A warning about blade fuses.

[Colvic Watson]

Last winter I replaced all my blade fuses with the type that has a small LED light to indicate it has blown. They are very useful in tracking a fault but this weekend I was installing a new wiper motor and without realizing it I momentarily shorted the connection. I couldn't get he darn wiper to work, out came the multi-meter and I tested and tested, 13.5v every time. I got ready to take the wiper off and return it when I opened the fuse panel to check something else - sure enough the fuse for the wiper had a bright red light showing. It turns out that the LED allows current through, more than enough to register on the meter.

Not just a problem for fault finding but a real problem if you have a continuous short as some current will continue to flow - enough to cause a fire?

 

[pteron]

The LED probably passes 5mA or so. Unlikely to cause a fire.

 

[nigelmercier]

One reason I've avoided these (gimmicky) fuses.

 

[elton]

Virtually impossible to cause a fire with that current; 5 to 20 mA (at 12V) or thereabouts. These LED fuses are a brilliant innovation IMO, and a great example of lateral thinking.

I wonder how anyone would think the LED illuminates, if it didn't draw current.

 

[Avocet]

That's a common fault even without those fuses though. The number of times I've had a tail light on a car not working (or something like that) but have had 13-odd Volts at the bulb holder! It's just that there's a high resistance somewhere and it shows voltage when checked with a high impedance voltmeter, but not when you try it with a test lamp or something that draws a bit of current. Unless there are micro-electronics involved, I prefer to use a test lamp for fault fining.

 

[elton]

That's a common fault even without those fuses though. The number of times I've had a tail light on a car not working (or something like that) but have had 13-odd Volts at the bulb holder! It's just that there's a high resistance somewhere and it shows voltage when checked with a high impedance voltmeter, but not when you try it with a test lamp or something that draws a bit of current. Unless there are micro-electronics involved, I prefer to use a test lamp for fault fining.

It helps to have at least an O Level understanding of basic electrical theory before fault-finding a car's electrics.

There is no mystery, or inexplicable behaviour in the way these LED fuses work. Also, many vehicles now implement the CANbus system for control of lights and everything else, so old-school fault-finding techniques simply don't apply.

 

[doug748]

Well thank you Lazy Kipper.

For those of us without an O Level understanding of much at all, a useful warning.

 

[Woodlouse]

This also seems a lesson in playing with electronics without isolating first. When I do that the fuse is always the first stop for fault finding.

 

[VicS]

Also remember that digital multimeters take so little current that they will often show a normal voltage reading through a bad connection when a moving coil meter would not.

Easy to make a fool of yourself if you are not aware of this.

For this reason a test lamp made from a low power 12volt bulb on a couple of leads is often better for fault finding. The meter with a bulb in parallel a bit of a fiddle but even better perhaps.

 

[Avocet]

It helps to have at least an O Level understanding of basic electrical theory before fault-finding a car's electrics.

There is no mystery, or inexplicable behaviour in the way these LED fuses work. Also, many vehicles now implement the CANbus system for control of lights and everything else, so old-school fault-finding techniques simply don't apply.

You don't have to worry about doing the CAN controller(s) any harm as long as the test lamp load is significantly more than the circuit would normally feed. In the case of a tail light, you're fine using tail light bulb as a test lamp. Even if you don't, the worse that's likely to happen is you'll blow the fuse. The CAN controller will have been designed to cope with fuses blowing. Obviously, if the fault isn't a good old-fashioned high resistance or broken wire and it DOES lie within the controller, then yes, it's a totally different game.

 

[elton]

You don't have to worry about doing the CAN controller(s) any harm as long as the test lamp load is significantly more than the circuit would normally feed. In the case of a tail light, you're fine using tail light bulb as a test lamp. Even if you don't, the worse that's likely to happen is you'll blow the fuse. The CAN controller will have been designed to cope with fuses blowing. Obviously, if the fault isn't a good old-fashioned high resistance or broken wire and it DOES lie within the controller, then yes, it's a totally different game.

I wasn't really thinking of damage to the controller; rather the possible confusion to any "traditional" electrician in tracing feeds and supplies.

 

[Colvic Watson]

Good to know the current is small. And thanks for the one acerbic comments I tried to find an O level course in basic electronics but it seems they stopped doing these about 25 years ago, so no luck there.

It's been a heads up for me in the use of multi meters and a reminder to self to check the fuses as well as check with a multi meter. I do like the LED blades, as soon as I opened the panel it was there shining brightly, I can imagine on a dark and stormy night that would make things easier.

 

[elton]

Good to know the current is small. And thanks for the one acerbic comments I tried to find an O level course in basic electronics but it seems they stopped doing these about 25 years ago, so no luck there.

It's been a heads up for me in the use of multi meters and a reminder to self to check the fuses as well as check with a multi meter. I do like the LED blades, as soon as I opened the panel it was there shining brightly, I can imagine on a dark and stormy night that would make things easier.

O Level Physics should cover it. At least it should; O Level Electronics hadn't been conceived when I did Physics in 1974.

 

[oldgit]

Good to know the current is small. And thanks for the one acerbic comments I tried to find an O level course in basic electronics but it seems they stopped doing these about 25 years ago, so no luck there.

It's been a heads up for me in the use of multi meters and a reminder to self to check the fuses as well as check with a multi meter. I do like the LED blades, as soon as I opened the panel it was there shining brightly, I can imagine on a dark and stormy night that would make things easier.

Yup..... my fuse board contains around 100 fuses. Now its so easy to locate the dead one and change it.
No more finding torch usually with weedy nearly flat battery gloom output ,pulling out card/reference sheet to find out which fuse on which board in theory has blown, finding correct one out of 10 or so per fuse box then pulling the flipping thing out and reinserting a the new one.
If you have an eidetic memory.. lucky old you... I cannot usually remember where the torch resides

 

[oldgit]

Let there be light....Godsend in my gloomy dark cupboard

Yup..... my fuse board contains around 100 fuses. Now its so easy to locate the dead one and change it.
No more finding torch usually with weedy nearly flat battery gloom output ,pulling out card/reference sheet to find out which fuse on which board in theory has blown, finding correct one out of 10 or so per fuse box then pulling the flipping thing out and reinserting a the new one.


If you have an eidetic memory.. lucky old you...... I cannot usually remember where the torch resides

 

[David2452]

LED "blow and glow" fuses do obviously allow a small current but no more than testing with a multi meter, there is no danger from using them and after a short it is patently obvious they are blown unlike other fuses, gimmick they certainly are not, an aid to rapid fault finding, they are. A multi meter of of little use for testing current availability checking like previously said. A simple bulb is of far more use, even if the fuse is intact corroded terminals and cable can set up resistance sufficient to restrict current but still allow a good voltage reading with the usual digital multimeter.

 

[nigelmercier]

LED "blow and glow" fuses ... an aid to rapid fault finding ...

It appears not, hence this thread

I don't see the point. If my VHF appeared to have no power, I only need to check one fuse: the one with VHF written above it.

 

[Colvic Watson]

But that's not what happened, the wiper didn't work but the wires to it had 13.5v. I now know how little current is needed to make the meter register and next time I'll check the fuse straight away. As it is the LED fuse was excellent; but I had expected when they blow the diode would come on but the current would not pass through it and down the wire, in fact I'm not sure why it has to, the current can reach it from the battery, why it has to feed it on to the circuit that it's protecting I don't know. But asking that question will leave me open to some know-it-all who'll say I'm not fit to use Her Majesty's waves. The majority of the replies have been really helpful and as usual I continue my education by asking a question on YBW.

 

[pteron]

The current must flow in a complete circuit - the fuse only has two terminals, so the current flows into one, through the LED (generating light) and then through the load to ground.

 

[Colvic Watson]

The current must flow in a complete circuit - the fuse only has two terminals, so the current flows into one, through the LED (generating light) and then through the load to ground.

doh, thanks.