LED resistor question:

[Chuey]

A couple years ago, I bought a pair of LED turn signal lights from Electrosport. Since 'm using some LED lights as turn signals on my Cafe Racer, I am using an electronic flasher. I don't know the specs of the electronic flasher but had read that using one would negate the need for resistors in line with the LED lights.

I contacted Electrosport and asked them why only a few of the lights in the turn signal work. I told them about my set-up and Trent from their company responded with some help. He asked if I had hooked them up to a 12volt power supply. I thought that meant the battery, so I ran jumpers from the wires directly to the battery. All the LEDs lit up! I called him and told him the result. That's when I found out that the battery isn't actually a 12volt power supply. Oh well, at least nothing bad happened and it showed that the lights actually work, pointing out that it can be fixed. Trent said that I should put resistors in line for the turn signals. Thing is, all my other LED lights are now working, and without resistors. There is one pair of normal bulbs in the turn signal set-up. They are my side flashers.

I'm taking Trent's word that putting in some resistors will fix the problem but I would like to understand a little more about why some lights work fine in my set-up and others don't (at least, without resistors). I didn't want to ask Trent because he took care of his responsibility to me as a customer by answering how to fix the lights that his company sold to me. Thanks for any help.

Chuey

[Deleted User 72]

Unless the lights were sold with the caveat that resistors would be needed to work properly, I don't think he has taken care of his responsibility.

[Rob Frankham]

Hi Chuey,

Unfortunately, its not easy to give a hard and fast answer to your question because what you say doesn't really make sense (electrically anyway). Equally, the advice given by the company doesn't, on the face of it, make sense and i'm far from convinced that the information you were given is correct. All I can do is give some generic information about LED bulbs.

If a LED bulb is rated for 12 volts, it should not need any resistor in SERIES to make it work. This is what you discovered when you put the battery directly across the bulbs (and, yes, a battery is as good as a 12 volt supply... thats what the bulbs are meant to work on after all). The reson why resistors are used with 12 volt LED indicators is to make them work properly with a stock relay and, in this case, the resistor is wired in PARALLEL to the bulb (For those who are interested, this is because stock relays require a current drain equivalent to a stock bulb to work properly. The LED uses much less current so the resistor is needed to draw the extra current and 'fool' the relay into thinking it is driving a stock bulb). There is no way you should need any resistors in a circuit using LED bulbs and a relay designed to drive them.

My reading of your problem is that some of the individual LEDs in your bulbs are not lighting. I would normally expect this to occur if the supply voltage from the relay to the bulb was low.This would imply that:

• Your relay is defective.
• Your relay is not able to cope with the additional load from the conventional bulbs in the circuit.
• There is some other reason (wiring?) why the voltage to the lamps is low.
• Your relay isn't intended to drive 12volt LED units

Suggestions:

• Remove the conventional bulbs and see if that makes a difference.
• use a multimeter to check voltages at the relay input, the relay output and the bulb positive terminal. You will be looking for a reduced voltage
• Check the spec of the relay to ensure that it intended to supply 12 volts out to the LEDS and isn't intended for some lower voltage units.

Sorry I can't be more specific... hope it helps...

Rob

[Duane Ausherman]

Rob, as usual a very well thought out answer. Nothing to add.

[Chuey]

Thanks, Rob.

I had had so much trouble with the LED lights on this bike that my latest tactic was to buy some nice looking small oval LED trailer marker lights to use as turn signals at each end of the bike. Those lights work great. That makes me think that the flasher unit works correctly. I maybe should have said that it is a two pin flasher unit. I am not using the dash pod, and have surmised that the third terminal on the flasher unit is for the dash lights.

When I checked the turn signal lights in question, I simply unhooked the wires from the terminal board I'm using and ran extension wires down to the battery. They lit up properly at that point.

I will take the flasher out of the headlight bucket and see if it says whether it is for LEDs or not.

Chuey

[ME 109]

Rob, as usual a very well thought out answer. Nothing to add.

Yes, we are very fortunate to have people on Boxerworks who not only have knowledge, but care enough to share.

[Major Softie]

Rob, as usual a very well thought out answer. Nothing to add.

Yes, we are very fortunate to have people on Boxerworks who not only have knowledge, but care enough to share.

AND, can put it into clear language. That quality seems to be ever more rare than the others.

[jjwithers]

Rob has a great answer.
I went through this when i built my Cafe bike a few years back. the turn signals and brake light are set up with LEDs.

My front 'flush mount' LED turn signals seemed to cause a bunch of problems. Some LED lights have resistors built into them so they can plug and play with a typical 12v system. others are cheaply made and don't have the proper resistor (my front turn signals).

When the fronts were plugged in, the turn signals wouldn't work, and all 4 turn signals would light up solid. no blinking.
when they were unplugged, the turn signals worked.
I ended up buying 2 resistors (one for each side) and wiring them with the front turn signals. Problem solved. I had to experiment with which resistor rating to use though.

I got them here (at bottom of the page):
http://www.superbrightleds.com/cgi-bin/ ... -turn.html


This link really helps if you want to build your own LED board:
http://led.linear1.org/1led.wiz
You could easily make your own LED tail light in a BMW housing with parts from radioshack or e-bay. Just type in the specs of the LEDs and it tells you the resistors you need. Then cut up a board to fit in the housing, and solder it all together.

[Rob Frankham]

As it happens...

You don't actually need resistors to run LED bulbs... in fact you get much more light per amp if you don't.

This is going to need a bit of explanation and a little bit of Arithmetrick but its not difficult so bear with it.

First you need to look at the LEDs you are using. There are hundreds of different types of LED, each with a different spec so you'll need to know what you've got.

For the sake of argument, lets look at a fairly standard 3mm LED that has a max forward voltage of 2.5 volts and a max current rating of 25ma. If we are using this with a supply voltage of 14 Volts then we would need a serial resistor of around 460 Ohms.

If, however, we use two LEDs (of the same type) and connect them in series, the max forward voltage across the pair becomes 5 Volts and we would need a resistor of around 360 Ohms. The important point is that the current drawn from the system remains at 25ma but we get twice as much light.

Taking this a step further, if we take six similar LEDs and connect them in series, the max voltage across the chain of LEDs becomes 15 volts, which is actually higher than the supply voltage. For this reason, we no longer need a resistor at all and we get nearly six times the light with the same current draw.

The only slight drawback is that, in the unlikely event that any one of the LEDs should fail, we will lose all six.

In an application where you are using a large number of LEDs (say 24 for a rear lamp bulb replacement) it makes very good sense to link them in groups of six (for the type of LED under discussion) since this will give you a sixfold reduction in current for the same light output as 24 individually resistored LEDs.

Hope that makes some sort of sense...

Rob

Just to clarify, the above relates to 'bare' component LEDs, not to groups of LEDs assembled as replacements for automotive bulbs, which will already be connected in this way or have integral resistors. It will also not apply to component LEDs that you can buy that are rated to 12 volts. These will also have integral resistors.

Rob

[hal]

Thanks Rob, very well described!