I have a carbon film resistor that got so hot that it melted itself
free of the circuit board. Can the resistance (of the resistor) be
permanently increased when this happens or does a resistor just become
"open" when that happens? The reason I ask is because I have two
resistors in parallel that go:

Black | Brown | Black | Gold

I think the resistance should measure ~1/2 Ohm, but my ohm meter reads
33 Ohms. Does this mean they are damage or did I do something wrong?

Thanks,
Scott, KC2PIH

wrote:

I have a carbon film resistor that got so hot that it melted itself
free of the circuit board. Can the resistance (of the resistor) be
permanently increased when this happens or does a resistor just become
"open" when that happens? The reason I ask is because I have two
resistors in parallel that go:

Black | Brown | Black | Gold

I think the resistance should measure ~1/2 Ohm, but my ohm meter reads
33 Ohms. Does this mean they are damage or did I do something wrong?

Thanks,
Scott, KC2PIH

Usually when I do that the color code becomes Black | Black | Black | Black.

Yes, if the resistance element burns off a bit without going completely
away the resistance can rise. In your case I suspect that you're
measuring other resistances in the circuit. Even if you're measuring
the actual resistance, 66 times the design value may as well be open.

You need to replace the resistors, and before you do that you need to
find and clear whatever fault it was that made them char.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

Actually, I did a little more reading and the resistor is about .39"
with a blue case. I think its actually a 1/2w metal-film, flameproof
resistor (not a carbon film). Learning is fun!

The resistors being measured are unhooked completely from the circuit
- so I know its not something else. Finally - I know what smoked these
resistors already

-Scott, KC2PIH.

If black, brown, black, gold was the original color code--well, it's a
pretty strange code for a resistor, starting with black. But
sub-10-ohm resistors should have a gold or silver third band, not
black, so maybe it didn't change all that much.

My suspicion is that it _might_ be a flameproof resistor as you say,
but it's quite likely that it's a metal-oxide resistor. The flameproof
ones tend to act like fuses (to keep not only themselves but things
around them from flaming!), but the metal-oxides have a long reputation
of being able to not only melt the solder but burn a hole clear through
the board before they give up as resistors. Most of the ones we use
have a pale blue case that gets somewhat paler if they've run very hot
for a while. I've done some experiments with them, dissipating up to
ten watts in a two watt resistor for long periods, and noting that yes,
they do change value some, but nothing like a metal or carbon film
would. They will survive a long time (hours at least) at five watts
with only a few percent value change. Yours could indeed have been
orange-orange-black-gold before they got so hot that the paint changed
color. I'm curious if you found a schematic or something else to tell
you what the design value really was.

Cheers,
Tom

I can't get an original schematic. Yeah I guess the colors markings are
ruined. There are bunches of this type of resistor in this circuit.
They are all baby-blue, with very bright markings. This resistor was
HOT. The blue is almost greyish now. Maybe I'll unsolder the two
resistors and see if the values match exactly. If so, I guess that
*might* be an indication that they are still good.

Scott, KC2PIH

K7ITM wrote:
If black, brown, black, gold was the original color code--well, it's a
pretty strange code for a resistor, starting with black. But
sub-10-ohm resistors should have a gold or silver third band, not
black, so maybe it didn't change all that much.

My suspicion is that it _might_ be a flameproof resistor as you say,
but it's quite likely that it's a metal-oxide resistor. The flameproof
ones tend to act like fuses (to keep not only themselves but things
around them from flaming!), but the metal-oxides have a long reputation
of being able to not only melt the solder but burn a hole clear through
the board before they give up as resistors. Most of the ones we use
have a pale blue case that gets somewhat paler if they've run very hot
for a while. I've done some experiments with them, dissipating up to
ten watts in a two watt resistor for long periods, and noting that yes,
they do change value some, but nothing like a metal or carbon film
would. They will survive a long time (hours at least) at five watts
with only a few percent value change. Yours could indeed have been
orange-orange-black-gold before they got so hot that the paint changed
color. I'm curious if you found a schematic or something else to tell
you what the design value really was.

Cheers,
Tom

Yeah, the "bunches of" is another indication that they are metal-oxide
parts. It would be pretty unusual to have bunches of flameproof parts
in a circuit, I think. The metal-oxides are good parts, but you do
have to be aware that they can survive being really hot better than a
lot of things around them can.

Cheers,
Tom

wrote in message
oups.com...
I have a carbon film resistor that got so hot that it melted itself
free of the circuit board. Can the resistance (of the resistor) be
permanently increased when this happens or does a resistor just become
"open" when that happens? The reason I ask is because I have two
resistors in parallel that go:

Black | Brown | Black | Gold

I think the resistance should measure ~1/2 Ohm, but my ohm meter reads
33 Ohms. Does this mean they are damage or did I do something wrong?

Thanks,
Scott, KC2PIH


That value is ONE OHM +/- 5%
http://www.dannyg.com/examples/res2/resistor.htm

g. beat

Resistors can certainly change value when heated. As for your color
code... that resistor must have gotten very hot indeed to have the
first band as black. That color doesn't make sense in that position...

www.telstar-electronics.com