Strip light replacement

[marlinowner]

The -ve input on the dimmer will have to be connected to earth to power the dimmer circuit itself, I would guess that the -ve output on the dimmer can left disconnected.

[Oldiebut goodie]

Only leave disconnected if it is a common negative - not all of these devices are common. Easy enough to check with a meter, just check that there is zero resistance between the 2.

[ghost123uk]

Do I just connect the red wire in and out, forgetting the earth, or do I connect the negative in and out from the dimmer both directly to earth ?

The dimmer will require an earth. So, best practice would be a +ve and a -ve (earth wire) into the dimmer, and the same from the output of the dimmer to the LED's. Earth wires are easy to do because all / anny part of the vans metalwork is an earth. Self tapping screws come in handy here

[MagicWagon]

I don't really want to run an extra wire from the dimmer to each of the lights as it would be difficult. I'm hoping to use the wires that connect to the lights I am replacing. However, each light already has two wires, one of which is already, it seems, connected to the body, so do you think if I connect a wire from the -ve output of the dimmer to the body that would be enough. I guess if it is a common negative on the dimmer I only need to connect one -ve to the body. As you can tell, my schoolboy physics is a dim and distant memory.

[marlinowner]

And if you are going to connect the -ve output to earth, same as the input, then you can just bridge them together instead.

[ghost123uk]

Just for clarity, yes Magicwagon, what you describe will be fine.

E D I T = turns out some of these dimmers use the -ve line to do the controlling, so in that case "common'ing" the negatives to the body will not work and may even damage the unit. Thanks to MidLifeCrisis for noticing this anomaly in the way they work.

[MidLifeCrisis]

I don't really want to run an extra wire from the dimmer to each of the lights as it would be difficult. I'm hoping to use the wires that connect to the lights I am replacing. However, each light already has two wires, one of which is already, it seems, connected to the body, so do you think if I connect a wire from the -ve output of the dimmer to the body that would be enough. I guess if it is a common negative on the dimmer I only need to connect one -ve to the body. As you can tell, my schoolboy physics is a dim and distant memory.

FYI - I got a remote control dimmer off flea bay that was actually modulating the negative signal to dim the LEDs (rather than the positive) - so just be sure to check what your dimmer is actually doing?!?!

[ghost123uk]

^^^ Good observation there

[MagicWagon]

I have hooked up my new dimmer to a meter, and it seems the two positives have no resistance between them, whereas the negatives have a few hundred ohms resistance between them, and that doesn't vary as I turn the knob.
Does this mean I should actually treat the +ve terminals as the -ve and vice versa, i.e. connect the -ve to the lights and the +ve to earth ?
Also, would you expect resistance to vary as I turn the knob ?
Thanks

[MagicWagon]

Well, I thought I'd go ahead with hooking up the lights with the dimmer, and assumed the +ve and the -ve on the dimmer are the wrong way around as the +ve have 0 ohms between them (see my previous post). I connected the red and black which was feeding one of my old lights to the input, and the new LEDs to the output (red to -ve and black to +ve). The lights come on, but turning the knob has no dimming effect. I tried switching the inputs and outputs, but it does exactly the same. Should I try connecting the red wires to the +ve terminals and black wires to the -ve terminals as that would just be connecting those wires together (as I already tested the two +ve terminals are connected (0 ohms). I don't want to start a fire ! Any other suggestions welcome, or must the dimmer be faulty.

[Oldiebut goodie]

I don't know why you are messing about swapping inputs for outputs and positive for negative etc.. Just wire it up as it should be. You seem to have done everything that you shouldn't so far so try the correct way!

[ghost123uk]

The knob will not show any alteration on you meter scale, it's not a simple potentiometer, it's a a delicate circuit that sends pulses of different lengths to the LEDs. As mention above by MidlLifeCrisis, it does this via the negative wire, so you cannot "common" this negative wire via the body, it has to go straight to the LED strip.

the dimmer be faulty.

It will be faulty now. You will have blown the delicate circuit components by wiring it up the wrong way round (for some reason)

E D I T = Spelling !

[marlinowner]

Surely there is a reasonable chance it is protected against reverse polarity?

[ghost123uk]

Surely there is a reasonable chance it is protected against reverse polarity?

Possibly, but remember, this will be cheapo Chinese made, so it might not have any protection
Plus, your standard "crowbar" diode relies on a suitably (low) rated fuse to be in line.
It might have an in line diode I suppose, but it night not.

[MagicWagon]

Thanks for all the help and comments. I think I had a mental blank on wiring it up the way it is marked as I knew that would be the only way that I could not use it. I tried it that way now and it does indeed work. I will not therefore be able to use it. I need to be able to use the existing wiring which uses the chassis as common. Running new wiring will mean too much disruption to existing lining etc. Does anyone know of a dimmer that works like that ? If not I'll have to forget having a dimmer.