Hi

I have a motorbike with a 12v generator and regulator. I have wired in two 12v 3W LED daytime running lights. They work perfectly with the ignition on and engine NOT running. If I start the engine, the LED lights randomly go on and off. I have tried wiring the lights direct from the battery and also post regulator, the same problem exists if the engine is running. I have fitted a carbon film resistor (1.1k ohm) to the positive wire. This has stopped the lights from randomly going on and off randomly. The problem I now have is that the lights are fairly dim. Is there a way to calculate what size resistor in terms of watts and resistance? Please explain in layman’s terms. Thanks in advance. Paul

 

Welcome to AAC!

Draw us a block diagram or schematic of how you have the LEDs connected.

 

Welcome to allaboutcircuits.com, Bowish1.

It sounds like the 1k resistors are in series with the light and that solved the problem but caused another. You seem to have good intuition about this kind of problem.

It might be that the voltage is too high -did you or can you check the voltage across the battery when the engine is running?

Maybe a better guess is that there is a lot of electrical noise on the battery from the generator and or ignition system. For that you could put a choke in series with the lamps and a capacitor across the lamps.

 

Welcome to allaboutcircuits.com, Bowish1.

It sounds like the 1k resistors are in series with the light and that solved the problem but caused another. You seem to have good intuition about this kind of problem.

It might be that the voltage is too high -did you or can you check the voltage across the battery when the engine is running?

Maybe a better guess is that there is a lot of electrical noise on the battery from the generator and or ignition system. For that you could put a choke in series with the lamps and a capacitor across the lamps.
View attachment 158092

Thanks for your help. I’ve checked the voltage and it’s 12.8v. What would you recommend re the choke and capacitor specs?

 

 

The LED is dim because you've limited the current to around 1mA.

Flashing makes no sense unless the voltage is dropping below the turn on voltage of the LED when the engine is running. If it the problem was over voltage caused by the generator/alternator, the LED would be brighter.

How stable is the voltage coming out voltage regulator?

Does flashing mean on/off? Or does it mean brighter/dimmer?

 

Without the resistor the led is bright, but randomly turns on and off. I appear to have a constant 12.8v but don’t have an oscilloscope to look at how smooth it is etc

 

I appear to have a constant 12.8v but don’t have an oscilloscope to look at how smooth it is etc

Do you have a few hundred μF capacitor rated for more than 25V that you can put on the output of the regulator?

 

Do you have a few hundred μF capacitor rated for more than 25V that you can put on the output of the regulator?

I can purchase one. Would I fit it in series?

 

I can purchase one. Would I fit it in series?

In parallel with the regulator output.

Do you have anything in your parts bin to try? It might not work and buying in quantity one isn't economical.

The battery should be able to provide a nominal 12V when the engine is running; even if the regulator output dips.

 

Is the 12.8 volts the you measured DIRECTLY AT THE LED TERMINALS. I am thinking that the point on the chassis that the LED negative is connected to does not have a good path back to the battery negative and it is the vibration from the engine that is causing a intermittent connection between parts of the chassis. If you have a battery charger you could try putting the battery on charge with it still on the bike to see if the fault comes back when the battery voltage reaches 12.8 volts.

Les.

 

Thanks for your help. I’ve checked the voltage and it’s 12.8v. What would you recommend re the choke and capacitor specs?

Not knowing what parts to which you have access I suggest going for the largest choke (as many tens of millihenries) as you can get that doesn't have so much resistance that it dims the lights noticeably) and thousands of microfarads.

The capacitor goes in parallel with the lamp, preferably right at the lights rather than back at the battery.

You can try just the single capacitor that dl324 suggested and if that doesn't quite do it, try adding the choke.

 

Hi All. I’ve tried numerous capacitors in parallel with chokes etc. Nothing is making any difference. I borrowed a oscilloscope to see what is happening. What should be DC voltage looks like AC with the peaks cut off, also there’s voltage spikes. Speaking to someone who knows the bike a bit more, he’s saying that the rectifier/regulator converts AC to DC. I’ve tried another rectifier/regulator but that’s still dishing out noisy DC. I’m lost as what to try next, any help would be really appreciated.

 

Post a picture of the waveform and tell us where the measurement was taken.

A bad ground could cause the symptoms you're describing.

 

Without the resistor the led is bright, but randomly turns on and off. I appear to have a constant 12.8v but don’t have an oscilloscope to look at how smooth it is etc

pls check the batt negative pool voltage and pulses related to the frame (chassis)
Try to connect the minus led lead directly to the minus on battery.

Picbuster

 

most likely it is caused by poor input filtering of the LED driver you have. I think it is worth to buy a different one.