is there a way that i am supposed to wire up 12v LEDs? i have put a couple on the switch that i am using for my compressor, and it keeps blowing them. I am just runing them from the delivfery side of the switch to the relay, then to earth. They are supposed to be rated for 11/15 v What am i doing wrong??
TIA
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LEDs
Moderator: -Scott-
Temporary Australian
LEDs
Last edited by Vulcanised on Wed Jan 03, 2007 5:18 pm, edited 1 time in total.
There is no "I" in Team, but there are 5 in Individual Brilliance
Temporary Australian
firtsly putting a LED straight across a supply will blow it apart, it will act like a dead short and shoot little bits of glass or plastic around. (i found that out in year 8 science) you need to put a bleed resistor in series with the LED to limit the current to what they are rated for. also LED's are polarity sensitive, the longer leg is positive (99%of the time).
how many leds do you have?
assuming one blue/red LED on a 12V supply you would use a 450ohm resistor in series with it. a small 0.25W resistor (the little sausage looking ones) will suffice
that is assuming a standard (non high intensity) LED with a Vd of ~3V and max current of 20mA.
tell us what type and how many and ill work it out
how many leds do you have?
assuming one blue/red LED on a 12V supply you would use a 450ohm resistor in series with it. a small 0.25W resistor (the little sausage looking ones) will suffice
that is assuming a standard (non high intensity) LED with a Vd of ~3V and max current of 20mA.
tell us what type and how many and ill work it out
Last edited by ausoops on Wed Jan 03, 2007 6:14 pm, edited 1 time in total.
Temporary Australian
no shooting debris anywere to be seen 
I thought it may have needed a resistor, but i hate auto electrics, i was told by a backyard expert that you can wire them directly in....... never trust a back yard 'spert eh!!
in MY year 8 science, LED's weren't even around I'm talking 1974 
damn i'm old!!
I thought it may have needed a resistor, but i hate auto electrics, i was told by a backyard expert that you can wire them directly in....... never trust a back yard 'spert eh!! in MY year 8 science, LED's weren't even around
I'm talking 1974 damn i'm old!! There is no "I" in Team, but there are 5 in Individual Brilliance
Temporary Australian
Temporary Australian
it worked the first time then stopped working...... I'll check it out when i can.ausoops wrote:ok you have 12V led (i haven't seen many of those before) so it should work without a resistor, if it didn't work the first time you connected it, change the connections around and it should work. if not post up and we'll try something else
There is no "I" in Team, but there are 5 in Individual Brilliance
Temporary Australian
Got a diagram? Sounds like the LED is in series or in-line with the coil used to activate the relay, in between the relay and switch. It needs to be in parrallel with the relay or switch as LED's only has small currents passing thru them.Patrolden wrote:i have it wired to the relay side of the switch on the dash. the relay is in the back.ausoops wrote:i re-read your first post, are the led in series with the relay coil?
Something to remember when using LED's as indicators across relay coils... is called Back EMF.
When a relay coil de-energises, the collapsing magnetic field creates a large reverse voltage spike. (try it for yourself, get a relay, hold the coil contacts with your fingers, wet your fingers if you are game, and feel the spike as the relay turns off!).
This reverse voltage spike is enough to destroy LED's very quickly, you need to get a relay with a built in diode to clamp this spike to a safe level (make sure you observe the polarity on these types of relays, ARB supply them with their lighting looms), or you can put a diode (1N4001, 1N4004, etc) across an existing relay to stop the spikes... either way, you then need to make sure you don't hook the relay coil the wrong way around!
I found this out the hard way with a blue LED in my spotlight switch... it worked when I first switched the spotties on, when I turned the relay off, the LED died....
BTW, I use 1K 1/4 watt resistors in my LED indicators, they are still quite bright and don't get too stressed out with too much current.
Regards,
Michael.
When a relay coil de-energises, the collapsing magnetic field creates a large reverse voltage spike. (try it for yourself, get a relay, hold the coil contacts with your fingers, wet your fingers if you are game, and feel the spike as the relay turns off!).
This reverse voltage spike is enough to destroy LED's very quickly, you need to get a relay with a built in diode to clamp this spike to a safe level (make sure you observe the polarity on these types of relays, ARB supply them with their lighting looms), or you can put a diode (1N4001, 1N4004, etc) across an existing relay to stop the spikes... either way, you then need to make sure you don't hook the relay coil the wrong way around!
I found this out the hard way with a blue LED in my spotlight switch... it worked when I first switched the spotties on, when I turned the relay off, the LED died....
BTW, I use 1K 1/4 watt resistors in my LED indicators, they are still quite bright and don't get too stressed out with too much current.
Regards,
Michael.
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