Pilot light

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I have taken some pilot lights out of a broken stove. These lamps indicated whether the element was on or the element was hot. I think they are what you call Neon lamps eg. Small glass bulb with thick Metal bars in the bulb.

I want to use one of them as a pilot light for a 120vac light telling me that the switch is on . (120vac.)

The light from the stove is in a nice little plastic package, that when I took a part it looks like this. Very simple.

          |             (0)Lamp

It is NOT an LED and says on it 250v. Clearly it is meant for 120-250 volts.

To test it I just connected 120 volts to the two terminals. However it did not light.

Is the above this some sort of shunt? Do I need a load to make it work.  


  MyLamp   Load

thank you


Re: Pilot light
snipped-for-privacy@gmail.com says...
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It is difficult to tell by your drawing how it actually is.
It looks like you have two resistors in series with the neon bulb across  
one of them.  

Most neon bulbs need about 90 volts to fire off (light) then around 70  
volts to keep them lit.  A series resistor is needed to keep the current  
to a save level.  

If you have the two resistors in series and the bulb across one of them  
you only have about 1/2 of the 120 volts or 60 volts which is not enough  
to fire off the bulb.  Even allowing for the 1.414 peak voltage of a  
sine wave over the rms value you just barely do not have enough voltage  
to fire off the bulb.

Try unhooking the resistor in parallel with the bulb, but leave the  
resistor that is in series with the bulb in place to limit the current  
when you hook it to 120 volts.

Re: Pilot light
You are right.  The first resistor is in series and the other I would call  
in parallel I think? Second resistor runs from hot to neutral "bridging" th
em.  Sadly my diagram when saved on web got screwed. If you copy and paste  
into notepad it may show up more clearly.  However your conclusion may be r
ight. IF this light used to have 240 running an element in a stove through  
it then maybe removing one resistor will give it enough to start the lamp.

I will try and get rid of second Resistor and get back with results.

Re: Pilot light
On Tue, 7 Jan 2020 10:45:16 -0800 (PST), Steve Wolf

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One hundred K resistor in series with one leg of the lamp is about
right for 120VAC.


Re: Pilot light
snipped-for-privacy@gmail.com says...
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have 240 running an element in a stove through it then maybe removing one resistor will give it enough to start the lamp.
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If you remove one resistor, make sure the wiring is from hot wire,  
resistor, bulb, neutral.  That puts it in series.  

If you remove the resistor that is across the bulb , the bulb will  
probably explode in one bright flash as there is nothing to limit the  
courrent to a safe value.

Re: Pilot light
Yes I understand.

Re: Pilot light
On 1/7/2020 4:59 PM, Ralph Mowery wrote:
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Wrong - backwards.  Not what you said above.  Do remove the one across  
(parallel to) the bulb.  Do not remove the one in series with the bulb.

Re: Pilot light
I have removed the resistor in parallel so it is now simply a lamp with one resistor in series. Works fine. I might try a 100k R as one person suggested to get a little brighter light. It's fine but flickers a little.

Thanks for all your help.

Re: Pilot light
Quoted text here. Click to load it

Glad you caught that.  It should have been do not remove the series  
resistor or the bulb will probably explode.  

The bulb probably takes about 90 volts to fire off.  When used on the  
240 volt stove the bulb is like an open circuit to start with.  So the  
resistors form a simple voltage devider and puts 120 volts on the bulb.  
It fires off and then drops to around 70 volts.  The series resistor  
limits the current to a save value for the bulb and the parallel  
resistor takes a small ammount of current, but can probably be ignored.

If hooked across the 120 volt circuit you only have 60 volts across the  
bulb.  Not enough to fire it off .  If the parallel resistor is removed,  
then there  is not a voltage devider, so the bulb gets the full 120  
volts for a fraction of a second then the bulb fires and has 70 volts  
across it and the resistor had the 120-70 or 50 volts.  

Re: Pilot light
Thanks for the physics. Always amazes me how complex a simple 2 or 3 component circuit is.

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