Electrical engineers....need help with LED turn signal lights..........
01-18-2002, 02:06 PM
#1
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Electrical engineers....need help with LED turn signal lights..........
I have a friend who has been promising to help me with this for 2 weeks now, and I am giving up on him. So, time to ask the forums:
I have a pair of the clear/blink yellow LED turn signal lights from Mark P, and need help determining the resistance of the LED bulb vs the standard filiment bulb.
The problem is that since the LEDs have so little resistance compared to a filament bulb, once you install the LED bulbs, the turnsignals blink very fast as if one of the bulbs was burned out. I realize this is due to the less resistance in the circuit with the new bulbs.
So, I need to find out what is the resistance of LED vs. filament bulb, and ADD that much resistance to the wires to bring the circuit back to normal resitance, and therefore normal blinking speed.
I thought I would just hook up my multimeter to both style bulbs, and the ohms would be easy to figure out. That didn't work. Then someone said you need to have the bulbs hooked up to a 12v supply, then measure the resistance. I tried this with no luck either....
So, how do I solve this problem?
Thanks
I have a pair of the clear/blink yellow LED turn signal lights from Mark P, and need help determining the resistance of the LED bulb vs the standard filiment bulb.
The problem is that since the LEDs have so little resistance compared to a filament bulb, once you install the LED bulbs, the turnsignals blink very fast as if one of the bulbs was burned out. I realize this is due to the less resistance in the circuit with the new bulbs.
So, I need to find out what is the resistance of LED vs. filament bulb, and ADD that much resistance to the wires to bring the circuit back to normal resitance, and therefore normal blinking speed.
I thought I would just hook up my multimeter to both style bulbs, and the ohms would be easy to figure out. That didn't work. Then someone said you need to have the bulbs hooked up to a 12v supply, then measure the resistance. I tried this with no luck either....
So, how do I solve this problem?
Thanks
01-18-2002, 02:22 PM
#2
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I'm no EE, but.
What if you hooked the lights to a constant 12v source, then measured the voltage across the LED's.
Or, disconnect from the 12v, and hook an ammeter inline, to measure how much current the LED's draw. compare these numbers with the stock bulb.
Or, disconnect from the 12v, and hook an ammeter inline, to measure how much current the LED's draw. compare these numbers with the stock bulb.
01-18-2002, 02:41 PM
#3
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P=I^2R=VI...
if my calculation is right with 14.4v and 21W dissipation, you should would be drawing 1.45A.
so get a 10 ohm power resistor rated for atleast 21Ws and you should be good.
wire it in parallel. Get two from partsexpress.com.
so get a 10 ohm power resistor rated for atleast 21Ws and you should be good.
wire it in parallel. Get two from partsexpress.com.
01-18-2002, 02:44 PM
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Got that right :)
The first option won't work, it will still be 12V.
The second option will work: measuring the current.
Volts = Amps x Resistance
If you know volts and amps, you can figure the resistance out.
If possible, measure current and volts at the same time. You should have a good digital volt meter to do this.
The second option will work: measuring the current.
Volts = Amps x Resistance
If you know volts and amps, you can figure the resistance out.
If possible, measure current and volts at the same time. You should have a good digital volt meter to do this.
01-18-2002, 02:44 PM
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I am an EE, and
Mike's just about right.
LED's don't have resistance per se. Diodes behave completely differently than resistors or filaments. But measuring their equivalent resistance is a similar process.
A resistor will have a current through it proportional to the voltage on it.
A filament will have a resistance that goes up as it gets hot. So when cold then have almost no resistence (like when you use an ohm meter to test them) and the resistence climbs to some value as they turn on and get hot.
A single diode (or LED) has infinite resistance if you try to run current through it the wrong way. In the "right" direction, it will conduct no current until the voltage gets above some threshold - like 0.7V or so, more for an LED, like 15 volts. In LED lights they stack a bunch of them in series so that 12-14V gets turn-on voltage for all the LED's in the series (like if each LED switched on at 4 volts they might have 3 in series, and many sets of 3-in-series to make a light).
In each of these cases, the thing to do is hook them up to a 12-14V source so they are on. Measure the current going in. The resistance is V/I.
- Z
LED's don't have resistance per se. Diodes behave completely differently than resistors or filaments. But measuring their equivalent resistance is a similar process.
A resistor will have a current through it proportional to the voltage on it.
A filament will have a resistance that goes up as it gets hot. So when cold then have almost no resistence (like when you use an ohm meter to test them) and the resistence climbs to some value as they turn on and get hot.
A single diode (or LED) has infinite resistance if you try to run current through it the wrong way. In the "right" direction, it will conduct no current until the voltage gets above some threshold - like 0.7V or so, more for an LED, like 15 volts. In LED lights they stack a bunch of them in series so that 12-14V gets turn-on voltage for all the LED's in the series (like if each LED switched on at 4 volts they might have 3 in series, and many sets of 3-in-series to make a light).
In each of these cases, the thing to do is hook them up to a 12-14V source so they are on. Measure the current going in. The resistance is V/I.
- Z
01-18-2002, 03:09 PM
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You don't have to understand it, just do what I said... ;)
Or what MikekiM said.
Compute resistance by putting 12V on the light, and measuring the current that results. R = V/I
Or, you want the various lights you might be contemplating to draw the same current as the stock lights do when powered by 12V.
If the car puts 12V to the light, and the light then draws the same current as stock, then the car doesn't know you swapped the light with something else.
And the car won't be angry at you...
;0
Compute resistance by putting 12V on the light, and measuring the current that results. R = V/I
Or, you want the various lights you might be contemplating to draw the same current as the stock lights do when powered by 12V.
If the car puts 12V to the light, and the light then draws the same current as stock, then the car doesn't know you swapped the light with something else.
And the car won't be angry at you...
;0
