On Mon, 1 Mar 2010 13:53:13 -0800 (PST), George Herold
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 300µA:
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
:
E² 240²V
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:
E² 200²V
R = --- = ------- = 13,333 ohms ~ 13k
P 3W
and if there was some circuitry downstream from the lamp, which shorted,
then the resistor would dissipate:
E² 240²V
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:
JF