11.2v with fuse removed.

[Rob.P]

Hello all. I’ve just fitted a small fuse board next to and fed from my auxiliary battery, with a fuse in I get 12.7v as expected but with no fuse fitted I’m getting 11.2v. It has LEDs next to the fuses which apparently light up if the fuse blows. I would have assumed 0V with no fuse but am I missing something or is it something to do with the leds?

 

[Alster]

It’s because of the leds. As your multimeter is a low impedance (probably 10 meg ohm) it’s showing the voltage‘bleeding’ through the LEDs. If you put a small load on it the voltage should disappear. So should be nothing to worry about.

 

[sparkzer]

..your missing something todo with the LEDs how those fuse boards work, is in effect the LED bridges the fuse holder. lets go with an example....Imagine an output from one of the fuse holders gose to a bulb, then the other side of the bulb goes to 0V chassis ground.

Put the fuse in, +12V flows through the fuse, to the bulb and the bulb lights up. the LED will do nothing as on either side of the fuse is +12V. If the fuse blows on the input to the fuse (and one side of the LED) there is +12v and on the other is the wire to the bulb, which in effect is connected to ground. as the LED draws such tiny current a path to ground is found via the element of the bulb, and the LED will glow. if you remove the bulb (or its blown) the LED also wouldnt light, so they are not foolproof, but does give a quick indication if the only fault is a blown fuse.

 

[Rob.P]

Thanks for the explanations to you both

 

[Dellmassive]

It's a normal reading for the fuse boxes with LEDs. As above there is no current to power anything when fuse removed.

 

[Skyliner33]

Just been thinking about this, is there something in the fuse box that limits the current that will flow through the LED? Surely it can't be the resistance of the LED itself?

 

[Dellmassive]

It's just the led diode and a resistor.

It uses the connected circuit for a 0 volt to illuminate the LED.

But with no circuit connected you just get a reading of a voltage on a multimeter but with no current flow in the light one show up.

Without a few of these questions pop up recently.

Nothing to worry about as you can see the circuit does actually go dead when you pull the fuse..... xHamster voltage raining on the multimeter that gets some people anxious.

I've got one at home I'll have to pull the back off and take a picture of the layout..

 

[Skyliner33]

It's just the led diode and a resistor.

It uses the connected circuit for a 0 volt to illuminate the LED.

But with no circuit connected you just get a reading of a voltage on a multimeter but with no current flow in the light one show up.

Without a few of these questions pop up recently.

Nothing to worry about as you can see the circuit does actually go dead when you pull the fuse..... xHamster voltage raining on the multimeter that gets some people anxious.

I've got one at home I'll have to pull the back off and take a picture of the layout..

Thanks. No need to pull the back off. That just confirms my thoughts it must be a load resistor in series with the LED.

 

[roadtripper]

For a bit more physics the forward voltage of a red LED is around 1.8v so the reason you see a slightly lower voltage than the main battery voltage is that you are effectively measuring the main battery voltage less the voltage drop across the LED.

Red LED tend to be the lowest forward voltage, with green/yellow next and Blue/White highest.

Sometimes knowing the rough voltage drops is useful diagnostics. If the drop is around 2v it's likely going through an LED, if it's around 0.7v it's likely going through a standard diode. In power electronics you tend to find Schottky diodes that have a drop of around 0.2v - more expensive but with a lower voltage drop less energy is lost as heat across them.

Also can be useful if you have to replace an LED on a prebuilt light fitting, so long as one of the chains is still working. A lot of modern "chip" type LED packages used in lights have several LEDs in series in them so you need to know how many to replace. You can sometimes see them by looking through sunglasses to drop the brightness but the other trick is to use a multimeter across one of the "chips" to see the voltage drop across it and divide by 2.