Need Help Need some advice - current limiting resistors for directionals?

trepetti

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I am cleaning up the wiring on my FZ and I am having trouble figuring out if I need any current limiting on my LED. Attached is a drawing of my directional / pod light wiring. My directionals are Watsens, and are currently wired in with a 10 ohm 1/2w resistor in series with the anode. The resistors came with the Watsens (got them used) and I am trying to see if I need them. Everything works, but a 10 ohm resistor in a 14.4 v circuit will dissipate almost 21 watts, and as you can imagine the 1/2 watt resistor gets HOT. Because it only heats up when the directional is on, there have been no problems to date. I do not have any electrical specs for the Watsens.

So here is a question for the electrical gurus out there. The 10 ohm resistor only consumes 1.44 amps at 14.4 volts, so it is not doing much current limiting. Who is of the opinion that I need to have ANY resistor on the anode?

Bonus question: The oem plugs from Watsen have a regular PN diode forward biased on the cathode of the LED. There may be instances where something like this is needed, but looking ay my schematic I see no need to block any reverse current at the LED. Can anyone think of why they are there?

Thanks
 

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Motogiro

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Yes, keep the series resistors in line. I've seen another Euro LED company that does the same. It creates a voltage drop to probably keep the LED junction in it's safe operating temperature. You could bump up to a 1 watt which would keep the resistor cooler.

This is obviously different than parallel wire resistors used for load matching for non electronic flashers.
 
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trepetti

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Yes, keep the series resistors in line. I've seen another Euro LED company that does the same. It creates a voltage drop to probably keep the LED junction in it's safe operating temperature. You could bump up to a 1 watt which would keep the resistor cooler.

This is obviously different than parallel wire resistors used for load matching for non electronic flashers.
Thanks Cliff. I am still thin on my knowledge of electronics, so I was thinking that a 10 ohm resistor does not do much current limiting, and so could be discarded. I ordered some 1 watt resistors to replace the 1/2s in there now.
 

Motogiro

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Thanks Cliff. I am still thin on my knowledge of electronics, so I was thinking that a 10 ohm resistor does not do much current limiting, and so could be discarded. I ordered some 1 watt resistors to replace the 1/2s in there now.
Yes if those are stock that come with those LED's, keep em. I'll see if I can find some data/specs on those....
 

trepetti

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I agree with that 10 ohm thought. It's hard to know what resistance is already incorporated in the LED assembly. 10 ohms is more like a load matching resistor. I found a handy calculator. https://www.hobby-hour.com/electronics/ledcalc.php
I also looked for some info on the Watsons and was not successful.
I guess I will need to grab the multimeter and measure the forward voltage and current draw and reverse engineer the requirements. Thanks....
 

Motogiro

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I guess I will need to grab the multimeter and measure the forward voltage and current draw and reverse engineer the requirements. Thanks....
Except we don't actually know the designed rating for forward current on those devices. We can measure what the forward current is but that doesn't tell us what what it's intended forward current was designed for. The Watsons may not require a dropping resistor but we dont know that yet and so must consider that the 10 ohm are series current limiting.
 

Motogiro

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Why not just use a flasher unit that is made to work with LED lights. So much easier.

The resistors we're discussing are in series for creating a voltage drop and limit forward current in the LED unit.
The load resistors are used to emulate the load of an incandescent lamp to help the flasher keep correct flash rate. Those resistors are wired in parallel and not in series.
I think all flasher relays should be electronic. Most of the electronic relays are well rated and easily carry incandescent loads.
 
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