Resistor for neon indicator lamp

 

I've got a tankless electric water heater from eemax. that stopped
working the other day.  eemax won't provide any component level
support, but I've ordered a new board for $45.00.

When I opened up the unit, after switching off the circuit breaker on
the 240V AC line, I observed that a big (maybe 3-5 watt) (metal film?)
resistor was discolored and was an open circuit.  The markings look
like 100 ohms, but because of the discoloration it's hard to be sure.
I say metal film because the resistor is pale blue in color.  The
resistor feds a neon indicator bulb... (And probablly more of the
circuitry.)

My question.  Is 100 ohms a good value as a current limiting resistor
for a small neon lamp running off 240 V AC?  (60 Hz if that matters.)
I'm not sure what the I-V curve for the lamp will look like.  (Is the
one shown here OK?)

http://en.wikipedia.org/wiki/Neon_lamp

Thanks for any advice.

George H.

 


Does yours look like the one in the wiki article?  If so, I would vote
for a value between 100K and 470K.  At 100K, a half-watt resistor
would be marginal, and I would go for one rated for 1 watt.  Within
this range, the value is probably not too critical.

 


Yes the lamp looks like the picture in the wiki article.   If I
believe the curve there, then I've got something like 160-200 volts
across the lamp at 20mA.  So I've got (call it) 80 V to drop on the
resistor at 20mA.. I get 3 k ohm at a watt or two.  Hmm maybe what
looks like brown black brown is a discolored red black red.  (I always
have hated the way red and brown look almost the same on the pale blue
body of the metal film resistors.)

100k seems like it won't allow enough current through to keep the lmap
lite.  (But I've never worked with neon lamps.)

George H.

 

George Herold wrote:

  I don't believe the voltages in that curve in Wiki - I would plan on 70
volts and peak current around 3 milliamps (peak voltage of 240 VAC is
about 340 volts).

  If the glass bulb's overall length is 3/4 inch, then the neon lamp is
probably an NE-2H. I would use an 82K or 100K 2-watt resistor for an
NE-2H at 240 volts AC. I would err on the high side (100K) to maximize
life of the lamp.  I suspect the toated resistor is a 100K 2 watt whose
yellow band was browned by being toasted.

 - Don Klipstein (don@misty.com)

 

On Mar 1, 5:48 pm, d...@manx.misty.com (Don Klipstein) wrote:

Excellent, Thanks Don.  I like the faded yellow stripe idea.  The bulb
is about 1/2 that size.  It's standing straight up and the tip of the
glass is 0.4" (10mm) above the circuit board.

George H.