# Use of 120 VAC neon mini lamp on 240 VAC circuit

Discussion in 'Electronic Basics' started by Serious Machining, Sep 20, 2005.

1. ### Serious MachiningGuest

I would like to use a Radio Shack # 272-704 120 volt neon lamp as an
ON/OFF indicator on a 240 volt circuit (single phase, no neutral).
Based on the Ohm's Law calc's I've done, it appears I need to add 889K
ohms resist in the circuit for this 0.032 watt (0.00027 amp) lamp.
I'm considering using two Radio Shack # 271-1133 470K 1/2 watt
resistors in series, connecting one to each lead of the lamp assembly.
The 940K resistance (893K @ 5% tolerance) is more than the 889K per the
calculation, but this 120v lamp only needs a min of 70v to fire.
Is there any thing I'm missing in my thought process that someone in
this group may recognize as a "newbie's bonehead blunder" ?
Thanks for any forthcoming assurance or constructive advice !
- Dennis Anderson

2. ### petrus bitbyterGuest

Dennis,

(Almost) all neon indicator lamps fire at around 90V, light at around 70V
and extinguish at around 60V. I guess the "firevoltage" to be the normal
operating voltage. So the 120V lamp should already have a current limiting
resistor inside. To account for the extra 120V at 270uA you need a
444k/0.032W resistor. So one 470k/0.25W resistor will do fine. If the light
is too dim you can try a 390k resistor as well.

petrus bitbyter

3. ### petrus bitbyterGuest

Dennis,

(Almost) all neon indicator lamps fire at around 90V, light at around
70V
and extinguish at around 60V. I guess the "firevoltage" to be the
normal
operating voltage. So the 120V lamp should already have a current
limiting
resistor inside. To account for the extra 120V at 270uA you need a
444k/0.032W resistor. So one 470k/0.25W resistor will do fine. If the
light
is too dim you can try a 390k resistor as well.

petrus bitbyter

4. ### Serious MachiningGuest

Pieter,
Thank you for your prompt and thoughtful response,
Dennis A.

5. ### ChrisGuest

The Radio Shack 272-704 neon lamp has a clear plastic case (with the
thread built-in -- don't overtighten!). If you look at the side of the
lamp, you'll see the resistor. It's about 47K, I believe.

Here's how it works. The Ne bulb has a 90V trigger voltage, and after
firing, it stabilizes at about 70V. For 120V line voltage, the lamp
will see a peak of 170V. While the lamp is on, only 70V is across the
lamp, which means that a peak of 100V will be across the resistor.
That will mean you'll have about 2mA peak going through the resistor
(which is a good, safe value).

With 240VAC, there's a peak voltage of about 340V. 70V of that will be
across the Ne bulb, so you have to choose a total R so the peak current
will be around 2mA. Ohms Law comes in handy here:

270V / 2mA = 135,000 ohms

Since you've already got a 47K internal to the lamp, try an 82K to get
close to the above. It's possible 390K additional resistance at 240VAC
might not look as bright as the lamp with 120VAC.

Good luck
Chris

6. ### Ralph MoweryGuest

The neon bulbs are not very current (voltage) sensitive. As long as the
resistance is low enough for them to light and high enough to keep the
current under the maximum you are ok. Anywhere inbetween it is just a
manner of how much light you want and how long you want them to last. The
pocket voltage testers go anywhere from about 90 volts to 600 volts with the
same resistor.
They are just brighter with the higher voltage . As they are only used for
a short period of time the life of them is not usually a problem.

7. ### petrus bitbyterGuest

Chris,

I don't have the lamp, only the figures provided by Radio Shack.

|(272-0704) Specifications Faxback Doc. # 34728
|
|Wattage Consumption:..............................................32.4 mW
|Insulation Resistance:................500 Volts, DC, 100 M-ohm., Minimum
|Dielectric Strength:..........................1500 Volts, AC, one minute
|Life:...................................................Over 25,000 hours
|Unit:.......................................................120 Volts, AC
|Fire Voltage:................................................70 Volts, AC
|Fire Current:.....................................270 micro Amps, minimum
|Hole, Drill:....................................................7/16 inch
|Lugs:..........................................................Solderable
|Housing:........................................................Black ABS
|Body:..................................................................PC
|
|Specifications are typical; individual units might vary. Specifications
|are subject to change without notice.

They state clearly 120V/34.2mW and that's where my calculations were based
upon. Nevertheless you may be right. Can't trust those specifications, can
you? After all they say that they may "change without notice". From my own
experience I know these neon indicators come in current ranges from 0.22mA
to over 1mA. The low current types are advised to be used with a 220k series
resistor at 120Vac and with 750k at 240Vac. Accordingly the high current
types are advised to be used with 56k and 180k. So I stick at my advise to
try the 470k first and lower it when the light is too dim. If you can really
see a 47k or 56k resistor inside, you can go down to 150k or even 120k.

petrus bitbyter

8. ### Serious MachiningGuest

I picked up a pack of the Radio Shack 470K ohm, 1/2 watt resistors
(271-1133) they all checked out at 464K +/- 1K ohms, a deviance of only
1.3% for a tolerance of 5%.
I pulled the red lens body off the 272-704 neon lamp and saw the
built-in resistor. The four colored bands appear to be: orange,
orange, orange, gold - which should yield 33,000 Ohms @ 5% tolerance.
I checked the brightness of the neon bulb in four increments:
1) As is, no addt'l resistor, @ 120 volts: Medium brightness,
detectable in lit room.
2) As is, no addt'l resistor, @ 240 volts: Very bright, calls attention
to itself.
3) One 470K resistor added @ 240 volts: Less bright than #1, s/b usable
in dim light.
4) One 470K resistor added @ 120 volts: Lit fine - just very dim,
barely visible.
I think I'll wire it up as per #3 since I expect it will be "ON"
perhaps as much as 5 hours per evening, 6 months of the year. Despite
this amounting to around 900 hours per year, the bulb should last 28
years - probably more since it won't be as bright as the "rated" output
- although the frequency of its ON/OFF cycles might offset this.
I'll probably wire up the second neon lamp as per #4 to serve as a
"light switch locator" on a 120 volt circuit. It will always be "ON",
should still last a very long time, and should be easily enough to
detect at close range in the dark of night.
Thanks also to Chris and Ralph for their follow-up postings - Chris for
reminding me to consider the actual 'peak voltage' associated with the
'averaged' 120 and 240 volt AC mains. The prediction that test #3
above would not be as bright as test #1 was right-on. Ralph's comments
gave me the courage to conduct test #2 - very useful for the
semi-quantifiable comparison of the four tests.
- Dennis A.

9. ### petrus bitbyterGuest

Dennis,

Apparently Radio Shack *did* change the specifications without notice. This
indicator will consume over 200mW instead of 32mW. To achive the same
brightness you found at 120V you may will have to add a 82k or even a 68k
resistor while using 220V. But if you're content with the current
brightness, keep it as it is now.

petrus bitbyter

10. ### Serious MachiningGuest

Pieter,

nice of you !
I will experiment with some of the other 10K & 100K Ohm series of
resistors
offered by Radio Shack. It would be interesting to actually be able to
meter
the light output intensity, but I suppose one can settle for making an
estimate
of the brightness by means of mathematical formulas. Even that would
be fun.
Take care and have a good day.

Dennis A.

11. ### Serious MachiningGuest

Pieter,

nice of you !
I will experiment with some of the other 10K & 100K Ohm series of
resistors
offered by Radio Shack. It would be interesting to actually be able to
meter
the light output intensity, but I suppose one can settle for making an
estimate
of the brightness by means of mathematical formulas. Even that would
be fun.
Take care and have a good day.

Dennis A.

12. ### ehsjrGuest

Now that you know what's in there (the resistor value),
you can easily determine the total resistance needed: double
the existing resistance. The resistor acts as a current
limiter. You compute it based on imagining the neon as shorted.
The ~70 volts across the neon is not needed in the computation,
because of your research (disassembling the indicator). So 33000
ohms at 120 needs to be changed to at least 66000 ohms at 240.
I'd go a bit higher and put a 47K or 68K in series to give you
80K - 101K total.

Ed

13. ### Serious MachiningGuest

Ed -

Thank you for your detailed explanation on how to determine which
resistor to add to the circuit - yes, I was wondering what sort of
consideration in the calculation was to be given to the 70 volts.

when deciphering the color coded banding, but I'll save that for
a later date. I don't want to get booted out of this group after
being a member for only 3 days ! Peace be to everyone.

- Dennis A.

14. ### Rich GriseGuest

Why not? Isn't the neon dropping ~70V while it's illuminated? I'd
subtract that from 120, giving 50V across the 33K, for ~1.5 mA.
To get 1.5 mA through 240 - 70 = 170V, you'd need 113.333K total
resistance. 120K is the closest standard value, or, to put in
series with the 33K leaves 80K, which 82K is the closest standard
value.

Cheers!
Rich