How do I run 20ish LEDs with a LED Driver built to run 1 large LED ?

June 20, 2011 8:49 PM

I want to drive 20ish LEDs, should I run them as series or parallel and why ?

I am looking to make a bike light for a friend with a "Bucktoot" LED driver left over from another project . the questions are; would there be any reason(s) I can't run 350 mA worth of LEDs with this driver ?
Also ,would I want to run the LEDs in a series or in parallel and why ?

posted by 70klicks to sports, hobbies, & recreation (8 answers total)

The datasheet for the thing (linked from sparkfun's page) shows it being used with 6 LEDs in series, so it can certainly run more than one at a time.

Whether you want to do them in series or parallel would depend on what kind of LEDs you have. It'll be best if all of your LEDs are identical. The principle is that by putting LEDs in parallel, the current is divided up among them, while by putting them in series, they all get the same current, but the voltage requirement adds up. According to sparkfun, you need to give the converter at least 2 volts more than the LEDs need, and no more than 28 volts or less than 5.

So here's some examples:

If you have 20 average LEDs, which might want about 17 mA each, put them all in parallel so the total current adds up to about 350, which is what the driver wants to supply.

If you have 6 high-current LEDs which want about 350 mA each, put them in series so the 350 mA goes through them all. That's the example in the datasheet. You probably couldn't have more than about 6-8 LEDs in series, because the total voltage drop will add up. If each LED drops 3V, then 8 of them add up to 24V, which is about the most that the driver can supply. And you'd need to put in 26V to make sure it has the 2 volts headroom it wants.

If you have a bunch of 50 mA, 2V LEDs, you could have 7 in parallel, so the current adds up to 350, and run the converter off 5V or four 1.5V batteries, since the converter needs 2 volts more than the LED voltage drop. You could also make 7 strings of 3 LEDs each, as long as you can give the converter about 8 or 9 volts to work with.
posted by moonmilk at 10:06 PM on June 20, 2011 [1 favorite]

thank you ! that was exactly what I needed to know. =]
posted by 70klicks at 10:21 PM on June 20, 2011

You also need to figure out what kind of LEDs they are. If they are the fancy high efficiency ones, they need to be current limited. (Which is the reason you need a driver circuit, rather than just a battery connector. If you plug one of those LEDs into a car battery, they will burn out quickly, even if the voltage is well within specs.)

But if they are the normal kind of LEDs, they may not like the varying voltage that the constant current driver will put out. You can put a normal LED up to a car battery (assuming it is rated for 12v) and it will happily light up and remain lit forever. If that's the case, then you don't need a driver, and you design the circuit based on what power source is convenient and the voltage ratings of the LEDs.
posted by gjc at 5:16 AM on June 21, 2011

Let's be super-clear about what that thing does. The Bucktoot is a 350mA non-adjustable current source. That means that it will do its damndest to push out 350mA regardless of what's connected to it. If you want to wire 20 LEDs in parallel you should bag the Bucktoot and just directly use the voltage source you were going to use to drive the Bucktoot -- it's got a minimum voltage requirement of 5V, which is enough to turn on any color LED you choose provided it's 20 in parallel. Unfortunately the caveat I'm about to describe applies to this configuration too...

The trouble with hooking a bunch of LEDs to it in parallel is that you have no way to influence how that 350mA divides among the LEDs. If you use identical parts, ideally from the same batch, then you can more-or-less reasonably assume the current will be divided equally among them. However, if it doesn't divide equally, you're in trouble, because LEDs tend to see runaway failure -- one gets more current than all the others, heats up, takes more current, heats up more, etc, until it blows out. Now that LED is an open circuit and you're dividing up your 350mA among 19 LEDs instead of 20. Repeat until they're all dead (this usually happens in the span of about 2 seconds when it finally happens).

In series, you have basically no choices other than the 1W Luxeon they show in the datasheet (or bigger) -- the LEDs will all see 350mA. Any number of normal 5mm LEDs in a chain will turn into shrapnel instantly. Same goes for the 1cm square superflux LEDs they use in brake lights (rated for about 40 and 100mA, respectively).

Also, all LEDs need to be current limited somehow. And current sources are probably the most reliable way to do this; the bit about varying voltages above is a red herring.
posted by range at 8:40 AM on June 21, 2011 [2 favorites]

Do you want to use this specific driver, or do you want to light 20 LEDs? This driver is the wrong choice for an array of low-current LEDs (15-20 mA). If you're going to use it, you should build your bike light using a single 350 mA LED like a Luxeon Rebel. If you're going to use an array of low-current LEDs, you should probably use the LED array wizard to figure out how to wire them up given your constraints. You'll need to know your battery voltage, the expected current draw (this is probably 20 mA), and the voltage drop per LED (this depends on color, usually either 1.9 or 3.1 volts).
posted by Mars Saxman at 11:23 AM on June 21, 2011

Oh, yeah: the number of LEDs you can run in series depends on the voltage. Each LED drops a certain number of volts; the total voltage drop must be lower than the battery's output. This is true whether you are using a current regulator or just simple resistors.

This regulator is a "buck" converter and can only reduce voltage (as opposed to a "boost" converter). The regulator itself drops 2V. The number of LEDs you can run in series equals the input voltage, minus 2, divided by the voltage drop per LED; throw away the remainder.
posted by Mars Saxman at 11:27 AM on June 21, 2011

I'll defer to people who know more about this stuff than me - listen to them, not my first answer!
posted by moonmilk at 4:46 PM on June 21, 2011

Also, all LEDs need to be current limited somehow. And current sources are probably the most reliable way to do this; the bit about varying voltages above is a red herring.

Not "normal" radio shack ones. Maybe they are current limited internally? But you can hook those up to anything and they will happily not explode.
posted by gjc at 8:01 PM on June 21, 2011