LED or having one fail open circuit may then kill the working LED as it sucks all the current instead. Not a good solution.

* When an LED fails does it act like an open circuit or a short circuit? I imagine that this would slowly change the characteristics of the current divider and might lead to a cascading failure.

Good question. I think in the majority of cases LEDs don't "blow" open or short circuit. Rather they just get dimmer with time.

Traffic lights have current monitoring on their LAMPs for safety reasons. With some earlier LED lights that current monitoring system was a problem. As you see in the description of the old light above there were ways for the thing to fail such that the LEDs are dark but it still consumes current. The fault was not detectable.

Martin_H

Thanks for all the helpful responses. I definitely feel like my local stop light has been demystified.