Hey! Electronics guys!
Discussion
Explain to an idiot (who actually is an aircraft electrician by trade, believe it or not) something about LEDS?
I'm making some tail lights for my track day car, and want to put some LED's in parallel. Now I am aware you need a ballast resistor, but if I have say 20 LEDs in parallel, do I need one for each LED, or one for all the LEDs? I think I only need one for all the LEDs in parallel, as the ballast resistor is only there to give some resistance to a circuit that would otherwise appear to have no load. Help, I feel stoopid.
I'm making some tail lights for my track day car, and want to put some LED's in parallel. Now I am aware you need a ballast resistor, but if I have say 20 LEDs in parallel, do I need one for each LED, or one for all the LEDs? I think I only need one for all the LEDs in parallel, as the ballast resistor is only there to give some resistance to a circuit that would otherwise appear to have no load. Help, I feel stoopid.
There's no reason not to run them in series as long as you keep the total voltage under the supply voltage.
For example if your LEDs are designed to run at 2V then you could run five in series for a total Voltage drop of 10V (12V supply), or 6 for a drop of 12V (14 V supply). You will probably want a lot more LEDs than that so you would run multiple strings of LEDs in parallel.
You will need a series resister to limit the current. Calculate the Voltage drop you need across the resister, divide this by the total current draw (current per LED times number of strings), that gives you the necessary resistance.
>> Edited by greenv8s on Thursday 6th May 18:46
For example if your LEDs are designed to run at 2V then you could run five in series for a total Voltage drop of 10V (12V supply), or 6 for a drop of 12V (14 V supply). You will probably want a lot more LEDs than that so you would run multiple strings of LEDs in parallel.
You will need a series resister to limit the current. Calculate the Voltage drop you need across the resister, divide this by the total current draw (current per LED times number of strings), that gives you the necessary resistance.
>> Edited by greenv8s on Thursday 6th May 18:46
nobbysworld said:
Explain to an idiot (who actually is an aircraft electrician by trade, believe it or not) something about LEDS?
I'm making some tail lights for my track day car, and want to put some LED's in parallel. Now I am aware you need a ballast resistor, but if I have say 20 LEDs in parallel, do I need one for each LED, or one for all the LEDs? I think I only need one for all the LEDs in parallel, as the ballast resistor is only there to give some resistance to a circuit that would otherwise appear to have no load. Help, I feel stoopid.
Hey Darrent,
LEDs take so little current that normally you only need a resistor with LEDs to drop the supply voltage down to typically about 2.1 volts - although it is actually there to limit the current.
Your best bet is to get some hold of some 12volt LEDs then you can hook up as many as you like in parallel.
(they're quite common now. Try sticking '12V LED' into the search box at www.maplin.co.uk - about 29p each)
If you decide to go with cheaper 2.1 volt jobbies then you can hook the whole lot up in parallel with one resistor.
I can try an figure what value it needs to be if you want but its a while since I've done the sums and it can depend on the curernt rating of the actual LEDs.
Hope that helps and remember to wire them up the right way round - log lead is usually cathode (battery -ve)
If you have LEDs in parallel you need a separate resistor for each LED. If you have one resistor for all of them, the one which happens to have the lowest Vf will hog the current and pop. Then the one with the next lowest Vf will hog the current and pop, etc. etc.
Vf is generally around 1.8V for red LEDs, 2.2V for orange, yellow and green, 3V or 3.6V for blue and white.
If can be assumed to be 20mA (0.02A) for each LED.
Resistor value used = (Vsupply - Vf)/If, so for a red LED on a nominal 12V supply that's (14.4 - 1.8)/0.02 which gives 630 ohms; the nearest preferred value, erring on the safe side, is 680 ohms.
It may well be easier to string a lot of LEDs in series and use one resistor for each series string, in which case Vf in the above equation becomes the sum of all the Vfs. The trouble is that Vf is pretty well constant but Vsupply varies with revs and state of charge of the battery; you can take the limits as being 11.5V to 15V. This is quite a wide range, and the result is that the LEDs will be quite noticeably dimmer when the battery is low and you're not revving.
As regards polarity, the conventional markers - one lead longer, flat on the side of the package - tend to be unreliable and change between manufacturers etc. It's much easier to simply look at the shapes of the wires inside the LED package. One of them ends in a little short bit, the other ends in a long bit bent over at right angles. The long bent bit is ALWAYS negative, EXCEPT in high-intensity RED LEDs, where it's positive - that's only for high-intensity red LEDs; in normal-intensity red LEDs and high-intensity non-red LEDs, it's negative.
On my motorbike the indicator and stop/tail lights are made with Lumileds (www.lumileds.com) which are beefy enough to replace a colour-filtered 21W bulb with a single 1.2W Lumiled and still give the same light output. They're dead cool if you're short of juice (which is always the case on an MZ). They also need heatsinking. The above values for Vf and If do not apply, and it's better to use a constant current source rather than a resistor anyway.
Vf is generally around 1.8V for red LEDs, 2.2V for orange, yellow and green, 3V or 3.6V for blue and white.
If can be assumed to be 20mA (0.02A) for each LED.
Resistor value used = (Vsupply - Vf)/If, so for a red LED on a nominal 12V supply that's (14.4 - 1.8)/0.02 which gives 630 ohms; the nearest preferred value, erring on the safe side, is 680 ohms.
It may well be easier to string a lot of LEDs in series and use one resistor for each series string, in which case Vf in the above equation becomes the sum of all the Vfs. The trouble is that Vf is pretty well constant but Vsupply varies with revs and state of charge of the battery; you can take the limits as being 11.5V to 15V. This is quite a wide range, and the result is that the LEDs will be quite noticeably dimmer when the battery is low and you're not revving.
As regards polarity, the conventional markers - one lead longer, flat on the side of the package - tend to be unreliable and change between manufacturers etc. It's much easier to simply look at the shapes of the wires inside the LED package. One of them ends in a little short bit, the other ends in a long bit bent over at right angles. The long bent bit is ALWAYS negative, EXCEPT in high-intensity RED LEDs, where it's positive - that's only for high-intensity red LEDs; in normal-intensity red LEDs and high-intensity non-red LEDs, it's negative.
On my motorbike the indicator and stop/tail lights are made with Lumileds (www.lumileds.com) which are beefy enough to replace a colour-filtered 21W bulb with a single 1.2W Lumiled and still give the same light output. They're dead cool if you're short of juice (which is always the case on an MZ). They also need heatsinking. The above values for Vf and If do not apply, and it's better to use a constant current source rather than a resistor anyway.
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