SandyC
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| posted on 28/10/13 at 10:09 PM |
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Maplin LED shiftlight question
Hi,
I've just fitted a maplin LED shiftlight, wired up to my MBE ECU. The LED has 3 wires. White +ve. Green for green LED's. Red for red
LED's. I've put the white to a switched live and joined the red & green together through a 130ohm resister to the ECU (switched
earth).
When I hit 7000revs the green LED's come on but the red's don't, what have I done wrong???
Thanks for any replies/ advice.
Sandy
Tiger Avon
C20XE
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MikeRJ
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| posted on 28/10/13 at 10:37 PM |
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You have effectively connected the two LED's in parallel, so the voltage across them will be limited by the LED with the lowest forward voltage
which is typically the green LED (around 2.2volts). Red LED's tend to have a forward voltage of around 2.4volts or higher, so with the green
LED connected across it the voltage will never get high enough to allow it to switch on.
The simple solution is to use a separate current limiting resistor on each LED rather than a single resistor for both.
130 ohms sounds very low for a single LED however, are these special high current devices or are there multiple LED's in series? If you post a
link to the LED which you are using I could check if this is correct.
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jabs
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| posted on 29/10/13 at 07:39 AM |
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anything like this ?
http://www.msextra.com/forums/viewtopic.php?f=101&t=39633
or this
http://forum.wscc.co.uk/forum/index.php/topic/65184-megajolt-shift-light/page-2
[Edited on 29/10/13 by jabs]
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SandyC
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| posted on 29/10/13 at 08:24 AM |
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Thanks for the replies. As you can probably guess i'm not good with electrics.
Jabs, yeah it's the same as the one in your first link.
Here's the link to it http://www.maplin.co.uk/productsearch?criteria=26mm%20LED%20Cluster
Again, many thanks
Tiger Avon
C20XE
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MikeRJ
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| posted on 29/10/13 at 08:44 AM |
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You can see in that (very badly formatted) web page that the forward voltage of the green LED array is 8.4v nominal, and for the red it's 10.5v
nominal so this explains the problem. The voltage across the LED will never get higher than the green LEDs will allow, so the red's won't
light.
Assuming 13.8v system voltage, and 30mA LED current:
Green resistor: (13.8-8.4)/0.03 = 180 Ohms
Red resistor: (13.8-10.5)/0.03 = 110 Ohms
By changing the relative green/red currents you will change the apparent colour, from green through yellow and orange to red.
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SandyC
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| posted on 29/10/13 at 08:53 AM |
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Thanks for the quick reply Mike,
I'll get out to the garage this afternoon and re-wire it with a seperate resistor for each color
Tiger Avon
C20XE
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