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.

In <ef993f27-eac5-404c-a3f8-2d1224188...@j27g2000yqn.googlegroups.com>,





George Herold wrote:
On Mar 1, 5:04 pm, "jf...@my-deja.com" <jf...@my-deja.com> wrote:
On Mar 1, 1:53 pm, George Herold <ggher...@gmail.com> wrote:> 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?)

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.)

  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 (d...@misty.com)- Hide quoted text -

- Show quoted text -

On Mon, 1 Mar 2010 13:53:13 -0800 (PST), George Herold





ggher...@gmail.com> wrote:
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.

Numbers more like 330K to 560K are more reasonable. Higher currents
will sputter the gas and make the thing fail sooner.

Better yet, use an LED.

John- Hide quoted text -

- Show quoted text -

On Mar 1, 5:48 pm, d...@manx.misty.com (Don Klipstein) wrote:
In <ef993f27-eac5-404c-a3f8-2d1224188...@j27g2000yqn.googlegroups.com>,
George Herold wrote:

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.)

  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.

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.

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?

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

On Mon, 1 Mar 2010 13:53:13 -0800 (PST), George Herold

ggher...@gmail.com> wrote:
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?  

---
No.

For a small neon lamp like an NE-2, (GE 3AD) with a recommended current
of 0.3mA and a nominal holding voltage of about 60V, peak, that's about
42VRMS, so the resistor would have to drop about 200V at 300 A:

         Vs - Vne      200V
    R = ---------- ~ ------- ~ 670k oms
           Ine        3e-4A

and it would dissipate:

    P = IE = 3e-4A * 200V ~ 0.06 watts

If that resistor were  100 ohms, then with 200V dropped across it
there'd be 200 amperes through it _and_ the neon lamp, which they'd both
hate for a little while.

OTOH, if the resistor is 100k it'll drop about 200V with a neon lamp in
series, which will let about 2mA through when the lamp fires.  

That'll cause the resistor to dissipate about 0.4 watts, which doesn't
explain why it got toasted, since even into a short it would only
dissipate about:

         E    240 V
    P = --- = ------- = 0.576 watts  
         R     1e5R

Just for grins, let's say it's a 3 watt resistor running full bore in
series with a load (like a high current neon lamp) drawing 5mA.

Then the value of the resistor would be:

         E    200 V
    R = --- = ------- = 13,333 ohms ~ 13k
         P      3W

and if there was some circuitry downstream from the lamp, which shorted,
then the resistor would dissipate:

         E    240 V
    P = --- = ------- = 4.43 watts
         R     13e3R

Not huge but, anyway, without a schematic it's all just conjecture.
---

(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

---
Only in a very general way; here's a much better one for your
application:

d7iqo5lphgp0k15o8fh2ccal5eoi0c2...@4ax.com

JF

On Mar 2, 12:23 pm, John Fields <jfie...@austininstruments.com> wrote:
On Mon, 1 Mar 2010 13:53:13 -0800 (PST), George Herold

ggher...@gmail.com> wrote:
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?  

---
No.

For a small neon lamp like an NE-2, (GE 3AD) with a recommended current
of 0.3mA and a nominal holding voltage of about 60V, peak, that's about
42VRMS, so the resistor would have to drop about 200V at 300 A:

         Vs - Vne      200V
    R = ---------- ~ ------- ~ 670k oms
           Ine        3e-4A

and it would dissipate:

    P = IE = 3e-4A * 200V ~ 0.06 watts

If that resistor were  100 ohms, then with 200V dropped across it
there'd be 200 amperes through it _and_ the neon lamp, which they'd both
hate for a little while.

OTOH, if the resistor is 100k it'll drop about 200V with a neon lamp in
series, which will let about 2mA through when the lamp fires.  

That'll cause the resistor to dissipate about 0.4 watts, which doesn't
explain why it got toasted, since even into a short it would only
dissipate about:

         E    240 V
    P = --- = ------- = 0.576 watts  
         R     1e5R

Just for grins, let's say it's a 3 watt resistor running full bore in
series with a load (like a high current neon lamp) drawing 5mA.

Then the value of the resistor would be:

         E    200 V
    R = --- = ------- = 13,333 ohms ~ 13k
         P      3W

and if there was some circuitry downstream from the lamp, which shorted,
then the resistor would dissipate:

         E    240 V
    P = --- = ------- = 4.43 watts
         R     13e3R

Not huge but, anyway, without a schematic it's all just conjecture.
---

(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

---
Only in a very general way; here's a much better one for your
application:

d7iqo5lphgp0k15o8fh2ccal5eoi0c2...@4ax.com

JF

Thanks John, I must admit it's hard to understand how a 3 watt 100k
ohm resistor across the 240 V lines would blow even if everything down
stream of it was shorted. (As your calculation shows.) A new circuit
board is in the mail, but this won't solve the mystery. The board has
been redesigned and is no longer analog but digital. Once I put the
new board in I'll look at the busted one and see if I can figure
anything out.

Say could you resend that link to the neon bulb I-V curve? What ever
you posted got turned into gobble-de-gook by google.

"d7iqo5lphgp0k15o8fh2ccal5eoi0c2...@4ax.com"


George H.

Thanks John, I must admit it's hard to understand how a 3 watt 100k
ohm resistor across the 240 V lines would blow even if everything down
stream of it was shorted. (As your calculation shows.) A new circuit
board is in the mail, but this won't solve the mystery. The board has
been redesigned and is no longer analog but digital. Once I put the
new board in I'll look at the busted one and see if I can figure
anything out.


Say could you resend that link to the neon bulb I-V curve? What ever
you posted got turned into gobble-de-gook by google.

Thanks John, I must admit it's hard to understand how a 3 watt 100k
ohm resistor across the 240 V lines would blow even if everything down
stream of it was shorted. (As your calculation shows.) A new circuit
board is in the mail, but this won't solve the mystery. The board has
been redesigned and is no longer analog but digital. Once I put the
new board in I'll look at the busted one and see if I can figure
anything out.

In <36079a9a-c27f-4b4b-859f-c5ea6260d...@s36g2000prf.googlegroups.com>,
George Herold wrote in part:

SNIP previously quoted material

Thanks John,  I must admit it's hard to understand how a 3 watt 100k
ohm resistor across the 240 V lines would blow even if everything down
stream of it was shorted. (As your calculation shows.)   A new circuit
board is in the mail, but this won't solve the mystery.  The board has
been redesigned and is no longer analog but digital.  Once I put the
new board in I'll look at the busted one and see if I can figure
anything out.

  I have seen resistors fail in ways that cause their resistance to
decrease until they get toasted and then go open circuit.

  My experience has been that this happens more with carbon composition
resistors.  Those tend to be cylinders with untapered ends.  Many times,
the composition is exposed.  The composition may absorb moisture and
experience a decrease in resistance, especially if it is operated only
intermittently.  This is more likely to be a problem where there is
exposure to moisture or high humidity or salty coastal air.

  And carbon's resistivity has a negative temperature coefficient.

  Preferably the resistor degrades rapidly and then goes open circuit
quickly by the time it allows anything else to get overloaded to a
damaging extent.

  A possible other explanation is that a severe line voltage surge caused
something in the resistor to carbonize.  The resistor becomes more
conductive and hotter.  The carbonization may then go into a runaway
situation, leading to sizzle-sizzle and then to *pow*, *poof* or *phut*.

 - Don Klipstein (d...@misty.com)

On Tue, 2 Mar 2010 12:34:55 -0800 (PST), George Herold

ggher...@gmail.com> wrote:
Thanks John,  I must admit it's hard to understand how a 3 watt 100k
ohm resistor across the 240 V lines would blow even if everything down
stream of it was shorted. (As your calculation shows.)   A new circuit
board is in the mail, but this won't solve the mystery.  The board has
been redesigned and is no longer analog but digital.  Once I put the
new board in I'll look at the busted one and see if I can figure
anything out.

Say could you resend that link to the neon bulb I-V curve?  What ever
you posted got turned into gobble-de-gook by google.

---
Ah, Google...

Google groups doesn't support binaries so I'll email it to you.

JF

On Mar 2, 6:53 pm, John Fields <jfie...@austininstruments.com> wrote:
On Tue, 2 Mar 2010 12:34:55 -0800 (PST), George Herold

ggher...@gmail.com> wrote:
Thanks John,  I must admit it's hard to understand how a 3 watt 100k
ohm resistor across the 240 V lines would blow even if everything down
stream of it was shorted. (As your calculation shows.)   A new circuit
board is in the mail, but this won't solve the mystery.  The board has
been redesigned and is no longer analog but digital.  Once I put the
new board in I'll look at the busted one and see if I can figure
anything out.

Say could you resend that link to the neon bulb I-V curve?  What ever
you posted got turned into gobble-de-gook by google.

---
Ah, Google...

Google groups doesn't support binaries so I'll email it to you.

JF

Wow. Thanks John

On Tue, 2 Mar 2010 20:05:50 -0800 (PST), George Herold





ggher...@gmail.com> wrote:
On Mar 2, 6:53 pm, John Fields <jfie...@austininstruments.com> wrote:
On Tue, 2 Mar 2010 12:34:55 -0800 (PST), George Herold

ggher...@gmail.com> wrote:
Thanks John, I must admit it's hard to understand how a 3 watt 100k
ohm resistor across the 240 V lines would blow even if everything down
stream of it was shorted. (As your calculation shows.) A new circuit
board is in the mail, but this won't solve the mystery. The board has
been redesigned and is no longer analog but digital. Once I put the
new board in I'll look at the busted one and see if I can figure
anything out.

Say could you resend that link to the neon bulb I-V curve? What ever
you posted got turned into gobble-de-gook by google.

---
Ah, Google...

Google groups doesn't support binaries so I'll email it to you.

JF

Wow.  Thanks John

---
You're welcome. :-)

If you're interested in neon glow lamps, GE's 1965 glow lamp manual
(pretty much the Bible) is here:

http://www.donsbulbs.com/cgi-bin/r/t.pl/library.html

JF- Hide quoted text -

- Show quoted text -