Capacitor needed to stop neon triggering?

Thread Starter

Paulypots

Joined May 17, 2019
15
Central heating boiler ("furnace" to any Americans reading!) is located in the garage and operated by quite sophisticated controls in the house (system designed by self). These involve a long run of 12 wires (three 4-core cables) laid in an underground pipe between garage and boiler.

Two neon indicator lights on the control panel have a habit of glowing dimly when current is flowing through adjacent wires - obviously a phantom voltage caused by capacitance. They even light up when completely disconnected at the boiler end! We are on 240V AC.

Theory tells me a capacitor across the neon would soak up any stray voltage but I don't know what type or value to use. Afraid of getting it wrong with mains voltages involved.

Is this a solution and, if so, what components would you suggest?

Thanks. Paul

*In case you're wondering why it is so complicated, there are four heating zones, one a room in the garage itself that can be activated remotely, and a frost protection system with feedback.
 

Thread Starter

Paulypots

Joined May 17, 2019
15
Sounds like the installer used the wrong type of cable. The cables should have been a shielded type. Or are they the correct type and the shields not grounded?

https://en.wikipedia.org/wiki/Shielded_cable
The installer was me! Cable is three lengths of unshielded four core. It never occurred to me to use shielded cable for a mains installation because the symptom would not have been noticeable but for the neons.
 

AlbertHall

Joined Jun 4, 2014
9,282
It takes very little current to make a neon glow. Do the neons include their own resistor or is it external to the neon?
I would suggest a resistor rather than capacitor but if the resistor is internal to the neon then the resistor would need to be able to cope with 240V and most don't so you would need two resistors in series to cope with the voltage - perhaps 47k. If the resistor is external you can connect the resistor directly across the neon and this means much less voltage (~70V) so a single 47k would do the job.
 

Thread Starter

Paulypots

Joined May 17, 2019
15
It takes very little current to make a neon glow. Do the neons include their own resistor or is it external to the neon?
I would suggest a resistor rather than capacitor but if the resistor is internal to the neon then the resistor would need to be able to cope with 240V and most don't so you would need two resistors in series to cope with the voltage - perhaps 47k. If the resistor is external you can connect the resistor directly across the neon and this means much less voltage (~70V) so a single 47k would do the job.
Albert - The neons have a built-in resistor so we're talking about a full-on 240V.

Why do you think a capacitor wouldn't work?
 

Thread Starter

Paulypots

Joined May 17, 2019
15
It probably would but you would also need to consider that it would need to withstand 240V AC and be suitable for permanent connection across the mains. A couple of resistors would be cheaper.
You think we're safe with those using ordinary small resistors?
 

LesJones

Joined Jan 8, 2017
2,497
I have seen this sort of problem with LED lighting (Mains voltage.) caused by capacitance between the live feed to the switch and the wire from the switch to the LED. I have used contact suppressors such as these a they capacitors (Wit a small series resistor.) rated for use across the mains. Some are packaged with flying leads so they are easy to add to the input connections of the LED lights.

Les.
 

Tonyr1084

Joined Sep 24, 2015
4,142
Why do you need neon lamps? I don't think a regular incandescent bulb would glow without sufficient current. Whatever cross-talk (inductance) you're getting probably isn't enough to light a regular lamp.
 

Thread Starter

Paulypots

Joined May 17, 2019
15
I have seen this sort of problem with LED lighting (Mains voltage.) caused by capacitance between the live feed to the switch and the wire from the switch to the LED. I have used contact suppressors such as these a they capacitors (Wit a small series resistor.) rated for use across the mains. Some are packaged with flying leads so they are easy to add to the input connections of the LED lights.

Les.
Interesting, Les. Thanks.
 

Thread Starter

Paulypots

Joined May 17, 2019
15
Why do you need neon lamps? I don't think a regular incandescent bulb would glow without sufficient current. Whatever cross-talk (inductance) you're getting probably isn't enough to light a regular lamp.
Neons are so easy - no need to drop voltage, last almost for ever. Anyway it's done now!
 

Thread Starter

Paulypots

Joined May 17, 2019
15
Yes
I missed that the resistor is built-in. :oops:
I'm wondering if the coolest (literally) solution would be to have Albert's two resistors in series but tap the neon from between the two. That way the neon would be closer to its trigger voltage (normally about 90V). Since power dissipated is the square of the voltage, the resistors would be doing very little work.
 
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