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Sherlock Ohms

The Failed Fuse Fooled Me

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Jim_E
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Platinum
You're not the only one!
Jim_E   8/7/2013 9:01:50 AM
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If it makes you feel better, you're not the only one who has misdiagnosed a blown vehicle fuse.  I was fooled by a newer mini ATM type fuse which didn't look blown, but really was.  Like you, I now test them with my meter.

 

GTOlover
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Platinum
Re: You're not the only one!
GTOlover   8/7/2013 9:44:12 AM
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I will ditto that! However, being a little more thick-headed I thought the chance of a fuse burning out like that again would be slim. So in my most rebelious attitude of youth, I continued the practice of looking at fuses instead of testing them with a meter. Then one day, (I think I was working on my riding lawn mower) the same situation happened. After a couple hours trying to get the garden tractor to start, I got the meter and tested the 30 amp fuse. I was humbled and now grab the meter whenever I suspect a fuse!

What can I say, I am a slow learner of the obvious!

Rob Spiegel
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Blogger
Re: You're not the only one!
Rob Spiegel   8/7/2013 10:48:08 AM
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This proves a hunch that I've long held that you can't tell everything my looking at a car fuse. I've looked at a ton and thought, "I can't really tell if this is OK or not."

tekochip
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Platinum
Re: You're not the only one!
tekochip   8/7/2013 10:51:30 AM
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We had a product with a 1/10A fuse.  There was no way to see the element without a magnifying glass, so a quick continuity test was the only way.  When I ran the service department the first thing I always did was check the fuses and power traces for continuity, then pull the fuses to make certain that the customer hadn't replaced a 1/10A with a 20A. 

 

It seems that the customer would always install a larger fuse until the wiring harness burned.  As soon as the Magic Smoke was let out, though, they would send the unit in to have the smoke refilled.

GTOlover
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Platinum
Re: You're not the only one!
GTOlover   8/7/2013 1:21:05 PM
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Is the magic smoke sold by volume or weight? :-)

rjnerd
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Iron
Re: You're not the only one!
rjnerd   8/7/2013 5:05:31 PM
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I remember fuses like that.  A data aquisition system I used to use had its inputs protected with fuses that had possibly lower current ratings.  Like yours, you couldn't really see the wire, but checking had its own issue -   One problem quickly appeared, back in the days of the simson 260, you ran a good chance of blowing the fuse with the ohmmeter's battery.  If you used a vtvm, that was ok, it had high enough impedance, that the fuse would survive, but there were some that still swore by their old friends.


We wound adding a fuse test fixture to the front of the thing -- it was just a holder with a 1k resistor in series, to keep the current down.

tekochip
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Platinum
Re: You're not the only one!
tekochip   8/7/2013 8:08:16 PM
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Yup, happened to me too.  I think it was a Huntron Tracker, or something like that would take the fuse out!

Contrarian
User Rank
Gold
Not just fuses, either
Contrarian   8/8/2013 9:26:41 AM
Had a problem once when you put on the brakes, the headlights came on, and vice-versa.  I scoured the wiring diagram looking for any possible path and could not see how it could be possible, short of a physical cross connect.  I started checking for shorts in the bulb sockets and that's when I found an 1157 dual filament bulb that had a broken filament that was touching the adjacent filament and would readily provide enough current across the brake circuit to the running light/headlight circuit and itself would still illuminate.  You don't normally suspect a working bulb so it missed initial inspection.

szyhxc
User Rank
Iron
Re: Not just fuses, either
szyhxc   8/11/2013 8:20:03 AM
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Connie,

I have seen this a lot with dual filiment bulbs.  Now whenever I have lighting issues and the system is working (not the fuse) but working erratically, the first thing I check is the dual filiment bulbs in the eratic circuit.  Usuallyit is one of the primary bulbs in the function (brakes, turn signals etc.)

Gaffer Gaggs
User Rank
Iron
Re: Not just fuses, either
Gaffer Gaggs   8/16/2013 4:49:59 PM
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Several decades ago I was working on a gas plasma laser power supply, on a tight shipping schedule. As we were trying to get the display system and the 2 lasers out the door on a Friday afternoon for a Saturday night show one of the lasers's power supplies would not strike the tube the interlocks wouldn't allow the water cooled series pass bank to energize. After an hour of poking around I discovered that someone had reversed the water flow (in to out and out to in) why they did such a thing I will never know but when we got it to fire up it popped two of the three the 35 amp fuses, So we sent some one off to the electric supply store before they closed for the weekend to get a box of 10 because we needed some for the shop anyway. 

We replaced all three fuses, powered the unit up all three fuses popped as we brought the current up to opperating power. There were 30 transistors on the water cooled series pass bank so we started to pull them and test them because "they were usually the problem". An hour or three later after finding no bad transistors we powered up the unit with fresh fuses and again as we brought up the current control up all three fuses popped again.

We were now running out of time, so the owner called out to a friend and borrowed a power supply we kept trying to get the other power supply going to no avail blew all of the new fuses. The barrowed power supply arrived and worked, the show went on!!!

Saturday morning I went to the same electric supply house and picked up 2 boxes of 10 fuses, hooked up a different laser head to the supply and blew all of the fuses again. We needed the supply later in the week so I continued to trouble shoot the supply after all Saturday 12 -14 hours and all day and night Sunday I had blown all of the fuses But had cleaned and checked EVERY THING in that power supply at leased 3 or 4 times. 

Monday morning on the way in to the shop I stoped by the electrical supply house to pick up another box of fuses. In talking to the counter man I discovered that they had been notified about a bad batch of fuses that morning by a call from the factory. Sure enough the boxes of fuses that I had spent all week end blowing/testing were in the bad batch!!! The electrical supply house went to a competitor and brought over a different brand of fuses the supply worked perfectly for several years to come. One lost weekend that I will never forget!!!

Larry M
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Platinum
Re: Not just fuses, either
Larry M   8/16/2013 5:03:31 PM
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Contrarian wrote: "found an 1157 dual filament bulb that had a broken filament that was touching the adjacent filament"

Yup, seen that one before. Luckily was thinking about the problem at work, so forced to figure it out before spending any time with the car.

I had an 70 Opel GT. One of the fuses (10A, wide element) fatigue-cracked through. You couldn't see daylight through the element when it was under the dash, but it had the peculiar property that it would heal and carry a low current but open with higher current. When all current was removed it would heal again. Something like: you could play the radio but if you turned on the wipers both the radio and wipers would quit. If the ignition was turned off for a few seconds you could repeat the sequence. After several minutes I noticed that the fuse element wasn't flat; both ends of the waisted part of the element formed a V with vertex in the center. I removed the fuse from the clips and held it directly in front of my 100-watt trouble light and could see the hairline crack.  Several minutes to find--less than a minute to fix.

TexasTJ
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Iron
Smart vs. wise
TexasTJ   8/8/2013 9:52:18 AM
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I can't remember where I heard this, but I've tried to impart it to my daughter:
"A smart person learns from his mistakes, a wise person learns from the mistakes of others."  You've given us a chance at wisdom.

Analog Bill
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Gold
Re: You're not the only one!
Analog Bill   8/8/2013 10:39:25 AM
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Over my career (nearly 50 years), I've seen this dozens of times. Normal temperature cycling of fuses often causes a microscopic break in the tiny filament in the fuse. The take away lesson is a very general one: "Don't assume anything ... especially true with a visual inspection!".  -- Bill Whitlock, analog circuit designer (and former drag-race car builder)

William K.
User Rank
Platinum
Re: You're not the only one!
William K.   8/17/2013 7:35:39 PM
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Fuse failure from thermal cycling is why military vehicles hace circuit breakers instead. And does anybody remember the "battle switches" that were used to bypass the fuses in the much older military equipment? That was in the era before they had circuit breakers.

Now, we see that a lot of the higher reliability fuses have coiled elements and an inert powder to support the fusible element. The powder also serves to absorb the heat and cool the plasma when the fuse opens a high overcurrent fault.

Cabe Atwell
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Blogger
Re: You're not the only one!
Cabe Atwell   8/27/2013 2:04:00 PM
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Something along those lines have happened to everybody at one time or another and I am no stranger to the fuse situation either

racerjerry
User Rank
Iron
Re: You're not the only one!
racerjerry   8/8/2013 10:47:38 AM
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Actually, this is quite common.  During first troubleshooting steps, I always check auto fuses for power at each side of the fuse using a TEST LIGHT with a sharp pointed probe.

kenish
User Rank
Platinum
Re: You're not the only one!
kenish   8/8/2013 6:01:38 PM
I had a "hidden" fuse break happen to me in high school; been there/done that!

This sounds like a common problem, so it's time to go to market with my Fuse Tester.  A fuse clip, momentary switch, and AC plug all connected in series.  Install the suspect fuse into the clip.  Plug the tester into a wall outlet and press the switch.  If you see a blue flash the fuse *was* good!  :)

Engineering trivia:  AG (as in 3AG size) stands for "Automotive Glass"

 

Astro-Eric
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Blogger
Whew !!!
Astro-Eric   8/9/2013 12:19:28 PM
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Whew!  I'm glad that I'm not the only one that has experienced this, or had similar troubles.  Even though this was more than 20 years ago, the lesson is still fresh in my mind. 

I appreciate all the interesting responses...


Eric

rich.gunderson
User Rank
Iron
A fuse can imitate active components
rich.gunderson   2/21/2014 11:09:47 AM
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Having worked  as a serivce manager for a well known marine electronics installer and servicing dealer in Portland for years, I came across many unusual failures over the years, especially since the owner was one of the original Tektronix employees who started out working for radio station KGW in Portland and he owned several repeaters with a large customer base. I kept a bag of "trick" fuses I had found over the years. Some of the fuses I remember:one that acted like a diode, another was a variable resistor, whose resistance varied from 20 ohms to about 150, depending on the amount of current going through it. Another one oscillated, although it was a mechanical vibration of the end of the filament against the fuse end cap inside the device, it worked like a UJT in the circuit, varrying frequency with current. For kicks, I put the variable resistance fuse in series with the oscillator fuse on a breadboard with a 12 volt source, that really made an intersting output of a small op-amp circuit when energized. I offered them to a friend who was an electronics instructor at ITT locally to use for his students final lab, which was troubleshooting problems he put into an AM/FM reciever they had all built. He said he did not want to stay all night, because he knew it would drive them to a loop trying to find the problem which was one of my "whacky" fuses.  Rich Gunderson, Airgas Rental/Repair Portland, Oregon

lbrac
User Rank
Iron
Me too!
lbrac   8/16/2013 5:44:46 PM
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On the way home one evening many years ago, my motorcycle quit running and I coasted into a parking lot to troubleshoot. It would crank over and try to fire, spit and sputter when cranking, but wouldn't run. Daylight was fading quckly. I checked the fuses and they appeared to be intact. I feared an engine failure. By then it was nearly dark, so I crossed the street to call home for a ride, planning to deal with it the next morning.

I was working at a motorcycle dealership at the time, so I called a coworker that lived near my home. He brought a trailer the next morning and we loaded and took it into the shop. One of the mechanics checked it while I got other things going for the day. He found the fuse element was broken but the ends of the element would touch until they were heated by current flow. Then they would distort and open the circuit.

The mechanics at the dealership joked about installing a 30 day (or some other time frame) fuse into a customer's bike so they would have to bring it back in for repair, not knowing it was just a fuse. Of course they didn't have any such fuses.

That was many years ago now and I've come across similar problems in automotive and industrial applications a number of times. It seems the element fatigues from repeated cycles of heating and cooling when conducting current. This expansion and contraction causes the element to break but the ends do not separate until heated by current flow. I've also seen similar problems with light bulb elements that test good with an ohmmeter but open when heated.

ChuckCooper
User Rank
Iron
Intermittent Fuse
ChuckCooper   8/27/2013 3:00:44 PM
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Years ago I ran into an intermittent problem.  Of course I didn't check the fuse since there is no such thing as an intermittent fuse! During several rounds of troubleshooting, everything worked fine, but when replaced, the system would fail.  We would again remove it, remove the cover, set it on the bench and all systems checked out.  We spent hours looking for a loose wire or bad connection. Finally we pulled the fuse and it looked OK.  More testing ensued.  We pulled the fuse again and tested it and it was bad.  I tapped it with a finger and one end of the fuse wire vibrated.  I did it again and it shook but suddenly stopped vibrating.  When tested, the fuse now showed good.  After several repetitions, it was possible to orient the fuse so the broken wire would make contact, then rotate it, tap it, read open reorient and connect, and then test good.  Apparently, the orientation when operating was the open state, and when on the bench, it was rotated to the fail state.  Ever since then I not only test, I also tap it once or twice while reading the meter.  Of course it's never happened again, but you never know.

William K.
User Rank
Platinum
Re: Intermittent Fuse
William K.   8/27/2013 9:00:34 PM
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CC, that is why I check the supply voltage, not only do fuses go half bad, but sometimes connections fail, like splices inside cable ducts. No, there are not supposed to be splices in cable ducts on panels, but I have seen them and seen them fail. A bit tough to spot sometimes. 

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