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Help I need some advice

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New Member

Hi all,

Thought this site would be the best way to get an answer that has been on my mind for some time now.
I use to have some break-out boxes which turned on a neon when the fuse blew, but because it's been sometime since I've built some, I've lost the wiring diagram and wanted some help.
The way I remember doing it was to use a resistor across the live and neutral of which when the fuse blew would drag the supply to the neon, but I carn't remember where the resistor went and what value it was.
The unit consisted of 4 IEC cables, 4 coloured neons, 4 fuses and 2 blue 16 amp plugs.
The reason for me wanting this unit is to be put onto disco/stage lighting rig, so that I can fault find at a glance whether a unit has blown a fuse or is faulty in some other way.
I know alot of you will say use a relay and switch the mains to the neon when the fuse blows but I've thought of that already and the size of box I want to use will not allow for it (plus I would have to build a PCB for it anyway)
Any help on this matter would be gratefully recieved.

Many thanks


The way I've seen it done is to put a resistor in series with a neon and wire them across the fuse. If/when the fuse blows the full mains supply voltage will appear across its ends and the neon will light.

That assumes that the load is still in circuit after the fuse blows, ie. there is not a contactor that drops out and removes the load.


New Member
In reply

Thanks for the reply.

It seems logical enough to do that, but I've got two other questions for you.
Firstly, What side of the rail would I put the resistor and secondly what would be the value of the resistor to use.

My only concern is that there will still be a slight voltage leakage coming from the resitor and it would make the neon glow slightly.
What I remember from the unit I built, was that the neon was dead until the fuse blew.

Anyhow once again thanks for the reply, and if you can shead any more light on the subject I'd be grateful.


I don't know what you mean by 'which side of the rail would I put the resistor'. If you mean where do you physically put it, then wherever is convenient.

I'm going from memory now, but I think the value is about 0.5megohms for a neon fed from 240v mains - or about 220K for 110v.

With the fuse intact and passing current the neon would not be lit because it needs about 80v across it for the neon gas to become ionized. So there will be no leakage current through the resistor. When the fuse blows the voltage across neon and resistor will go up to full mains voltage. The neon then lights and the voltage across it will drop to around 20v(?). Without the resistor the neon would pop, so the resistor is there to limit the current.

I hope that helps.
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