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?)

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Thanks for any advice.

George H.

Reply to
George Herold
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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.

Reply to
jfeng

100 ohms is much too small. It's more likely to be 100 k ohms or even higher.
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Reply to
Gareth

Assuming the neon bulb is still okay, why not measure the break down? I'd start with perhaps 100k with your 240V. If the breakdown is around 100V, the current will be in the area of 1mA, which is probably safe enough. And 100mW, or so, so you won't burn something up.

At say 2 watts, you would be talking in the area of 10k, my guess. Not 100 ohms.

Jon

Reply to
Jon Kirwan

It is, if you want the neon lamp to light up whitish purple for less than an second, then explode.

You need something more in the range of 150K to 200K, possibly even larger.

Reply to
William Sommerwerck

on. =A0Is 100 ohms a good value as a current limiting resistor

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.

Reply to
George Herold

An NE2 needs a 22k dropper for 120 volts, do the math.

Reply to
Meat Plow

Yikes! You should be looking at more like 1mA through the lamp. Are you sure about the connections in the circuit?

Reply to
D Yuniskis

Thanks, I'm beginning to think I should just leave it alone and wait for the replacement PCB. Now sense 'frying' someting else.

George H.

Reply to
George Herold

Thanks Jon. This value seemed way to small to me too.

George H.

Reply to
George Herold

Well I can see that the big (3 to 5 watt) resistor feds the neon lamp. I don't know what else the circuit does.... I didn't bother to try and trace the whole thing out. Perhaps I can look at it more closely tonight and see if I can make something more out of it.

Thanks,

George H.

Reply to
George Herold

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Thanks I'll look up an NE2 and see what I can make out of it.

George H.

Reply to
George Herold

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 ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

100 ohms in series with a neon lamp at 240 volts AC: Not that I would try this for the answer, but I suspect both the neon lamp and the resistor will likely be severely damaged. Either or both may explode, and the neon lamp may implode.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

100K would me more like it.. "100,000"
Reply to
Jamie

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

Reply to
John Larkin

ion.

70

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.

Reply to
George Herold

Well somehow this must also feed power to some other part of the circuit. There's also an LM324 and small triac Q601e3 on the pcb.

George H.

Reply to
George Herold

That sounds just a little small for the A1B (mini NE-2) and the A1C (mini NE-2H), so I suspect your lamp is one of those. Given the size of the resistor, I think it's the latter.

If your neon lamp kicked the bucket along with the resistor (more likely it survived, at least well enough to glow from 240 volts AC), Radio Shack may still carry these (a litle incorrectly calling them NE-2H).

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

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No.

For a small neon lamp like an NE-2, (GE 3AD) with a recommended current
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Reply to
John Fields

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