Steaming light problems

[Adelaide]

Not the worlds greatest electrician!!!
I have 12 to the deck fitting, 12 v at the bulb connectors, have tested the bulb it works, I put the bulb in, no volts suddenly. Have cleaned all connectors, tried a new bulb. A bit stumped now, any help appreciated. Many thanks.

[Unity]

Check your connectors to voltage and ground. Sounds like a wire is barely connected somewhere so you get a reading without a load but once load is there the resistance of the (likely single strand touching) wire results in very low voltage.

Are you reading 0v when under load, or something higher but not 12+v?

[Tornadosailing]

I think you've posted in the wrong forum. Maybe admins will move it for you.

What you've described is classic voltage drop. First you need to make sure which side of the circuit you're losing. Because you're at the fixture, odds are you're using the wires there for both the power and ground test. A loss of voltage there could indicate a voltage drop on the power side or ground side.

You'll need to get a ground source back to the battery. This will mean a long wire from your meter ground lead to the battery. Once you have a ground, check voltage at the fixture on both the power wire and the ground wire.

If you have little or no voltage at the power wire, start chasing that side of the circuit. Test the voltage at each switch and each connection. When you find the one with good voltage in, but very little out, you've found the culprit.

If you have good voltage on the positive wire, but also have voltage on your ground wire, then the ground side is the culprit. Simply trace the ground circuit in the same way, looking for where the voltage drops from whatever you were reading at the fixture to less than half a volt.

Good luck

[Privilege]

What unity says...
A dead battery might still read 12v until you put a load on it, then it will drop down straight away. Your wires are the same in that if there is a short somewhere or corrosion, they will read 12v until you put a load in the circuit. It's not a battery problem...just dodgy wiring.

[Adelaide]

Thank you. I’ve found the dodgy connection. Now I know, I think, so if you have a bad connection and put a load on it the resistance cuts out all power, is that right. Many thanks for your help.

[Tornadosailing]

Originally Posted by Adelaide

Thank you. I’ve found the dodgy connection. Now I know, I think, so if you have a bad connection and put a load on it the resistance cuts out all power, is that right. Many thanks for your help.

In any circuit the voltage is used up between the power source and ground. This is true no matter what is using the power. In a series circuit the power is used proportionally between loads. Two equal bulbs wired in series will each consume half the power, resulting in full voltage present at the first bulb, and half the voltage at the second bulb, and no voltage at the ground side. In the case of two unequal bulbs (or loads) they will consume power proportionally. The higher watt bulb will consume more power, and more voltage loss will be seen at this bulb than the smaller bulb.

This is essentially what you had. The bad connection was acting like a high load on the system, and consumed so much of the available voltage that there was none left for the bulb.

While this explanation isn't 100% technically correct, it's easier for people to visualize brighter or dimmer bulbs than resistance to current flow.

[Tornadosailing]

The reason you had good voltage with the bulb out is that without current flow, there is no voltage drop. Full voltage will be available on the entire power side of the circuit.

[Unity]

One more tidbit of info ... a poor connection doesn’t always result in no voltage. More common is lower voltage like 10 volts when your batteries are providing 13. That causing all sorts of havoc like electronics being unreliable, LED lights flickering and more.

Your volt meter is your best friend - measure voltage at the batteries, and at your distribution panel, and they should be very close to the same (if not, check those connections and/or wire gauge). Then look at voltage at the place where you have problems. A drop of >3% on electronics or >10% on things like pumps indicate a wiring problem (either bad connector or insufficient wire gauge).

On my Beneteau 523 I was stunned to find a high gauge bus wire from the battery switch to the panel resulting in about 1.5v drop (>10%!). I ran a new low gauge wire and many problems went away (including my refrigeration working much better).

[Adelaide]

Really useful, thank you everyone for your replies. I leave you a little bit wiser (good thing) a little bit older (who knows).