MULTIPLE LED HELP!

In the top three Google hits for "wiring multiple leds":

Google is your friend.

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I think that's not the case; they look like standard LEDs that require the usual resistor.

They lied to you.

Graham

Hi, Alex. The recommended forward current for these LEDs is 10mA. At that current, you'll have an average forward voltage of 2.0V.

Try this (view in fixed font or M$ Notepad):

| | 100 LEDs 25 X 390 ohms | ___ | .--o-->|-->|-->|-->|--|___|-----. | | | ___ | | | o-->|-->|-->|-->|--|___|-----o | | | ___ | | | o-->|-->|-->|-->|--|___|-----o | | | ___ | | | o-->|-->|-->|-->|--|___|-----o | | | ___ | | | o-->|-->|-->|-->|--|___|-----o |12V| | ___ | | +| o-->|-->|-->|-->|--|___|-----o | --- . . . . . . . | - . . . . . . . | | . . . . . . . | | | ___ | | | o-->|-->|-->|-->|--|___|-----o | | | ___ | | | o-->|-->|-->|-->|--|___|-----o | | | ___ | | | '-->|-->|-->|-->|--|___|-----o | | | | | | | '-------------------------------' | (created by AACircuit v1.28.6 beta 04/19/05

For the last diode string, use three LEDs and a 620 ohm resistor.

Remember that you'll need about 10mA for each of the 25 strings, so that makes a total of at least 250mA, or 1/4A for your 12V power pack. Remember, more current capability for your power pack is OK. Have fun.

Good luck Chris

Wouldn't strings of 5 LEDs and a 200 ohm resistor be more efficient? That's 200mA total.

/me wonders if 17 strings of 6 leds (170mA) with no resistors would work, or if it would be too sensitive to voltage fluctuations.

You could use constant current sources in place of resistors for better efficiency.

Graham

Wouldn't you have the same power loss? If the current regulator is causing the same current, then the LEDs have the same voltage drop, so the voltage drop across the current regulator be the same?

My thought was to add more LEDs so that the voltage drop across the non-LED device is less, so less heat loss from that particular component.

With a current regulator you can probably put more leds in the chain and not worry about brightness varying with battery voltage so lower losses are possible.

Fine but the battery/or other source is unlikely to give an exact 12V or be stable with load/input voltage. Constant current sources solve this problem and can run with very low volts across them.

Graham

Well, I was originally asking about adding another LED to the chain, which would cause less heat loss in either a resistor or a current regulator.

I'm not sure why you think a current regulator would have less heat loss than a resistor, for the same battery voltage drop and current. I realize a current regulator would be more stable and predictable than a resistor, but that wasn't what I was asking about. I was only asking about adding more LEDs.

No, it wouldn't have less heat dissipation than a resisitor for any given number of leds but it would make it more practical to use more leds in any chain so it can save energy that way.

Graham

Hi, DJ. The basic problem, of course, is variation in LED forward voltage (Vf). The OP's LED shows a typical Vf of 2.0V, which means it can probably vary between 1.85V to 2.15V. When you're making an LED display, you want the same light intensity for all the LEDs, so you try to have each LED pass the same forward current.

Now ignoring the statistical chance that a string of LEDs will have both low and high Vf LEDs which cancel each other out, you can look at the variations in LED current for different length strings like this (view in fixed font or M$ Notepad to have the table line up properly:

LEDs Series R Nominal I Min I Max I

1 1000 0.01 0.00985 0.01015 2 800 0.01 0.009625 0.010375 3 600 0.01 0.00925 0.01075 4 400 0.01 0.0085 0.0115 5 200 0.01 0.00625 0.01375

The number of LEDs in the resistor string is a compromise between cost (resistors are less than a penny apiece even in hobbyist quantities -- it's really board space and number of holes if you're doing thru-hole) and current matching. Even assuming a regulated 12V, you're looking at a worst-case 2:1 variation in current in the strings if you use a resistor instead of a constant current source -- clearly unacceptable.

I think Graham meant that, with 5 LEDs in a string, you'd pretty much be required to use a current source to keep your display from looking blotchy. For 100 LEDs, having 20 ea. 5-LED strings instead of 25 ea.

4-LED strings would save power.

Cheers Chris

Thanks, I figured it was something like that.

Spot on !

Graham