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From: John Fields on 2 Mar 2010 12:23
On Mon, 1 Mar 2010 13:53:13 -0800 (PST), George Herold
<ggherold(a)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:

d7iqo5lphgp0k15o8fh2ccal5eoi0c29h0(a)4ax.com

JF
From: George Herold on 2 Mar 2010 23:04
On Mar 2, 6:28 pm, d...(a)manx.misty.com (Don Klipstein) wrote:
> In <36079a9a-c27f-4b4b-859f-c5ea6260d...(a)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...(a)misty.com)

Thanks Don, This certianly wasn't a carbon composition. (Though I
was digging through a pile of 1-2 Watt 100-300k ohm carbon C's at work
today.) It doesn't seem like putting 100k in there can hurt anything,
though I don't expect it to work either... I figure something else
might have happened also.

George H.
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