Simple circuit question about diode.

[Stevien1]

I've had a 5th injector setup on my van for years now and the other day I was cleaning up some wiring under the dash and found that the ground side of the injector has B+! I found this on the wire from the pressure switch to the neg terminal of the LED indicator. The ground side loses 12v with the 5th injector unplugged, so I'm guessing that the coil inside is causing this. I'd like to put a diode in to keep the ground side from seeing 12v. Here's what the circuit looks like:



If that's the correct place for a diode- what kind to should I get? Thanks!

[turbovanman²]

Maybe your injector is no good, I had my 5th injector setup just like that with no problems or maybe the switch is no good??

[RoadWarrior222]

Bear in mind you get a forward voltage drop when you use a diode, 0.6V for a general purpose silicon diode.

[lametec]

Why would you want to keep the "ground side" from seeing +12V? This is normal behavior of a circuit that's not connected to ground.

Since there's no current through the circuit, there's no voltage drop, and you get 12V.

[Stevien1]

The 5th inj still works, but my new indicator bulb doesn't like 12v on the ground side. I've swapped 5th injectors and pressure switch with no change. I have no idea what kind of diode I need, or which way to face the "band".

[lametec]

Your bulb doesn't care about +12V on the ground side as long as the positive side is also connected to the same +12V. No current flows from +12V to +12V.

Even if you were to put a diode in your circuit, the result would be the same.

Like I said, +12V on the "ground side" of the injector is perfectly normal when the circuit is not connected to ground (pressure switch off). No amount of swapping injectors or switches will change this.

The only issue I see with your circuit is that there's not current limiting resistor for your light (LED).. I'm gonna assume it's a 12V LED, with a built-in resistor?

[Stevien1]

Your bulb doesn't care about +12V on the ground side as long as the positive side is also connected to the same +12V. No current flows from +12V to +12V.

Even if you were to put a diode in your circuit, the result would be the same.

Like I said, +12V on the "ground side" of the injector is perfectly normal when the circuit is not connected to ground (pressure switch off). No amount of swapping injectors or switches will change this.

The only issue I see with your circuit is that there's not current limiting resistor for your light (LED).. I'm gonna assume it's a 12V LED, with a built-in resistor?

Thanks for clearing it up, I sent my last reply before looking at your first one.

I had a green LED in there for years and wanted to switch to a more visible red one, but it doesn't work. The old green one still works, so I'm guessing my new red LED is defective. I got them both from the local auto parts place.

Like the saying goes- "if it ain't broke, fix it untill it is!"

[lametec]

Could it be the green LED is a 12V diode, and the red one isn't? 12V would certainly have fried it if that's the case. Of course, installing the diode the wrong way would also cause no light.

[SwiftTech]

Did the LED come with a resistor? If not, even with it hooked up right you will be putting too much current through it and will burn it out. LED's need a resistor to act as a load device in the circuit, or else they won't work right.

[Stevien1]

The new red LED has a sticker on it that says 12vDC, so I'm assuming it has a built in resistor. I left the old green LED in for now.