Partially Blown Fuse?

[knobtwiddler]

Hi,

Does it strike you as plausible for a fuse to 'partially' blow, whereby it's introducing significant resistance and causing V to drop?

An item came back to my workshop recently, where the client was telling me of symptoms that were consistent with one rail (it's got bipolar +/-17v rails @ 600mA each) having low voltage. I suspected a PSU fault and connected PSU to a programmable load, set @ 600mA. When the load was turned off, the voltage was bang on 17V, but when I put load on, the fuse (a 1A Slow-Blow) blew immediately. Since replacing it with a new fuse, I haven't been able to copy any of the symptoms that the client reported. I wondered if an MLCC (rail to rail) had gone short, weakening the fuse, having it blow on our load - but I could not find any signs of a shorted MLCC, not visually, smell-wise or current draw.

TIA.

NB - the item in question was manufactured in 2022, so 'fuse fatigue' that might happen with older gear should not be an issue.

[G6Tanuki]

I once came across one of the sand-filled 13A-plug fuses that had 'blown' sufficiently well that it wouldn't pass enough current at 240V to power a wall-wart, but tested OK on an analog ohmmeter with a 30uA taut-band meter.

Cracked it open, the sand was metallized but there were still the two ends of the fusewire visible [about 1/8 inch gap between the ends where it had blown].

So yes, fuses can play tricks.

[Craig Sawyers]

As can toasters. My wife, puts bread in, operates lever - breaker pops in the consumer unit. Tries again - same effect.

"We'll need a new toaster" she says. Nonsense, say I.

So I empty the crumb tray, then turn it upside down over the sink, give it a few sharp bangs, and out pops two pieces of charcoaled crust.

Problem solved.

Craig

[ben]

There was also a case I read about of a stereo system (or was it a VCR?) where someone had used the wrong type of fuse (type rather than rating I think) in a secondary circuit. This altered the corresponding power rail and caused odd fault symptoms.

[TonyDuell]

Years ago I was repairing a switch-mode PSU (actually part of a computer monitor). After replacing the failed components, including the fuse, and doing a ringing test on the transformer, I powered it up. The fuse blew, but testing the components again found no problems. So I replaced the fuse and tried again. And it blew again.

Finally I tried a fuse of the same nominal rating and type (I think 1A antisurge) from a different source. It held and the unit worked perfectly. It turned out that the switch-on surge current, to charge the mains smoothing capacitors, would blow some brands of antisurge fuse but not others.

[Vintage Engr]

I have had similar experiences as G6Tanuki, and on more than one occasion. It can be very misleading !

David.

[Outrun_uk]

I've had several occasions with SMD fuses having resistance under load, usually up to about 20 ohms which causes the device to show low battery or malfunction.

[Sideband]

Yes I've also had SMD fuses go high resistance. Can't say that I've experienced it with 'normal' fuses.

[gallowfields]

there is also the case with a grundig tv chassis where the 30volt fuse to feed the frame stage goes high resistance.[its usually a 1 amp anti surge the type with a coil in] causes lack of hieght . forget the cuc number

[jamesperrett]

Originally Posted by TonyDuell

Finally I tried a fuse of the same nominal rating and type (I think 1A antisurge) from a different source. It held and the unit worked perfectly. It turned out that the switch-on surge current, to charge the mains smoothing capacitors, would blow some brands of antisurge fuse but not others.

Fuse manufacturers usually quote the I2T characteristic for their fuses so you can choose how quickly you want your antisurge fuse to blow. I think I've mentioned this before but one project I worked on had the fuses chosen so that the one that should blow first would be the one that was easiest to change. If one of the difficult to access fuses blew, it could take a couple of hours to dismantle the system to get at them.

[MotorBikeLes]

gallowfields, you beat me to it with that.
Not a CUC chassis however, but GSC100 and 200 from memory.
Les.

[The Philpott]

Nearest i got to this was a 12v 5w lamp with a blown filament that had coated it's interior with a mirror finish. The resistance was quite precise (around 1.8k if memory serves correct)

We also had an oddity on the forum where a member had a failed NiCad battery that was showing counter-intuitive readings during testing with a DMM- it turned out a link between cells had corroded to green pulp, and it was semiconducting.

Dave

[knobtwiddler]

Thank you, all.

I think a summary would be: 'partially blown' fuse is rare, but not outside of the realms of possibility. I'd be stupid to blame it for the issue without looking at other potential culprits, but also stupid to discount it entirely.

It was a 20mm glass fuse BTW.

[bikerhifinut]

I had a fault on my iCom756pro1 HF transceiver where it would shut down when the transmit button was pressed, with these radios, like most modern transmitters there are safety cutouts if the output stage is overloaded/abused by inappropriate loading or shorts so the first place one generally looks is the antenna and associated wiring etc.
The last thing I thought about was the supply voltage dropping below that which was necessary for the Tx to work.
It was another local amateur who nailed it for me as he'd had the same issue on some kit where a PSU fuse was going high resistance or O/C under a heavy load. No visual indications and when metered tested normal.
I swapped the offending fuse out, a standard 20mm Glass fuse in an inline holder on the 12V supply cable into the connector on the back of the set and Bingo back on the air!
I have no idea what the mechanism was causing this, the best I can think is when 20A is passing through when say "Tuning up" the fuse element was heating enough to move off the metal end cap in much the same way a mechanical indicator relay works, except of course the fuse ain't a bimetal strip designed to do it.
Incidentally, why do certain manufacturers fuse both the +ve and -ve supply lines into the back of a radio? All I can think is its just in case there's a short between one side or the other as an over current situation is going to occur on both wires logically.
Andy.

[The Philpott]

-I have noticed that old exposed fusewire corrodes..i suppose if it corroded at the end inside a glass or ceramic fuse that same corrosion could cause the wire to curl as it heats up. (Stranger things happen at sea)

Dave

[Beardyman]

Yes, I see this phenomenon a lot in my job. Particularly very low value fuses (<500mA) of ceramic construction known as Picofuses (trade name). Some are deceptively low resistance but I have seen them as high as 10-15 ohms, enough to stop the circuit working correctly or cause random malfunctions. I think (and it's only my feeble assumptions) that the fuse initially blows with such violence that the inside of the tube becomes coated with a metallic film of sorts. Whilst a modern DMM shows just a few ohms, when more current is flowing the resistance goes up.

[kellymarie]

The reason some manufacturers put fuses in both DC leads is because its just possible that if there's a fault in the cars wiring on the negative side of the accessory system the rigs lead could be the only link back to the battery in winter its possible that it could end up carrying many tens of amps from lights blower motors etc something you don't want. It's an unlikely scenario but mite happen if your unlucky

[knobtwiddler]

Originally Posted by bikerhifinut

I had a fault on my iCom756pro1 HF transceiver where it would shut down when the transmit button was pressed, with these radios, like most modern transmitters there are safety cutouts if the output stage is overloaded/abused by inappropriate loading or shorts so the first place one generally looks is the antenna and associated wiring etc.
The last thing I thought about was the supply voltage dropping below that which was necessary for the Tx to work.
It was another local amateur who nailed it for me as he'd had the same issue on some kit where a PSU fuse was going high resistance or O/C under a heavy load. No visual indications and when metered tested normal.
I swapped the offending fuse out, a standard 20mm Glass fuse in an inline holder on the 12V supply cable into the connector on the back of the set and Bingo back on the air!


Andy.

The item in question has gone back to the client now. I am seriously hoping that the diagnosis was correct as he's in Shanghai... My experience mirrored exactly what you describe above. Thank you for taking the time to relay this experience.

[Red to black]

Two stock faults with consumer Electronics I can think of offhand caused by high resistance fuses are:
1) Philips L6 chassis ctv where a wickman type fuse went a fraction of an ohm high causing all sorts of problems such as tripping/ motor boating etc... to be fair the 5 volt rail was critical and fairly marginal at best and Philips did issue a mod too.

2) late model Sanyo VHS vcrs (also used by Nokia) VCR 95 mech I think, which would trip to stby when entering record mode only, this was down to one of those yellow cylindrical soldered in fuses about the same size and shape as a 1/4 watt resistor (location PR512 iirc) in the psu going slightly high resistance, in this intance I couldn't take credit for finding this fault as Sanyo technical gave us the answer the first time we had it.

[its ur aerial]

Yes I have two or three, during my 27 years in the TV trade, one I noticed out of the corner of my eye was glowing slightly orange, but red perfect when tested on the meter.
Always been 20mm glass types.
Ken, G6HZG.