My company has vehicle trailers with built in lights. They worked fine with the Ford vans we had but with the new Citroens the lights are always on. Dimly when they should be off and then brighter when they should be on. Some basic investigation with a multimeter showed that the various positives deliver around 8.5V when the light should be off and 12V when they should be on.

I am looking to block the current when the PD is below 9V to get the lights to function as expected. Is a zener diode with reverse breakdown voltage of, say 9V, going to do the job, or does the current only get driven at the voltage above breakdown (4V if source is 12V and breakdown is 9V, for instance). If so, maybe the zener is connected to the base of a MOSFET? Any help gratefully received, detailed diagrams and parts lists even more so! My understanding of electronics is modest at best.
Thanks in advance

 

Welcome to AAC!

Post a diagram of how the lights are connected. In particular, how they're switched/controlled. How much current do they draw? LED or incandescent?

The threshold voltages of discrete MOSFETs aren't controlled tightly. Unless you cherry pick parts, the circuit would have to work for a range of threshold voltages.

What are the area constraints for any solutions?

 

Some basic investigation with a multimeter showed that the various positives deliver around 8.5V when the light should be off

If the lights are being supplied/controlled with PWM voltages your meter is probably telling lies if it's being used on a DC range. What voltages do you measure if you use an AC range and connect the meter via a capacitor (say 100nF) to the test points?

 

Do the Citroens have some sort of light test built in to an ECU? If it expects incandescent lights and you have led lights it may do weird things. It's kind of out there I know, but who knows.

 

Hi,

The van does definitely have some kind of testing. It says specifically that there is a fault with the light board even if the light board should be off. Don't think I can specify incandescent or LED so I don't know if this knowledge will help but thanks.

...use an AC range and connect the meter via a capacitor (say 100nF) to the test points?

Will try this and get back you you

Welcome to AAC!

Post a diagram of how the lights are connected. In particular, how they're switched/controlled. How much current do they draw? LED or incandescent?

The threshold voltages of discrete MOSFETs aren't controlled tightly. Unless you cherry pick parts, the circuit would have to work for a range of threshold voltages.

What are the area constraints for any solutions?

Lights are LED. Area shouldn't be a problem, can mount a new housing to the trailer if necessary. Wiring diagram will be a problem. I have a standard trailer coupling at the back of the van and chocolate blocks on the trailer. These are the only points I have access to. There are separate positives for each light and a single negative. They split to the individual lamps by having multiple wires twisted into the far side of the chocolate blocks, very basic. I have space to mount my own equipment to the trailers but can't fiddle with the van. I'm thinking maybe relays instead of transistors of any kind?

Thanks

 

Lights are LED.

This may be part of the problem. Some autos have a lamp check feature for incandescent lights that can cause the symptoms you described.

Wiring diagram will be a problem. I have a standard trailer coupling at the back of the van and chocolate blocks on the trailer. These are the only points I have access to. There are separate positives for each light and a single negative.

That sounds typical for automotive circuits. What's a chocolate block?

How many lights are to be controlled? Do you have separate circuits for turn signals and brakes? Any other circuits? Does power for this module you're contemplating have to come from the light control signals themselves?

This is where a wiring diagram would be helpful. Otherwise, we need to play 20 questions.

 

How about making a short connector between your trailer connector and van with a couple dummy load resistors to simulate incandescent lights? They're common for folks who convert to led lights to make the turn signals flash at the right speed. That at least will let you know if you are on the right track or not.

 

This may be part of the problem. Some autos have a lamp check feature for incandescent lights that can cause the symptoms you described.
That sounds typical for automotive circuits. What's a chocolate block?

How many lights are to be controlled? Do you have separate circuits for turn signals and brakes? Any other circuits? Does power for this module you're contemplating have to come from the light control signals themselves?

This is where a wiring diagram would be helpful. Otherwise, we need to play 20 questions.

Hi again,

Thought chocolate block was a fairly well know term, maybe only my neck of the woods. I mean standard wire connectors, look like Lego bricks.

Light to be controlled = turn signals, break lights, fog lights, standard rear lights and a Licence plate light, all led. Each with their own 12V supply coming through the power lead. No others available. All the lights have their own negative and they are joined together to the common negative that drains back to the van.

Thanks again, sorry I don't have more info to give.

 

Do the lights have constant power as in they come on when you plug the trailer connector in or do you have to have the key on? The light check sounds very probable, but if the trailer is constantly getting enough power to turn on LED lights even dimly they will over time kill the battery... which makes me wonder if there's something else missing.

I also started to think due to the immense variables in trailer lighting it would be kind of pointless for a manufacturer to include a check for trailer light issues unless they have a specific lighting package that is supposed to be used.

Are the fog lights constantly powered with a switch on the trailer to turn them on or off? Maybe something there????

LED lights are the greatest and worst invention all in one. It takes little power to turn them on so they'll work with some of the most corroded and nasty wire, but at the same time even a tiny short or back feed somewhere will make them light up.

 

I'm thinking the error code you are seeing may be more to do with getting power somewhere it shouldn't be rather than a problem with the lights themselves... which of course doesn't make sense since the problem is going to be either a light or wiring.

 

I'm thinking the error code you are seeing may be more to do with getting power somewhere it shouldn't be rather than a problem with the lights themselves... which of course doesn't make sense since the problem is going to be either a light or wiring.

I hear you, can't say. All it tells me is "trailer fault, repair required" It does accept an incandescent light board and it correctly flags a fault if the coupling is loose, for instance.

 

Do the lights have constant power as in they come on when you plug the trailer connector in or do you have to have the key on? The light check sounds very probable, but if the trailer is constantly getting enough power to turn on LED lights even dimly they will over time kill the battery... which makes me wonder if there's something else missing.

I also started to think due to the immense variables in trailer lighting it would be kind of pointless for a manufacturer to include a check for trailer light issues unless they have a specific lighting package that is supposed to be used.

Are the fog lights constantly powered with a switch on the trailer to turn them on or off? Maybe something there????

LED lights are the greatest and worst invention all in one. It takes little power to turn them on so they'll work with some of the most corroded and nasty wire, but at the same time even a tiny short or back feed somewhere will make them light up.

 

Hi again, you need to turn the key. It behaves perfectly normally with an incandescent light board. Its just we have these trailers with in built lights and we'd like to use those lights rather than strapping on a separate lightboard

 

I hear you, can't say. All it tells me is "trailer fault, repair required" It does accept an incandescent light board and it correctly flags a fault if the coupling is loose, for instance.

Ahh... it does seem then it is the LED lights and the ECU not getting along.

The easiest fix would be some dummy load resistors added to the trailers themselves. They will add enough of a current draw to make them look like incandescent lights to the system. One on each of the circuits to ground and your problem should go away.

https://www.amazon.com/Zento-Deals-...ocphy=9015028&hvtargid=pla-568827705785&psc=1

 

Ok, that sounds likely. They're good and cheap so we'll give it a go. Thanks. Hypothetically though, would a circuit like this work? Obviously, this is very simplified and there's an ECU and some more complicated stuff going on with the LEDs but I think it should work the same way.... Hopefully

 

would a circuit like this work?

Makes no sense to me.

Your schematic drawn more conventionally:

 

Well, yes. That sure does look neater and more recognisable. But would it work?

 

But would it work?

No.

The relay coil current would have to go through D1. That would increase it's voltage and change the "divider" voltage. In any case, that's not the correct way to energize a relay.

 

Without knowing your business or the abuse your trailers see day to day I would go the simplest route possible. I've been on the side of the freeway enough times for light issues (along with tires, broken air lines, and even blown up air bags) to tell you that's not where you want to be working on your equipment. If they're a little dim when they should be off you can still drive home safely... if they aren't on when they should be then you have a problem.

 

Without knowing your business or the abuse your trailers see day to day I would go the simplest route possible. I've been on the side of the freeway enough times for light issues (along with tires, broken air lines, and even blown up air bags) to tell you that's not where you want to be working on your equipment. If they're a little dim when they should be off you can still drive home safely... if they aren't on when they should be then you have a problem.

All very sage advice and I absolutely will try the resistors 1st. Abuse wise, they take a lot but a basically never go on any fast roads or any distance from base. That said, down time is an issue so yeah keep it simple, for sure. Just wanted to know rhetorically if I was on the right track and if I knew what the hell I was talking about. Sounds like a no, but never mind!