high wattage resistor...what is its value?

[spitso]

G'day all

Trying to fix my benchtop power supply, i came across these two high wattage resitors which look ok, but when using my multimeter they are like 2ohms.

I am unsure what their values actually are (i beilieve they should be 80 ohm? 5w), below are 2 photos of the writting on the resistor.

cheers

 

[dknguyen]

To dissipate 5W at 80 ohms, it would take 20V@250mA.
To dissipate 5W at 2 ohms, it would take 3V@1.6A.

Depending on what your power supply's power, output voltage, and output current and where the resistor is in the supply which might help you determine what it's supposed to do (for example, how far away it is from the AC input and the DC output), that might help you make an educated guess on what the resistor value is supposed to be.

But yeah I agree with you that the writing seems to say it's 80 ohms. On the other hand, it's rather strange for BOTH resistors to breakdown and read the same value. It's also strange that it reads two ohms intead of something much lower (due melted wiring which has shorted) or much higher (due to melted wire causing an open). In any case, if either of those happened you'd probably be able to see it and it would indicate something else failed causing the resistors to fail since resistors don't have much that can go wrong with them in the first place.

I'm rather inclined to think the resistors are supposed to be 2 ohms based on that reasoning, but I don't know anything else about your power supply so I can't make a guess based on the things I described above.

But I'm gonna guess that being power resistors, you found them near the output of supply. And 3V@1.6A seems more like something that would appear on the output of a supply than 20V@250mA. But I'm totally pulling that outta my ass.

 

[colin55]

They are zero point eight ohms

 

[dknguyen]

The are zero point eight ohms

Oh the J in the resistor code?
Electronic color code - Wikipedia, the free encyclopedia

I thought it was an tolerance code, but I never read resistor colour codes. heh.

 

[spitso]

wow, never realized resistors are similar to capacitors in that letter value sense.

thanks guys

 

[Nigel Goodwin]

As colin55 said, they are clearly labelled as 0.8 ohms.

It would be EXCEEDINGLY rare for a resistor to go low in value, they go either O/C or high - the very rare exceptions were certain carbon composition types decades ago, where occasional ones overheated and reduced in value.

 

[colin55]

You have read the value of the resistor as 2 ohms with a multimeter because your meter is inaccurate on low values of resistance and the meter is measuring the resistance of the leads. The "J" on the resistor is the tolerance value and/or its temperature stability rating.

 

[winni142]

You have read the value of the resistor as 2 ohms with a multimeter because your meter is inaccurate on low values of resistance and the meter is measuring the resistance of the leads. The "J" on the resistor is the tolerance value and/or its temperature stability rating.

same here.

 

[crutschow]

You need to compensate for the zero resistance reading on your ohmmeter before measuring low resistances. Simply touch the two probes together and note the resistance reading. Then subtract that from the resistor measurement to get the resistor's value.

For values less than an ohm your accuracy would still not be better that ±0.1 ohm. Ohmmeters designed to measure low resistance accurately use a 4-wire (kelvin) measurement. Two wires apply a current to the resistance, and the other two measure the voltage drop.