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Fuses?


EmmienLightFan

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Which fuse would I need 

 

I have a 30amp 5v power supply, and on the 220v side, it has a 1amp fuse (Normal plug fuse) and the plug is a 16a "MK Commando". I think I have found the appropriate fuse holder and fuse for that

 

http://cpc.farnell.com/multicomp/mchtc-30m/fuseholder-screw-cap/dp/FF01813

And a 1amp fuse from a UK plug

 

 

I want to put a fuse on some of my other stuff, including on the output of the SMPSU, my Matrix, and my Arches, as well as in the junction box where the power for my pixels is distributed. 

 

Which type of fuse do I need?

I have seen a mix of vehicle fuses and fuses that look like plug fuses used by people on here.

Which do I use and which holders?

 

Also, I have a 30volt 4amp SMPSU as well, which could probably do with being fused, would the same fuses work for that?

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Em,

 

There are quite a variety of fuses and fuse holders out there.

 

First, you should select the fuse holder type you would want to use.  Make sure that they are rated for at least the max voltage and current of your circuits.  The holder you show rated up to 250 vac/vdc and 6.3 amps.

 

Then, you would select fuses that are appropriate for that holder. The fuse voltage rating must at least meet your circuit voltage (you can always go higher). The current rating should be 125% of your maximum circuit current. 

 

Hope this helps,

Rayburn.

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Em,

 

There are quite a variety of fuses and fuse holders out there.

 

First, you should select the fuse holder type you would want to use.  Make sure that they are rated for at least the max voltage and current of your circuits.  The holder you show rated up to 250 vac/vdc and 6.3 amps.

 

Then, you would select fuses that are appropriate for that holder. The fuse voltage rating must at least meet your circuit voltage (you can always go higher). The current rating should be 125% of your maximum circuit current. 

 

Hope this helps,

Rayburn.

Thank you very much. I understand a lot more now.

 

Could I use a 30 amp consumer unit fuse (250vac) for a 5 vcd circuit?

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Yes.  Fuses only care about amps (as far as their ability to blow).  They also have a maximum voltage rating.  The problem when the voltage rating is exceeded is that when the fuse blows, the current may just arc across the blown segment of the fuse resulting in current continuing to flow.  BTW, note that most fuses and circuit breakers have different voltage ratings for AC vs DC (DC is much lower).  Not a real issue when we're talking about 5 and 12 stuff, but when dealing with higher stuff (like electric cars for example) it's a major concern.

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Yes.  Fuses only care about amps (as far as their ability to blow).  They also have a maximum voltage rating.  The problem when the voltage rating is exceeded is that when the fuse blows, the current may just arc across the blown segment of the fuse resulting in current continuing to flow.  BTW, note that most fuses and circuit breakers have different voltage ratings for AC vs DC (DC is much lower).  Not a real issue when we're talking about 5 and 12 stuff, but when dealing with higher stuff (like electric cars for example) it's a major concern.

Ok. Thank you. I am deciding on the best fuses for different props now.

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Another question; could I use a fuse higher than 6.3 amps if I am using 12 volts if the fuse holder is rated at 6.3 at 250VAC?

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If the fuse holder is rated at 6.3 amps, you should not exceed that.

Ok.

 

 

If I use ATO fuses at 5v, will the LEDs to show the fuse has blown still work even though the LEDs were made to light up between 10 and 40vdc?

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I have not tried this. But would assume it will light but at a much dimmer level. There will be a current limiting resistor in series with the LED. Going back some years ago, I had an application were we had some LEDs indicating a switch was on. This was on a 30VDC circuit and we ended up with a 1.8K ohm resistor. I bet you will find such a resistor in your fuse.

 

So, yes, but much dimmer me thinks.

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The fuse thing is confusing me not that it takes much!  I am not disputing what is being said that a higher voltage rating is ok, but it does not make sense to me at the moment.

 

From my days of taking electrical in school, voltage + current (amps) produce energy frequently measured in watts.  This energy can be in the form of heat, light motion or a combination of forms.

 

If a fuse is rated at 250 volts and ten amps, then that fuse should be able to "pass" 2500 watts before it melts.  If I am correct it is the melting which opens the fuse.  Now if this same fuse can be used on lower voltages, would it still take 2500 watts to melt open when supplied with 120 volts or would it now be 1200 watts?

 

Could somebody please explain how it would take less wattage (the product of voltage and current) to melt the wire in the fuse at a lower voltage?

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GRG,

Yes, you remembering that Watts equals Volts times Amps (not V + A as you wrote).  However the only thing that volts has to do with a fuse rating, is the max voltage that the opening fuse can interrupt.  A fuse is a resistor - a very low value, but a resistor none the less.  When current passes through a resistor, you get a voltage drop.  It's that voltage that is multiplied by current to produce the watts that melts the fuse.  All the fuse cares about is the voltage across itself.  Let me give an example (with made up numbers - at 2.0 amp of current).

 

Voltage out of power supply:  13.000 V

Voltage at source end of fuse:  12.999 V

Voltage at load end of the fuse:  12.995 V

Voltage across the fuse:  0.004 V

Power dissipated in the fuse:  0.008 W (.004 V x 2 A)

 

Now lets change that to an overload condition where there are 20 amps flowing through the fuse:

 

Voltage out of power supply:  13.000 V

Voltage at source end of fuse:  12.990 V

Voltage at load end of the fuse:  12.950 V

Voltage across the fuse:  0.040 V

Power dissipated in the fuse:  0.8 W (.04 V x 20 A)

 

In fairly short order the fuse gets hot and melts open.  Now just after the fuse opens:

 

Voltage out of power supply:  13.000 V

Voltage at source end of fuse:  13.000 V

Voltage at load end of the fuse:  0.000 V

Voltage across the fuse:  13.000 V

Power dissipated in the fuse:  0.000 W (13.000 V x 0.0 A)

 

Now, you can see where the voltage rating comes into play.  Not a real issue with low voltage, but it is an issue as the voltage goes up.  The standard ATC automotive fuse is generally rated at 32 volts.  If you put 200 volts across that fuse, it will carry it just fine (as long as the current is not exceeded), however as soon as the fuse blows, the full 200 volts is across the fuse that it will very likely arc across the open in the fuse resulting in the power continuing to flow.  When you get into high voltage fuses (thousands of volts), all kinds of stuff has to happen in the fuse to make sure it will actually interrupt the current when the fuse blows.

 

Also note that at anything except low voltage fuses, they are rated for AC only, or have different voltage ratings for AC vs DC.  The reason is that AC power passes through zero volts many times per second as a function of it being AC power and therefore can handle a higher voltage at AC then DC which does not have zero crossing points.

 

What causes a fuse to blow is heat.  If a particular fuse can handle 1 watt of heat, that does not mean that it will fail instantly at 1.001 watts.  The higher the overload, the faster the heat buildup and the faster the fuse will blow.  The result is a curve on a time vs percent overload chart.  Most fuses will take a 10% overload for a very long time (maybe as much as hours), and most will blow almost instantly at something between a 500 & 1000 percent overload.  Also because it's heat that causes a fuse to blow, the fuse holder's ability to dissipate heat will also affect the time to blow.  This is more of an issue with larger fuses that do produce quite a bit of heat.  Also, the ambient temperature will affect the fuse.  Therefore, a fuse under the hood of your cold car in -25 weather will take a lot more current to blow that in the same car while driving across the desert in the heat of the afternoon.

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