Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Simple resistor question

Status
Not open for further replies.

fastline

Member
In your experience, can an axial lead wire wound power (2-10w) resistor reduce in resistance or do they usually just open if power handling is exceeded?

Also, has anyone ever seen a resistor reduce significantly in size? I have some that should be blue and 15mm long by 5.5mm OD. They are gray, cannot even read the bands, and are 4.5mm OD. I am thinking they have run hard for so long that they have reduced in size (burned away) to a point where they no longer have enough mass to meet their rated wattage and are getting too hot as a result.

I have attached a pic. Left resistor is the really roasty one, middle is what I think it should look like, RH is in the same resistor pack but not as bad. There are 5 of these that run in parallel next to each other. Actual power through them would be 2.2watts. they all measured 67.8 ohms but I really have no what those colors should even be...
 

Attachments

  • SDC11302.JPG
    SDC11302.JPG
    1.8 MB · Views: 141
Last edited:

killivolt

Well-Known Member
In your experience, can an axial lead wire wound power (2-10w) resistor reduce in resistance or do they usually just open if power handling is exceeded?

Also, has anyone ever seen a resistor reduce significantly in size? I have some that should be blue and 15mm long by 5.5mm OD. They are gray, cannot even read the bands, and are 4.5mm OD. I am thinking they have run hard for so long that they have reduced in size (burned away) to a point where they no longer have enough mass to meet their rated wattage and are getting too hot as a result.

I have attached a pic. Left resistor is the really roasty one, middle is what I think it should look like, RH is in the same resistor pack but not as bad. There are 5 of these that run in parallel next to each other. Actual power through them would be 2.2watts. they all measured 67.8 ohms but I really have no what those colors should even be...

Blue "faded" brown and black. 61 ohms and Gold -+5%. So, + 5 would be ruff guess 66 + ohms just for the wear and tear.
 
Last edited:

JimB

Super Moderator
Most Helpful Member
I cannot ever remember seeing a resistor which reduced in value, thats not to say that it cannot happen, but it would be unusual.

Your roast toasty resistors have had a hard life but I doubt that they have "shrunk".
The resistor you are comparing them with appears to be from another manufacturer amd the physical size will vary.

Actual power through them would be 2.2watts. they all measured 67.8 ohms
Do you mean that the power in each resistor is 2.2w, or the power in all the resistors together is 2.2w ?
These look like 2 watt resistors so they are slightly over-run.
If you are replacing them, I suggest that you use a resitor with a higher power rating.

If each resistor measures 67.8ohm, then it is a fair bet that they are 68ohm resistors, give or take the tolerance.

JimB
 

fastline

Member
That is what I was looking for Jim. In talking with my brother a bit, I think we kind of had doubts about the "shrinkage" but thought I would check. Also, in my experience, it seems resistors either work or they fail open real quick like.

The power I estimated was per resistor. They are all loaded up in parallel with the output. Why so much load??? I have no idea. 12V power supply card. Basically a modular PSU. Main board rectified to 330VDC, that is then fed to multiple DC-DC supplies.

I tend to agree on power rating. IN my research, it seems these could not be much more than a 2watt resistor and running at 2.2w, that is pushing it.. I planned to up to 5w or so. Hopefully that is the only issue. I simply fired the supply and found the resistors smoking a minute later and shut it down.
 

killivolt

Well-Known Member
Blue, gray, black 68ohms? but, the middle looks like brown on the resistor on the right? Maybe you pulled one different from the others? I don't know :D

Edit: I think your right JimB. How about Faded Green, Brown, Black. at 51ohms -+5%
 
Last edited:

fastline

Member
I agree with you guys and part of the reason for my questions. The banding colors just does not jive with the measurements. I can assure that all are measuring the same "value" but there are 4 of these in a parallel circuit in one section and one in another. That single one is the RH resistor in that pic and the only one you have any hope of IDing. Hell if I know but I have the mind to simply replace based on the values I measured and ignore the colors that are obviously toasty..
 

killivolt

Well-Known Member
I agree with you guys and part of the reason for my questions. The banding colors just does not jive with the measurements. I can assure that all are measuring the same "value" but there are 4 of these in a parallel circuit in one section and one in another. That single one is the RH resistor in that pic and the only one you have any hope of IDing. Hell if I know but I have the mind to simply replace based on the values I measured and ignore the colors that are obviously toasty..

If I'm correct then you will split the difference. My observation or JimB's conclusion. I think your right on it.
 
Last edited:

cachehiker

New Member
2.2W is enough that a 2W resistor will suffer from drift and shift. OTOH, the ratings from well known manufacturers with good reputations are typically on the conservative side.

I'd fix the issue if I found it but I've seen similar resistors pushed to almost double their rating last for years and years in applications where losing their stripes and having their resistance increase by 5% or more is no big deal.
 
Status
Not open for further replies.

Latest threads

Top