Neon indicator isolation

[Rick Hughes]

I recently posted Q;s on a project goals ... to have an indicator panel for 19 separate 220V AC signals ...

zone on pump on etc.

After advice here decided against using the published ccts for LED on 'mains', and will instead use individual wire ended neon indicators : of this type:

Just thought a sanity check worth while .... many years ago I used to put together disco lighting systems, for example on ,ulti channel chasers (lights run in sequence to the music) one of the issues was as well as the correct light being illuminated, lights that were off would also flicker at low level .... presumably through some breakthrough of the Triac circuitry.

Anyway purpose of Q is .... in this instance Neutrals will all be common connection, and each live will come form the appropriate 220 relay output that is being activated ... zone, pump, etc.

Are there any steps worth considering to prevent 'break through' .... unaware if this is an issue with Neon indicators.

All components are in a fully insulated box, but no screening.

[Rick Hughes]

I have seen some indicator panel ccts where they add an IN4007 in series with each neon/resistor .... but no explanation of why .... just wondering if that is to 'isolate' each neon ...... or is this unnecessary.

[mick]

Indicator-Leads-42-0312

If a neon is connected across a relay coil then that's ok, but if it's switched by a contact (not across a load) you need to add a resistor (about 330k-470k should be ok) across the neon otherwise you can get a slight glow from it with the contact open.

You don't need series resistors with the ones that you've chosen.

[meow2222]

you've got a load connected across each one, so not needed

no, they're built in. Rapid do similar cheaper ones btw

NT

[Rick Hughes]

Mick .. this is the cct I was trying to put into words :

do you mean to add a 470k resistor across the neon in addition to the series resistor it has built in?

so it would in effect be :

Any comments on need for a diode ?

[Rick Hughes]

On 08/02/2014 13:01, [e-mail] wrote: t need series resistors with the ones that you've chosen.

Never used RAPID before service is impressive, ordered yesterday arrived today. Web site very easy to use.

Agreed - I could have got 12mm solder terminal or 12mm spade terminal ones slightly cheaper, but 9mm will look nicer,

[meow2222]

zero need

NT

[Rick Hughes]

Dropped the diode :-)

But still have the 470K resistor across the neon lamp unit (incl series resistor) ?

[mick]

You only need the 470k resistor if the neon isn't connected across either a load or the relay coil. That would happen if you were using a contact or switch to switch just the neon.

[John]

Could be sensible to include one in case the load becomes disconnected and it looks like an output is on which is not. A few pence well spent

[Brian Gaff]

Do these neon's have built in resistors or are they for use on full supply voltage? In some cases I used to put a very high value resistor across them so electrostatic false illumination did not happen, but I found some with built in resistors seems not too clever that way. Another couple of things about Neon's. I had a Decimo clock where each segment was an orange neon behind a frosted area of a display. The neon's gradually started to go black inside the glass, presumably some deposit from the electrodes, I never did solve this. I was always replacing them. The other effect I noticed was that some of the neon's on a panel of indicators, could be switched on simply by putting a finger near them. I suppose this was due to them reacting to the electrostatic field.

Brian

[Rick Hughes]

I'll include as the 'signals' will only be there when output form switches or relays active ... inputs could then float.

[Rick Hughes]

Assume these need to be Carbon Film composition .... is 0.25 W enough ?

These are described as pulse capable high voltage.

[Rick Hughes]

Would assume the neon have a series resistor built in within the indicator assembly ... spec sheet does state "anti-luminescence resistor" - presumably to prevent the electrostatic effect you describe.

[meow2222]

You dont need them at all.

If you do fit such, use 0.25/0.3w carbon film not composition, and use 2 same value in series for adequate voltage rating

NT

[Rick Hughes]

IN the previous post(on LED) there was comment that for Resistor to withstand peak Mains voltage Carbon Composition should be used .... why are they not suitable here ? ... just keen to understand?

[fred]

In article , Rick Hughes writes

I think that is me you are misquoting there :-).

I suggested a specific 1/2W carbon composition resistor for the previous application as it was suitable for direct exposure to mains voltage (voltage rating) and because it could withstand pulsed overloads in _current_ and therefore power as was previously required.

It was it's bulk carbon composition construction that made is particularly suitable for pulsed current and power overload, and whilst that is not necessary for the your new application as a bleeder resistor, that method of construction doesn't rule it out either.

The 1/4W res you mentioned is not suitable as the datasheet (on the page you linked to) shows an isolation voltage of only 100V so not suitable for direct connection to the mains (across the neon) and it is unlikely that you will find a 1/4W resistor suitable for this app at reasonable cost.

The 1/2W resistor from the same family is suitable however as it has an isolation voltage of 500V and is only marginally larger than the 1/4W.

A neat solution may be to splice the res into the lead for the neon (parallel not series) such that it lies in-line with the wiring but you'll need to lay some heatshrink over the top to keep it tidy and insulate it.

[meow2222]

just higher price lower reliability. C comp are only used for high energy pulses now.

NT

[Dave W]

Same here. I used to be involved in TV design; carbon composition should be used, as a high voltage can arc across the spiral turns of carbon on the film type. Anyway, always look at the resistor specification to see if it can stand the peak of the mains voltage.

[Andy Wade]

You'd need to go out of your way to buy carbon composition resistors nowadays - they're a specialist item. For a readily available low-cost, half-watt, mains-voltage rated component I'd choose type VR37: