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Need a solid state fuse

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I think You didnt finish homework. You should read description of circuit.

Suggested circuit with description text :

https://www.circuitstoday.com/simple-elecronic-fuse


Initially the load current flows through SCR and resistor R1.The value of R1 is so selected that, the maximum load current multiplied by the resistance of R1 is equal to 0.7 volts. When the load current exceeds the maximum value the voltage drop across R1 becomes more than 0.7V and switches transistor Q1 ON. Now the transistor completely bye passes the load current and the current through triac falls below the holding current. This makes the triac OFF. When SCR is OFF there will not be any current flow through R1 and so the voltage across it falls to 0.This makes the transistor OFF, completely isolating the load circuit.The fuse can be resetted by pressing S1.When S1 is pressed the SCR is again triggered and remains latched to conduct the load current.



This is simple circuit, you can make it very easely and test it.
 

The circuit in post #16 should be reconsidered, I think. I'm not sure if it has other reasonable applications as is, but it's rarely suited to protect a meter current input.

Some requirements for the meter protection that can't be met by the present circuit
- bipolar operation
- working for 0 - rated current
- low voltage drop

To met the requirements, an active electronical fuse will need a power supply, basically feasible for a digital meter.

I have been researching the options for PTC fuse protection of current inputs in automation equipment. Without discussing the details, I can just say, it's really hard to achieve a shunt resistor protection that can keep up with regular fuses. ZekeR already mentioned it in post #4.

For an electronical solution, there's little chance below back-to-back MOSFETs with active controller.
 
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    tpetar

    tpetar

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If you need to measure current with that multimeter use a small resistor (small in resistance, large in wattage) in series in the circuit, and a voltmeter in parallel with the resistor. The rest is Ohm's law.
Click to expand...
What do you folks think of this tip?
 

boylesg said:
What do you folks think of this tip?
Click to expand...

Author of that text mean on current shunt. Shunt is not replacement for fuse or vice versa, but in some cases people underprice current in circuit and shunt burn like fuse, even fireworks is possible. :smile:
 
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jasonc2 said:
You could just run some wire from the internal fuse terminals outside the case and mount the clip there, but that could have potential safety issues if you don't cover it. You could also just tape your screwdriver and a bag of fuses to your DMM so it's always handy when you need it.
Click to expand...
Might be the easiest option but it is not really a safety issue as I don't have plans to ever test mains voltage. I am only using the DMM for electronic circuits.
 

boylesg said:
Might be the easiest option but it is not really a safety issue as I don't have plans to ever test mains voltage. I am only using the DMM for electronic circuits.
Click to expand...

Current shunt have influence on current in whole circuit. Inside some DMM you have shunt. Better variant is hall sensors.

See that wire in bottom right corner on right PCB, that is current shunt :

multimeter_internals.jpg


or see this in left bottom corner shunt :

**broken link removed**

Its made from resistive wire, often is used Canthal wire.
 
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As you are circling the original problem somehow, I feel tempted to clarify that the suggestion by jasonc2 is still the most appropriate solution in my view.
You could also just tape your screwdriver and a bag of fuses to your DMM so it's always handy when you need it.
Click to expand...

P.S.:
Current shunt have influence on current in whole circuit. Inside some DMM you have shunt. Better variant is hall sensors.
Click to expand...
Voltage drop can be an issue. But I can insure you from current measurement practice in industrial instrumentation, that it's really hard to keep up with the accuracy and long term stability of a simple shunt (like it's shown in the photo) without referring to expensive and bulky compensating current transducers.
 
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This can be best solutions, he will replace fuses until he change his habits and make more attention when using some instruments.

Voltage drop can be an issue. But I can insure you from current measurement practice in industrial instrumentation, that it's really hard to keep up with the accuracy and long term stability of a simple shunt (like it's shown in the photo) without referring to expensive and bulky compensating current transducers.
Click to expand...

Yes I know that metal/wire (for shunt) changing own material properties, specially under higher currents and higher temperatures, but I think that multimeters should be used just to measure some value, not to be installed and glued on some PSU and use it like some sticket voltmeter and ampermeter continuously. Also I saw that some manufacturers give time of usage parameter, after that time shunt need to recalibrate or change. Fluke like some other manufacturer of instruments have recalibration and certification like additional service with price around 200eur. I completely agree with your statement, I just show him that multimeter also have shunt inside. Our friend Boylesg is stucked with fuses, I think that here is need some good link to fuse distributer/dealer with discount on 1000 pieces.

:wink:
 
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boylesg said:
Taster? SMD?
Click to expand...

Tester. Unless tpetar's still thinking about corn flakes.

SMD = surface mount. Relevant glossary: https://encyclopedia.thefreedictionary.com/Surface+Mount+Device

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boylesg said:
... it is not really a safety issue as I don't have plans to ever test mains voltage ...
Click to expand...

While you probably won't kill yourself with an unprotected DMM fuse, I feel there are a few things wrong with this general statement that are worth pointing out, especially with the transistor circuits you've been experimenting with:

- "It's OK because I don't ever have plans to <insert unforeseen future here>" is not a good thought process in general.
- It's the current that gets you. Dry healthy skin has enough resistance to protect you but an interesting factoid: https://hypertextbook.com/facts/2000/JackHsu.shtml
- High current most definitely does not lie solely in the realm of mains. Also, if you're constantly blowing fuses you at *least* know that you're frequently running more than the fuse rating through the cartridge.
- Besides, you don't know you're testing e.g. "mains voltage" before you test it. That's why you're testing it. You wouldn't want to e.g. discharge a power supply capacitor into a sweaty hand, or run a transformer ac current through your palm and find that you can't let go of it - you only need about 5-10mA ac to get through your skin to cause muscles to contract and lose control.

Speaking generally though.
 
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Best solution for this problem based on readed discussion by me, can be disabling 200mA range on multimeter.

Solution can be immidietely be implemented with usage of scotch tape on 200mA contact to isolate him. Or easely by removing 200mA fuse from multimeter.

10A or 20A can be only solution for usage, and we can hope that measurement will be in that higher range what multimeter support.
 
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[offtopic] Re: Need a solid state fuse

jasonc2 said:
- "It's OK because I don't ever have plans to <insert unforeseen future here>" is not a good thought process in general.
- It's the current that gets you. Dry healthy skin has enough resistance to protect you but an interesting factoid: https://hypertextbook.com/facts/2000/JackHsu.shtml
- High current most definitely does not lie solely in the realm of mains. Also, if you're constantly blowing fuses you at *least* know that you're frequently running more than the fuse rating through the cartridge.
- Besides, you don't know you're testing e.g. "mains voltage" before you test it. That's why you're testing it. You wouldn't want to e.g. discharge a power supply capacitor into a sweaty hand, or run a transformer ac current through your palm and find that you can't let go of it - you only need about 5-10mA ac to get through your skin to cause muscles to contract and lose control.
Click to expand...

As an aside:

I was at a music festival once where a friend of mine had a small custom built board with a bunch of 50mA ~2V SMD LEDs on it, powered ultimately off a 9V battery. It was hot out and she drank a bit much and passed out for about 6 hours with the board, on, under her back.

When she awoke the current through the LEDs combined with sweat had burned her skin to the point where the board was literally stuck to her, and removing it involved tearing the skin that had fused to the board (which rather gruesomely left sizable bits of flesh burned to the board, which had burned about 2mm deep in places). It must have happened quickly as that board would eat through a fresh 9V battery in about an hour, and that's when it wasn't driving current through human skin. I was the person who detached it from her, with a boxcutter. It was not a pleasant experience. She still has a set of 12 scars matching the LED layout on her back two years later.
 
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I hope so that she is ok.

People buy the fuses you see what happens if there is no fuses, dont wait that multimeter melt and stuck with your hands.

If I cannot find some fuses in market I know who have it.

:smile:
 

That sounds awful.
Agree, the fuses (even 1000 pack as mentioned) sounds a cheap way to be safer with a multimeter. Also, most of the time the DMM is measuring voltage or resistance, so the times that you manually set it up for current, you will soon remember to check on a high current range first.
(and to be honest, most of the time if you're starting up a circuit for the first time, it is good to set a current limit on the power supply anyway, although often I will use a small-sized battery to first power up circuits - in the hope that the internal resistance would limit the current, but from that LED story it reminds that batteries can supply a lot of current..).

EDIT: btw, maybe you are blowing fuses when trying to measure voltage but the leads are still set to current?
In that case, some DMMs (at least my cheap fluke does) will remind you to change the lead each time you move the dial to/from the
current ranges to the other settings, it will print 'LEAD' on the screen.
 

boylesg said:
So I don't have to open open it up to change the fuse.
Click to expand...
Right. And this is a PTC self-resetting fuse:


boylesg said:
What rating was the original fuse and what was the rating of the PTC fuse you replaced it with?
Click to expand...

Original fuse rating: 2A slow
PTC fuse rating: 2A hold, 4A trip

Don't forget: this DMM fuse just has to protect the current shunt resistor, so it can be as "slow" as that one; the electronic part of the DMM (the input differential amp) has its own resistor+diodes protection.

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steveelliott said:
Erikl, are you sure the PTC is a MOV? MOV is a high voltage clamping device (like a zener diode), not current limiting device. A PTC current limiter device works generally by having pieces of carbon touching inside a special plastic matrix (the "polymer" part of the polyswitch name). Current is carried through the carbon pieces. When the carbon gets hot due to excessive current, the plastic matrix expands and the carbon doesn't touch anymore - no current.
Click to expand...

Steve, you're absolutely right, the PTC current limiter device is not a MOV. I probably mixed that up, because both devices are often called varistor, which means variable resistor (or better: varying resistor), and this applies to both device types.

Thank you for adjusting my comment!
 

tpetar said:
S1 is taster. Just one hole drilled on multimeter plastic case.

sot23.gif


Thyristor, transistor exist in SOT23 SMD case.

Resistor in SMD lets say 0805.


I hope that you didnt mean really to put 2n3055 transistor for this.



Dont let that this resseting fuse be bigger then multimeter. Yes you can also use TO92.
Click to expand...

S1 is taster - You lick it to reset the circuit?

I should tell you a story about the Transportation Safety Administration here in the US. I was shipping some 10cm dia x 20 cm long capacitors in my baggage for a test I was doing at a customer site. The TSA pulled my box and called me back to the check-in counter because they were concerned about the energy storage in the capacitors. TSA told the airline that it was their decision whether to accept the parts for shipment. I finally got the airline to accept the box after I licked the capacitors to show they were discharged.
 

steveelliott said:
S1 is taster - You lick it to reset the circuit?

I should tell you a story about the Transportation Safety Administration here in the US. I was shipping some 10cm dia x 20 cm long capacitors in my baggage for a test I was doing at a customer site. The TSA pulled my box and called me back to the check-in counter because they were concerned about the energy storage in the capacitors. TSA told the airline that it was their decision whether to accept the parts for shipment. I finally got the airline to accept the box after I licked the capacitors to show they were discharged.
Click to expand...


This is ok security measures of some air company, just as for lithium base batteries, I think that this is ok. Todays there is planty possibility to make some destruction things, maybe is better to skip airplanes or have own airplane. :smile:
 

[offtopic] Re: Need a solid state fuse

Dear all: Please hire me for freelance work so that I can afford my own airplane, fill it with the entire world's stock of 200mA fuses, and also turn my childhood dream of running an uncharged capacitor transportation and logistics company into reality.

Adding some humor, a friend of mine chanced upon this eBay listing a few weeks ago, which I believe is appropriate here (read the description): **broken link removed**. I have also attached a copy to this post for posterity.

eBay seller said:
There you go again: replacing that wimpy 5A fuse with another 5A fuse, knowing full well that it's gonna blow just like the last one. What you need is a fuse that will give your circuits the fortitude to withstand those transient high-load moments -- a fuse that won't simply melt away -- a fuse that will carry current even as smoke pours from your electrical goodies. What you need is 7.5 Amps!

Just because your electrical system was supplied with insulated wiring doesn't mean it has to stay that way. What is the #1 reason why small wires can't carry "big current?" Well, with all that insulation covering them, your wires simply get hot and melt. Your wiring needs plenty of fresh, cool air so your circuits can dissipate excess heat. Consider the electric company. Sure, they insulate wires attached to houses because they know some safety freak would complain. But when they leave the populated areas and start across the wide open spaces where nobody really cares, what do you see strung between the poles? Bare wires!

Now you can discover the "bare wire advantages" enjoyed by utility companies for decades. Simply replace those inadequate little fuses in all your circuits with these. If you accidentally pop one, put in two the next time. Or three. Or wrap some aluminum foil around the terminals. Then put some additional load on the circuit. Sure, it smells bad for a while, but you'll soon be free from that useless insulation once and for all.
Click to expand...

Clearly modern DMMs still aren't capitalizing on that "bare wire advantage". :lol: I won't sidetrack any more after this.
 

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Dear all: Please hire me for freelance work so that I can afford my own airplane, fill it with the entire world's stock of 200mA fuses, and also turn my childhood dream of running an uncharged capacitor transportation and logistics company into reality.
Click to expand...


We always searching for test pilots of experimental planes. You can bring what you whant in pockets. :smile:
 

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