pwm of led’s in dash lighting

I have a question on wiring this PWM module for my dash LED's to control dimming. It is my understanding that the PWM takes place on the negative of the output.

Here is the unit and suggested wiring.

As you can see both input and output are isolated from each other.My question is that I cannot easily seperate the two negatives, as I have a 1958 thunderbird, which utilizes the metal dash as the return (negative).Can I tie both negatives to the metal, without harming this unit, and will it still function properly. There is no trouble keeping both positives seperate.

That one heck of a chunky dimmer. I presume it's meant for a 12V house or van? Why not get a typical dimmer from a modern vehicle? Jap cars have used PWM dimmers since the 1980s. Once issue may be that they usually are -ve (GND) switching, but that could be simple to change with an additional MOSFET is you feel to inclined. As to your module, more info is needed. If it is truly an isolated output, then joining either output to either input should not matter. But the wiring diagram is insufficient on its own though it may be drawn that way for ease of wiring (as often done for building electrics). A DMM else opening up may determine if it has a common interconnection - ie, terminal bridge - but otherwise it's best to check its specs. (IE - volt-meter modules have equivalent wiring for power and input sensing but cannot be "commoned". Hence no probs for battery powered DMMs, but a dc-dc converter (isolator) is required if interconnected as in vehicle voltmeters.)

My guess would be that the positives are not isolated, there are just two separate terminals to make installation easier. An ohm meter would confirm this. More then likely the PWM is on the negative side, but it doesn't have to be - technically you can PWM one side, the other side, or both sides and get similar results. Kevin Pierson

Agreed. The circuit itself determines which side is PWM'd. Usually it is the negative side due to the availability and simplicity of using NPN or N-ch transistors & MOSFETs. I suspect most vehicle manufacturers would prefer the +ve side...

Use an ohm meter and you will see that the 2 negative terminals are common. According to the info on the picture, there is a separate output that is controlled by the potentiometer. If the grounds are common, as I think they will be, you will be fine. EDIT: I should have read Oldspark's comment before I wasted the time typing this.

Hello - I stumbled across this as I am facing the same issue. The problem I have, using what I believe is the same PWM as the OP posted, is that the power sides are "common" with dedicated (separate) grounds. Can anyone suggest a PWM that could be used in a vehicle application with a common (chassis) ground? I can't seem to locate one anywhere.

How hard could it be to cut the ground from the lighting portion of the cluster and connect all bulb grounds to the output ground of the PWM box?

Depends on the cluster. On the OP vehicle, the cluster is metal...just one wire per bulb. You'd need to isolate the whole dash, which will cause issues with other indicators..turn signal,high beam. Or replace the bulb sockets with isolated/insulated ones. Even on an older dash that uses a flexible printed circuit back, the bulb returns may be shared with gauges.

Agreed. The "simple" universal solution is +12V switching. Any multi-Amp P-ch MOSFET as per Q1 at electronics.stackexchange:p-channel-mosfet-high-side-switch with R1 = 1MOhms (100k to 1M) LH arrow +12V to dimmer output + "GND" to dimmer INPUT or vehicle GND. "ON/OFF" from dimmer output + "ON/OFF" from dimmer output taken direct to Gate (G) of Q1 (the vertical line) since Transistor T1 is not required. optional series 100 - 10k Ohm resistor between "ON/OFF" dimmer output + and the Q1 MOSFET's Gate (to current limit Gate failures). [ Transistor T1 can represent the output of a typical GND-switched PWM module. ] Note that the +12V dimmer supply now also supplies the dimmed bulbs (unless Q1's Drain is connected to another +12V source).