Discussion in 'Datasheets, Manuals and Component Identification' started by mzs, Jan 31, 2014.

1. ### mzs

1
0
Jan 31, 2014

Color code: orange, blue, silver (white?), gold

I have two of them in a water damaged "High voltage killer" (to avoid electric problems in cars during welding), connected to the suppressor diodes.

One is totally broken, and on the other I can measure about 3.2 Ohms resistance (desoldered).
Based on the color it should be 0.36 Ohms. I am really confused.

Any help would be much appreciated.

Thanks,
mzs

2. ### jcurrie

128
1
Feb 22, 2011
just guessing it might be a cap ? but if it is a resistor your correct.
jc

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,362
2,757
Jan 21, 2010
My guess is that it's a MELF or Mini-MELF resistor. I guess an inductor is a possibility, but not a capacitor.

Failure might cause the resistance to go high, and if it's in a current sense application, this would not precipitate further failure of the component.

Do you have images of the broken one and the one you have removed? Does the one you have removed look discoloured at all?

Can we see more of the board so we can try to determine what this resistor is used for. If it's a current sense and we can get sensible voltages (say 0.1V or 0.6V) for the expected current, that may allow us to determine whether 0.36 ohms is appropriate.

Does the device malfunction? (presumably it does) How?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,362
2,757
Jan 21, 2010
Oh, as an inductor it would be 0.36uH 5% (360nH). That's an odd value as well as being fairly large for this format (see here).

But, there are inductors with a similar DC resistance in this value in a small package (see here) so it's not entirely out of the question.

5. ### jcurrie

128
1
Feb 22, 2011
your right Steve don't know what i was thinking a cap would have 5 or 6 bands though i have never seen a tublar cap in surface mount.
jc

6. ### Jimmybravo

18
0
Jul 1, 2011
0.36 ohms, its a fusible resistor bro, it acts like a fuse.. You can change it to atleast 1ohms or lower values will be fine,thanks

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,362
2,757
Jan 21, 2010

1) The device measures 3.2 ohms when removed. If it's a fuseable resistor I would expect it to be open rather than 10 x the initial resistance. If 0.36 ohms represented a small load compared to the normal load (let's say 800mA to keep the dissipation under 1/4W) then if it rose to 3.2 ohms, the dissipation would likely skyrocket leading to rapid failure.

Let's assume 12V supply, and 800mA normal load. the load is around 14.6 ohms. Lets assume the resistor rises in value to 3.2 ohms, the current will fall to 670mA, but the power dissipation in this resistor will rise to 1.4W.

One possible exception is if this is a current sense resistor used to limit current, the voltage sensed would have to be less than about 0.3V (so not turning on a transistor). In this case the increasing resistance would result in lower power dissipation. I would not use a fuseable resistor in this application.

2) Replacing it with a fuseable 1 ohm resistor would lead to premature failure as the current required to fuse it would be about 40% lower

3) Partial resistor failures leading to large changes in resistance are normally associated with long periods at high temperature, not a single short event. I would expect to see some discoloration of the component (or even the board) if this were the case. This has not been described.

I did consider this may have been a 0.36 ohm resistor but I rejected that hypothesis because of the reasoning above.

I would be happy to hear an alternate reasoning because I think that an inductor in this package is pretty unusual.

8. ### Terabyte2007

2
0
Nov 16, 2013
It looks like some of the inductors I have. Do you have an LCR meter to test?