floating vs chassis tied light relays in OFF position

[kellogs]

For any vehicle, am I guaranteed to have a chassis connection on the + side of a light bulb when it is *not* lit ?

I am particularly interested in the rear turn/brake light bulb.

Thanks

 

[Nigel Goodwin]

Most bulbs are high switched, so when not powered there's a path through the bulb down to chassis - the main exception is the interior light, which is normally low side switched, so will have 12V on both sides of it when not lit.

What are you actually trying to do?.

 

[rjenkinsgb]

For any vehicle, am I guaranteed to have a chassis connection on the + side of a light bulb when it is *not* lit ?

Usually, it will show 0V or very near that, relative to chassis ground, in a vehicle using filament lamps.

It's not a *direct* connection to chassis, as the circuit is via the lamp.

It will usually be quite low resistance if it's a working (not blown) filament bulb, but may not show any connection with a normal multimeter in the case of LED lamps.

It also assumes the lamp ground is good.

 

[Diver300]

There are now a lot a lamp assemblies in cars that have some sort of electronics in them. Some have power connected to them all the time, and some signal that is not the power goes to them to tell them to turn on.

One example is LIN controlled multicolour ambient lights.

(On one of my cars, the lamps are at +ve when turned off, but it is quite old and is positive earth, so probably not what we are talking about here.)

 

[kellogs]

Just to clarify, I was referring to the GND connection marked "??". Is that "terminal" always grounded or can it also be left floating?

Thanks

 

[rjenkinsgb]

It would usually be left floating, for a simple on-off switching use.
I've never seen anything with it grounded for lighting applications.

Many switches do not have a normally closed terminal, just the two normally open switch contacts.

You would see a ground at the power side of a filament lamp with the switch off, due to the filament resistance to ground.

 

[Diver300]

I can think of a few reasons to ground a signal that is turned off.

It could be to prevent an LED light from glowing of flickering due to a monitoring circuit or other leakage. It could also be to prevent the monitoring circuit from flagging a fault when the LED is turned off.

If a normal DC motor is being supplied, shorting it when off will slow it faster. On reversing motors, the side that's not being connected to 12 V has to be connected to ground to complete the circuit, so two circuits like you have shown may be used.

There is very rarely harm in shorting out a load that's not in use.