Counterfeit fuse?

[atgeek]

Some time ago I bought from AliExpress 100 assorted glass fuse sold as quick blow type, from 0.2 to 15 A. I've just tested them with a bench power supply, the 0.5A blowed at 2A after about five seconds, the 6A at about 12A instantly. Is it normal for fuses to have such high discrepancy from the nominal hold current? Or maybe it's just because are poor made?

[PA0PBZ]

Have a look:

http://www.littelfuse.com/~/media/electronics/datasheets/fuses/littelfuse_fuse_217_datasheet.pdf.pdf

[Gyro]

All fuses have a current/time curve. The figures you quoted don't sound that unusual. The fuse rating is the 'carry' current. Depending on the fuse it normally takes around 10s or more to blow at twice rated current and somewhere around 0.1 secs at 10x (for fast blow). Without knowing the time/current curve for the specific fuses it's hard to get closer.

The normal worry with fake fuses is the ceramic body ones which are supposed to be filled with the right grade of silica sand, but in fake ones, often aren't. A glass fuse is low interrupt current (not HRC), so it's no more that a piece of fuse wire in a glass tube, there's no point in faking that. 

P.S. You should be careful not use glass fuses where HRC ceramic are required, eg. Mains use. The can't interrupt the prospective fault current (say, 1000A+) and will explode instead.

[atgeek]

Ok, thanks for the clarification

[RoGeorge]

One more note when using a bench power supply as a current source: There are big capacitors at the output of the power supply, near the power supply terminals. These capacitors are usually out of the control loop. As a result, when a load is suddenly connected, the current can go as high as the internal resistance of the output capacitors (and the resistance of the load) allow.

As an example, you can easily fry a LED if you set your power supply at 20V/20mA, then connect the LED (without any series resistor). The energy accumulated by a few thousands microfarads at 20V, combined with the low series resistance of the given electrical circuit, is enough to push a spike of tens or even hundreds of Amps through the LED.

[rsjsouza]

One more note when using a bench power supply as a current source: There are big capacitors at the output of the power supply, near the power supply terminals. These capacitors are usually out of the control loop. As a result, when a load is suddenly connected, the current can go as high as the internal resistance of the output capacitors (and the resistance of the load) allow.

Your statement seems a bit too generalistic, as I wouldn't assume that all bench power supplies fail to take into consideration the output capacitors in the control loop.

As an example, you can easily fry a LED if you set your power supply at 20V/20mA, then connect the LED (without any series resistor). The energy accumulated by a few thousands microfarads at 20V, combined with the low series resistance of the given electrical circuit, is enough to push a spike of tens or even hundreds of Amps through the LED.

Perhaps you are referring to a specific type of LED, since at these ratings you can pretty much fry any of the more ordinary types of LEDs, with or without capacitors on the output. 

[atgeek]

One more note when using a bench power supply as a current source: There are big capacitors at the output of the power supply, near the power supply terminals. These capacitors are usually out of the control loop. As a result, when a load is suddenly connected, the current can go as high as the internal resistance of the output capacitors (and the resistance of the load) allow.

As an example, you can easily fry a LED if you set your power supply at 20V/20mA, then connect the LED (without any series resistor). The energy accumulated by a few thousands microfarads at 20V, combined with the low series resistance of the given electrical circuit, is enough to push a spike of tens or even hundreds of Amps through the LED.

A solution could be discharge the output capacitor and then connect the load, am I right?

[rs20]

One more note when using a bench power supply as a current source: There are big capacitors at the output of the power supply, near the power supply terminals. These capacitors are usually out of the control loop. As a result, when a load is suddenly connected, the current can go as high as the internal resistance of the output capacitors (and the resistance of the load) allow.

Your statement seems a bit too generalistic, as I wouldn't assume that all bench power supplies fail to take into consideration the output capacitors in the control loop.

Wait, are you saying that a noteworthy fraction of power supplies detect the high current and actually violently sink charge out of the capacitor faster than the LED can drain it, until it matches the voltage of the LED? That seems to me like a very difficult control systems problem, if not basically impossible.

Seems like it'd be an awful lot easier to make a power supply without output capacitors.

[RoGeorge]

One more note when using a bench power supply as a current source: There are big capacitors at the output of the power supply, near the power supply terminals. These capacitors are usually out of the control loop. As a result, when a load is suddenly connected, the current can go as high as the internal resistance of the output capacitors (and the resistance of the load) allow.

As an example, you can easily fry a LED if you set your power supply at 20V/20mA, then connect the LED (without any series resistor). The energy accumulated by a few thousands microfarads at 20V, combined with the low series resistance of the given electrical circuit, is enough to push a spike of tens or even hundreds of Amps through the LED.

A solution could be discharge the output capacitor and then connect the load, am I right?

Indeed. In this case the initial current will be lower then the expected Amps setting, because a part of the generated current will be used to charge the output filtering capacitors.

To be more specific, my tests were made with a Rigol DP832, but I know for sure most (if not all) of power sources have a few hundreds or even thousand microfarads placed at their output, after the current sensor. To check if a particular lab source have this caveat, set a very low current and a big voltage, then power the output without any load. You will see (if this caveat is present) that the voltage will raise very slow and linear in time. You can even deduce the capacitance of the output capacitor by timing how long it takes to achieve the requested voltage.

[RoGeorge]

...Seems like it'd be an awful lot easier to make a power supply without output capacitors.

Usually a power supply is used as a voltage sources, so the designer's main goal will be to minimize the output resistance of the voltage source. The low output resistance is a very important parameter, especially for transient loads and AC currents. This might be not so easy to achieve with a series measuring shunt and a control loop with a limited speed. As a compromise, the designer can choose to put a low ESR capacitor right at the output, after the current sensor.

[RoGeorge]

As an example, you can easily fry a LED if you set your power supply at 20V/20mA, then connect the LED (without any series resistor). The energy accumulated by a few thousands microfarads at 20V, combined with the low series resistance of the given electrical circuit, is enough to push a spike of tens or even hundreds of Amps through the LED.

Perhaps you are referring to a specific type of LED, since at these ratings you can pretty much fry any of the more ordinary types of LEDs, with or without capacitors on the output. 

Not if you connect the LED first, then energize the power source output.
https://hackaday.io/project/7590-retardo-davinci/log/25376-rigol-dp832-power-supply-set-for-20-ma-can-kill-a-led

[rsjsouza]

One more note when using a bench power supply as a current source: There are big capacitors at the output of the power supply, near the power supply terminals. These capacitors are usually out of the control loop. As a result, when a load is suddenly connected, the current can go as high as the internal resistance of the output capacitors (and the resistance of the load) allow.

Your statement seems a bit too generalistic, as I wouldn't assume that all bench power supplies fail to take into consideration the output capacitors in the control loop.

Wait, are you saying that a noteworthy fraction of power supplies detect the high current and actually violently sink charge out of the capacitor faster than the LED can drain it, until it matches the voltage of the LED? That seems to me like a very difficult control systems problem, if not basically impossible.

Seems like it'd be an awful lot easier to make a power supply without output capacitors.

When I wrote that I was thinking on the lines of your last sentence but replied something completely nonsensical as you pointed out.

Most of the bench power supplies I have used do not have smoothing capacitors directly on the output.

[macboy]

At least your fuses blew at some reasonable current. The closer the current is to the nominal current, the long it takes, up to several minutes. At 2x the nominal current, most fuses will blow in some tens of seconds or so (less than a minute). At 3x the nominal current, it should take only a few seconds. At 10x the nominal current, any fuse should blow very quickly. (alway check the specs, these are just very rough estimates).

THIS is a counterfeit fuse:
https://www.eevblog.com/forum/chat/counterfeit-chinese-fuses/

I put 33 A through a "3 A" fuse long enough to completely melt the thing, but it kept on conducting. Any fus should have blown nearly instantly at 11x the rated current. Apparently, this is one of those newfangled Chinese-spec "no-blow" fuses. I guess they are used (by idiots) where the inrush current will blow even slow-blow fuses. Just don't count on them preventing a fire in case of trouble. 

[Neilm]

I have posted this before, but it shows a UK plug with a fake fuse trying to stop a short circuit.

[691175002]

I have posted this before, but it shows a UK plug with a fake fuse trying to stop a short circuit.

Shouldn't the circuit breaker open in that situation?  Is this just a problem with fake fuses, or glass fuses in general?

[Mechatrommer]

I have posted this before, but it shows a UK plug with a fake fuse trying to stop a short circuit.

or a fake video? the fuse must be like 100A or more during the blast. the enough energy to propel a heavy mains plug out of its socket is suspicious. a fake fuse with very thick copper is unlikely imho. or either they put The Black Sand in it in the demo video instead of silica sand just for the fun of it.

[rs20]

Seems like a pretty good fuse to me; it certainly cut power to the faulty device, isn't that what fuses are for?

[amyk]

That reminds me of the explosive disconnects used on HV mains transmission lines.

[rs20]

Yeah, and it's easy to identify which plug has the blown fuse in it too!

[Gyro]

That reminds me of the explosive disconnects used on HV mains transmission lines.

BMW use something similar on their rear mounted car batteries. I think they call it a pyroelectric disconnect (it has a separate trigger wire).

[joeqsmith]

I have posted this before, but it shows a UK plug with a fake fuse trying to stop a short circuit.

or a fake video? the fuse must be like 100A or more during the blast. the enough energy to propel a heavy mains plug out of its socket is suspicious. a fake fuse with very thick copper is unlikely imho. or either they put The Black Sand in it in the demo video instead of silica sand just for the fun of it.

May be a controlled test of sorts.  Personally I can believe it.  My own fuse powered rocket.  Who needs Black Sand.
https://youtu.be/xLDok9Sm07Q

[Mechatrommer]

May be a controlled test of sorts.  Personally I can believe it.  My own fuse powered rocket.  Who needs Black Sand.
https://youtu.be/xLDok9Sm07Q

some "practical" achievement there congratulation. but that rely on air expansion on a tiny sponge, try again with the open air plug like that. i'm sure the fake fuse manufacturer need to spend sometime to get the effect, i bet more expensive cost than the original fuse.

[joeqsmith]

some "practical" achievement there congratulation. but that rely on air expansion on a tiny sponge, try again with the open air plug like that. i'm sure the fake fuse manufacturer need to spend sometime to get the effect, i bet more expensive cost than the original fuse.

Forgot to say, I would guess your comment about the cost to set it up and film it was a more expensive than the cost of the original fuse was an understatement! 

I'm sure the fuse holder made a good enclosure.   I started out with fuses that were not contained. Like the Black Sand, you need to enclose it.


RANDOMTRONICS and I attempt to run a fuse inside an enclosed fuse holder.  This was my fuse tests to go along with his.

[Mechatrommer]

I would guess your comment about the cost to set it up and film it was a more expensive than the cost of the original fuse was an understatement! 

to be fair, what i meant is the cost of making a fake fuse alone thats capable of propelling a quality 3 pronged plug, compared to the cost of making one original fuse, and i meants the production cost in factory, not the cost of home diy. the cost of making a demo video, the hi-speed videocam, and damaged socket/plug is another story for you to waste. btw, lucky you no pcb projectile in the 2 later videos. i hope you are not the one who made the fake fuse fake video plug projectile above. if yes then you can elaborate, not extrapolate from your own setup.

[Neilm]

I have posted this before, but it shows a UK plug with a fake fuse trying to stop a short circuit.

or a fake video? the fuse must be like 100A or more during the blast. the enough energy to propel a heavy mains plug out of its socket is suspicious. a fake fuse with very thick copper is unlikely imho. or either they put The Black Sand in it in the demo video instead of silica sand just for the fun of it.

Produced by a testing house called ERA who were taken over by Intertek some years ago. It was part of an investigation into fires from faulty power lead extensions. As part of the testing they investigated the fuses - they also found similar fuses on sale in market stalls.