Some fuses will not open if overloaded by current if the fuse in the circuit has a higher working voltage than specified for the circuit.

Like Kubeek, I would like to see some proof of that. The working voltage of a fuse is more concerned with the retention of the "blown" fuse element than anything else. The higher a fuses voltage the stronger the containment. The amp rating is the current it takes to blow and has nothing to do with the voltage.

 

In my opinion the reverse diode is adequate. because the diode is conducting in the forward direction which instantly lowers the reverse voltage to the voltage across the diode junction, and the fuse will blow or open provided it has the proper operating voltage. Some fuses will not open if overloaded by current if the fuse in the circuit has a higher working voltage than specified for the circuit.

Power rectifiers usually have peak surge rating somewhere in the direction of 30x average rating. Vf can go well over 1V at these types of currents, so damage isn't impossible. The chances of survival improve if the diode fails short circuit - but I've seen one or two split apart.

 

Yes I do. A typical fast blow fuse by Littlefuse Co. Part# 0312002.HXP has the following rating FUSE GLASS 2A 250VAC 3AB 3AG. If the applied voltage us much less than 250VAC and the curent thru the fuse exceeds the 2 amp rating it my not open the circuit. The part about not opening the circuit was information I recall from reading their catalogue. I used to have a catalogue but at present do not have one. In the catalogue they mentioned that although the fuse element may melt somewhat it may not open. What I think is they were recommending that
one should not use the fuse to protect a smaller voltage. Like automobile fuses have a rating as I recall 32V, and they can be the same physical size as the 250V fuse. I would not use the 3AG/3AB fuse on a automobile ckt.

 

Yes I do. A typical fast blow fuse by Littlefuse Co. Part# 0312002.HXP has the following rating FUSE GLASS 2A 250VAC 3AB 3AG. If the applied voltage us much less than 250VAC and the curent thru the fuse exceeds the 2 amp rating it my not open the circuit. The part about not opening the circuit was information I recall from reading their catalogue. I used to have a catalogue but at present do not have one. In the catalogue they mentioned that although the fuse element may melt somewhat it may not open. What I think is they were recommending that
one should not use the fuse to protect a smaller voltage. Like automobile fuses have a rating as I recall 32V, and they can be the same physical size as the 250V fuse. I would not use the 3AG/3AB fuse on a automobile ckt.

I looked at spec's for the Littlefuse parts you referenced and did not find any mention about a minimum voltage.

Can you give a link, data sheet or application note showing that information. Thanks.

 

The simple fact of the matter is that you cannot buy fuses below a rating of several amperes that are not rated for moderately high (125 to 250) AC voltage. If you go to Mouser, for example, and look for a cartridge fuse rated at 250 mA, the lowest voltage that comes up is 25 VAC and there is exactly one out of 264 with that rating, and it isn't stocked.

I have spent a lot of time looking at fuse curves in detail. I don't recall ever seeing anything to suggest the fuses should be applied at some minimum voltage.

 

I did a lot of research yesterday and today and I could not find any data to back what I said about fuses my not open on a lower voltage.
I did find some data that lists fuse voltages and some fuse have rating for DC only. I am convinced I had a couple of brain farts.
I apologize for my posting of miss information.
Ned

 

I am convinced I had a couple of brain farts.
I apologize for my posting of miss information.
Ned

It happens to the best of us.

Few things are more disconcerting than knowing something and then finding out what you know isn't right.

 

Why no one thought that the diode could be a TVS diode?

 

I did a lot of research yesterday and today and I could not find any data to back what I said about fuses my not open on a lower voltage.
I did find some data that lists fuse voltages and some fuse have rating for DC only. I am convinced I had a couple of brain farts.
I apologize for my posting of miss information.
Ned

A fuse link melts - that takes power, so its not only current. The fuse link has a small amount of resistance that you have to develop a certain voltage across. an overcurrent condition such as running the protection diode into conduction will also drop voltage across any wiring it takes to get there. AFAIK: Voltage range is what it can isolate once it breaks the circuit.

Pretty sure Littlefuse have a decent library archive of appnotes.

 

A fuse link melts - that takes power, so its not only current.

I disagree. Current and voltage are obviously linked and we can get lost in the weeds over which matters most, but in the case of a fuse we don’t need to know anything except current. The behavior of the fuse will be identical and predictable based on the current passing through it. We don’t need to consider the voltage except when it might be high enough to arc across the gap of a melted element.

Yes, the voltage across the fuse is what determines the current through it, but fuses are characterized by their behavior against current, not voltage.

 

The fuse will prevent the power supply from reaching the expensive electronics before the fuse blows plus it is cheap.

 

I disagree. Current and voltage are obviously linked and we can get lost in the weeds over which matters most, but in the case of a fuse we don’t need to know anything except current. The behavior of the fuse will be identical and predictable based on the current passing through it. We don’t need to consider the voltage except when it might be high enough to arc across the gap of a melted element.

Yes, the voltage across the fuse is what determines the current through it, but fuses are characterized by their behavior against current, not voltage.

Bit like cold fusion then?

 

Bit like cold fusion then?

I don’t get it?

 

There were a bunch of BS claims about cold fusion that involved nuclear reactions taking place at room temperature with very few reactants. It is often associated with free energy BS.

 

There were a bunch of BS claims about cold fusion that involved nuclear reactions taking place at room temperature with very few reactants. It is often associated with free energy BS.

Oh I know all about cold fusion, but I don’t understand the context why Ian brought it up. I think he’s making a joke but it’s lost on me. Maybe he’s suggesting I’m free BS as well.

 

Oh I know all about cold fusion, but I don’t understand the context why Ian brought it up. I think he’s making a joke but it’s lost on me. Maybe he’s suggesting I’m free BS as well.

The fuse wire is metal - in order to break; it has to get hot and melt.

Methinks there might be a bit of I2R involved in there somewhere.

 

While this is true, a fuse will develop the voltage across it (therefore wattage) depending on the current through it, not the voltage of the overall circuit.

The wattage of a fuse is just like a resistor, it is what it is, regardless of whether you use it in a circuit that is 1000 volts or a circuit that is 10 volts.

 

While this is true, a fuse will develop the voltage across it (therefore wattage) depending on the current through it, not the voltage of the overall circuit.

The wattage of a fuse is just like a resistor, it is what it is, regardless of whether you use it in a circuit that is 1000 volts or a circuit that is 10 volts.

Unless you're cutting it fine with voltage headroom.