Peculiar Resistor Markings - Brown Red Gold Brown - Hayelp, Hayelp

[SgtRock]

Greetings EEVBees:

--I have a metal oxide 1 Watt resistor clearly marked Brown Red Gold Brown. Duh, beats me. It measures 0.77 ± 0.03 Ohms. Has anyone ever seen this sort of thing before.

"We are all born ignorant, but one must work hard to remain stupid." Benjamin Franklin

Best Regards
Clear Ether

[sacherjj]

Brown 1, Red 2, Gold 0.1x, Brown 1%

1.2 Ohm 1% Resistor is how I decode it.

Based on: http://www.hobby-hour.com/electronics/resistor_color_code.php

[amspire]

Sounds like it could be a purple-red-gold-brown resistor - 0.72 ohms 1%

If it came out of a board, it may have cooked a bit during its life which may have changed the purple to brown which could happen if the blue dye in the purple faded.

Richard.

[sacherjj]

Sounds like it could be a purple-red-gold-brown resistor - 0.72 ohms 1%

If it came out of a board, it may have cooked a bit during its life which may have changed the purple to brown which could happen if the blue dye in the purple faded.

Richard.

That would have to be Purple 7, Red 2, Silver 0.01, Brown 1%, right?  Otherwise it would be 7.2 Ohms.

[SgtRock]

Dear Richard and Sacherjj:

--Please accept my thanks for trying to help the old lone wayfaring stranger. I have looked at the resistor very closely with a 10x loupe and both of the brown stripes appear exactly alike with regard to color and composition. The gold indeed appears to be gold and the red red. See attached picture at the end of this post. I am using a 7.2 Megapixel camera and I need to get one with greater resolution, so I can shoot a closer macro. Suggestions (cheap suggestions)?

--My reluctant and tentative conclusion is that a 1.2 Ohm Metal Oxide resistor has declined in value to approximately 0.77 Ohms. Is that bloody likely? Or it could have been marked incorrectly, or a special house marking system was used. Heck, I really do not know what to think. Duh.

--I will check some of the other resistors.

"He didn't know where he was going. But he knew where he was wasn't it."
Lord Buckley 1906 1960
 
Best Regards
Clear Ether

[inse]

If the marking does not fit the ohmic value, have you considered it being an inductor (1.2uH)?
I have a couple of those myself...

[IanB]

See attached picture at the end of this post. I am using a 7.2 Megapixel camera and I need to get one with greater resolution, so I can shoot a closer macro.

7.2 Megapixels is more than adequate for all ordinary photography. It is only the marketing people who are trying to up sell you to more pixels than you need. Your picture is perfectly clear.

Could you try taking a picture from further away so the component can be seen in context on the circuit board? It might perhaps not be a resistor as others have surmised.

[T4P]

There seems to be a missing band .

[SgtRock]

Dear IanB:

--Thanks for your interest. As you can see by the picture attached at the bottom of this post, the item in question does indeed appear to be R22. The board is an SMPS, for an Apple Airport Router. The crappy capacitors have been replaced with SMPS recommended low ESR Nichicons and Panasonic FCs.

--The SMPS works, but occasionally goes into thermal shut down. Hence, I am trying to check some of the components. The other resistors seem to be well in spec, and the Power Thermistor seems to be working normally. I have already wired in a linear wall wart to keep the router running. I am trying to fix this one more or less and a learning tool. Suggestions from those who have been there are very welcome indeed.

To Dave S.:

--Thanks for your interest. Your post came in while I was typing this one. I cannot tell if that is a faded stripe, or if it is merely an artifact caused by the knee of the dog bone curvature. Under the 10x loop it appears much as it does in the attached picture, thin and grey.

“Horse sense is the thing a horse has which keeps it from betting on people."
W. C. Fields (William Claude Dunkenfield) 1880 - 1946

Best Regards
Clear Ether

[Rerouter]

1 2 x0.1 1% tolerance,

1.2 ohms

your resistor has walked, replace it

[SgtRock]

Dear Rerouter:

--Thanks for your conclusive conclusion. I will proceed on that basis. Do you, or anyone else for that matter, happen to know if Metal Oxide Resistors often drift low when stressed. It was the fact that the SMPS was going into thermal shutdown that led me to question the resistors and R22 seemed to be the one most likely to overheat. I will replace with a 2 or 3 Watt, and using shrink tube on the leads, move it further from the heat sink. Thanks again for your advice. It confirmed my tentative conclusion.

"Three weeks in the lab will save you a day in the library every time"
R. Stanley Williams 1951 -

Best Regards
Clear Ether

[amspire]

--My reluctant and tentative conclusion is that a 1.2 Ohm Metal Oxide resistor has declined in value to approximately 0.77 Ohms. Is that bloody likely? Or it could have been marked incorrectly, or a special house marking system was used. Heck, I really do not know what to think. Duh.

That is what is puzzling me. It is really unusual for resistors to drop in value, and since you quoted an uncertainty, I assumed you are confident with the measurement.

I guess it could have been manufactured with the wrong value, but it does look like it is meant to be 1.2 ohms.

Richard.

[Rerouter]

have been looking around for you, and while debatable whether an initial manufacturing defect or not, metal film resistors can drop in resistance when considerably overheated in pulses, or when in very eletromagnetic noisy enviroments,

i can only think its from the conditions encouraging dendrite growth or similar, and it is a rarer failure mode.

[slateraptor]

That is what is puzzling me. It is really unusual for resistors to drop in value, and since you quoted an uncertainty, I assumed you are confident with the measurement.

I'm in agreement with Richard; the failure mode of typical resistors tend towards hi-z, as opposed to a reduction in resistance.

@SgtRock: Did you measure that resistor using a 4-wire capable meter? If not, the resistor's low nominal value pretty much guarantees that your reading will be significantly off.

[SgtRock]

Dear Slateraptor:

--I appreciate your concern. No, I am afraid I do not have 4 wire capability. But then I am only trying to get a reading within a few percent, that being the published accuracy (at approx. 0.77 Ohms) of my old Fluke 87s, one of which is still in calibration. I do however have a number of very low number resistors with which to check the general performance of the Fluke 87s. Both meters , within stated tolerances, agree, and the low number resistors all read within published accuracy specifications.

--In the meanwhile I am on, to another problem. I soldered in a 1.19 ± 0.03 Ohm 2 Watt Resistor. And the
SMPS works just fine, sort of. If the power is interrupted the unit will not fire up again until it reaches room temperature. I speculate that the MOSFET has been stressed and its internal capacitors cannot hold a charge after it heats only very slightly. I will try replacing the MOSFET, and testing again. At this point is is only an experiment.

"I wear suspenders and a belt. I am a security man all the way"
Justin Wilson (The Cajun Cook) 1914 - 2001
 
Best Regards
Clear Ether

[amspire]

I doubt the MOSFET is stopping the supply starting.

I think you will have to find the part number of the regulator and look at a data sheet to see its startup characteristics.

Just as an example, your mains is 90v to 240V and the regulator needs perhaps 15V to operate. Where does it get it? Sometimes, it gets the 15V from a winding on the transformer, but then you need another source of power to start. So you could have a large resistor and series diode going to the mains to charge the regulator supply cap initially, and have the regulator draw no supply current until a startup threshold is reached. Once the regulator starts, it will it will get all ongoing power from the transformer winding. So you could have a thermal problem in this startup cicuit - a dry joint, a leaky capacitor, a faulty resistor, etc.

The other thing is it is trying to start, but the start fails, and the regulator has a startup timer, so it waits a second or so before another startup attempt. If the supply has a ON LED, you might see it blinking in this case. The supply need to deliver extra power at startup, and for some reason that ability to get the voltage up at the start has been affected. It could be something like the capacitors across the rectified mains are deteriorating, or it could even be something in the Airport that adds extra load when the Airport is warm.

Richard

[mikeselectricstuff]

If the power is interrupted the unit will not fire up again until it reaches room temperature

A very common cause of failure to start is the electrolytic on the DC supply to the controller on the mains side. This is typically fed via a high value (low hundreds of K) resistpr from the HVDC rail - when the supply starts, a more beefy supply from the transformer takes over.
When the electrolytic dries, it gets too leaky to get enough voltage to start the supply via the resistor, although once started it runs happily, so the typical symptom is failure to restart when power is interrupted after being on for a long time. 
This has been the problem on over half the dead SMPS's I've seen.
Faced with a a dead SMPS, look for a low voltage cap on the mains side and replace it. 
You occasionally also see the resistor go high due to voltage stress, so this is worth checking after you replace the cap.   

[IanB]

If the power is interrupted the unit will not fire up again until it reaches room temperature

A very common cause of failure to start is the electrolytic on the DC supply to the controller on the mains side. This is typically fed via a high value (low hundreds of K) resistpr from the HVDC rail - when the supply starts, a more beefy supply from the transformer takes over.
When the electrolytic dries, it gets too leaky to get enough voltage to start the supply via the resistor, although once started it runs happily, so the typical symptom is failure to restart when power is interrupted after being on for a long time. 
This has been the problem on over half the dead SMPS's I've seen.
Faced with a a dead SMPS, look for a low voltage cap on the mains side and replace it. 
You occasionally also see the resistor go high due to voltage stress, so this is worth checking after you replace the cap.

Just checking I understand this, for no reason other then general interest...

The SMPS has low voltage control circuitry, but until the supply is running there is no low voltage in the circuit to power the controller. So the circuit has to "bootstrap" itself somehow to get running. The way this is done is to bleed a little current from the (rectified DC) mains through a resistor into a low voltage capacitor which charges up. This capacitor supplies power to the control circuit so it can get going. Once the circuit gets going the startup resistor is not needed any more--but I presume it will still be passing current from the HV to the LV side and getting hot? So both that resistor and the startup capacitor are potential points of failure.

If this is so I never thought of that before, but I will file it away in my "snippets of useful information" folder in my brain.

[SgtRock]

Dear Mike, Richard, and Ian:

--Thanks for all the good advice. For the record testing is being done with a  14 Ohm dummy load which draws  nearly half the rated 1.8 Amps. Circuit behavior is nominal unless there is the slightest of interruptions in power. Then it will not start again until temperature drops to room temperature.

--Resistors have been checked, and one replaced. Capacitors are new Nichicons and Panasonic FCs, and appear to be in spec for capacity and ESR. The power thermistor has been checked and appears to be normal. PCB has been repeatedly inspected  and any suspected questionable solder joints have been resoldered. The power MOSFET and Schottky Rectifier are only being replaced as a last guess.

--Thanks again for all the inspired theorizing. I will not give up, and eventually the device will surrender due to exhaustion.

"But leave the Wise to wrangle, and with me
The Quarrel of the Universe let be:
And, in some corner of the Hubbub couch'd,
Make Game of that which makes as much of Thee.
"Omar Khyayyam" 1048 1131

 
Best Regards
Clear Ether