Large Scale Central

OK So now Fuses

Electronics
#1

When it comes to fuses, specifically the little glass ones, does it have to be a DC rated fuse for DC application or can an AC rated fuse be used. Lets say I want to protect a 12v DC circuit with a 2 amp fuse. Now I know that voltage ratings are the max voltage that can be stopped by the fuse once it is blown, for example a 32DCV fuse will stop up to 32DCV when it blows but at anything more than that it could arc across and not stop the current. So using a 32DCV 2amp fuse in my scenario is a good choice. But a much more common and readily available fuse is a 2 amp 120 ACV fuse. But in this case are volts volts and the rating is being applied because the more common usage of the fuse is in a 120ACV system?

What I don’t want is for the fuse to blow unnecessarily or worse yet not at all when it needs to. I can get the 32v fuse I know will work but I have to go looking for them or order them where I can get 120ACV 2 amp fuses all day long at ACE.

#2

Look at switches, you often find that the amp ratings are the same for 24 vdc and 120 vac, so I wouldn’t worry about it.

#3

if you are running a 12V supply any 3AG glass fuse will work. Where the problems arise is when the short circuit power is high enough to (not melt) vaporize the element and it condenses on the cool glass or casing and effectively plates it. If you have a car battery and use a 2A fuse to short it you can achieve this but a typical power supply will never source that much power

#4

Fuses do not blow with over voltage, they blow with over current. You CANNOT use a fuse to control over voltage.

They also are rated variously, often something like 200% in 10 seconds or something similar.

This actually is not a super simple subject.

What often works better is the polyswitch, which are effectively self-resetting slow blow circuit breakers, Aristo uses them in their locos:

http://elmassian.com/index.php?option=com_content&view=article&id=552:electronic-components&catid=22:general&Itemid=670

http://elmassian.com/index.php?option=com_content&view=article&id=548:rs3-improvements&catid=14:motive-power&Itemid=665

Greg

Greg

#5

I use polyswitches too. They work fine and never need resetting (once you take away the source of the problem!)

#6

Greg,

I know that fuses are current protection but from what I have read the voltage rating comes into play because at higher than rated voltage the arc can jump the gap created by the missing fuse element. That’s supposedly why you can’t go with a lower voltage fuse. My understanding of it is a 120v spark can possible jump the gap created by a fuse only rated for 12volts after the fuse has blown. So the voltage rating isn’t what the fuse is protecting against but may negate the fuse because the arc created at that voltage can jump the gap. Now thats how i have understood what I have read.

Now for polyswitches. Pete, you have recommended them before. I would like to understand this a bit more. I know what a circuit breaker does and that there are momentary circuit breakers that self reset. Is this something else?

#7

Greg,

thanks for the link to your sight. i am familiar with digikey and will be ordering stuff from them. Instead of a fuse I might see what they have for polyswitches. I like this idea better than the fuse or even the push button circuit breaker I put in my 2-6-0.

#8

Devon Sinsley said:

. . . .

Now for polyswitches. Pete, you have recommended them before. I would like to understand this a bit more. I know what a circuit breaker does and that there are momentary circuit breakers that self reset. Is this something else?

I think Greg recommended them too.

You can think of these as self-resetting circuit breakers. Greg posted something on the other thread about how they work. The big advantage is that the 1A polyswitches can be installed in parallel for higher current protection. I put about 6 on my Aristo Pacific !

Raychem “Polyswitch” # RXE-065S-1. General purpose circuit protectors for power supplies, alarm systems, speakers, motors etc. Unlike traditional fuses, they automatically reset when fault condition is cleared. Can be paralleled for higher ratings. Current: 0.65 Amps. Trip Current: 1.3 Amps. Max Voltage: 60 Volts. 0.33" diameter. UL, CSA listed. $0.20 each.

http://www.allelectronics.com/make-a-store/item/rxe-065/resettable-circuit-protector-0.65-1.3-amp/1.html

#9

Devon, in the copiers I work on, the DC circuits are protected, well, some of them are protected with fuses. We are talking DC 5 volts, DC 12 volts, DC 24 volts and DC -5 volts. Printed on all of the glass fuses is 120vac, and the current rating, 3 amps, 5 amps, 10 amps and 15 amps. For a fuse, amps are what counts. Theoretically (http://largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-wink.gif)(http://largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-wink.gif)a fuse is zero ohms, so there is no voltage drop across the fuse. so volts would not matter. What you are saying about arcing, I have not heard of, but I suppose its possible.

Poly-switches increase their internal resistance very rapidly when there is an over-current condition, and effectively open the circuit. They are in the one power supply I have, and they trip long before the inline fuse blows. The issue with the poly-switches in my power supply is, they don’t reset when the short is removed, I have to turn off the power and then they will reset. I think that is a sign that they aren’t working quite right, because my previous power supply would reset without turning off the power.

#10

OK looking at Digikey and their poly switches (Greg I can’t seem to find the exact one you specify) I see max current, hold current, and trip current. That’s just a bit confusing. Also I see time to trip. Which of these numbers would be critical? I would think that a fast trip time would be preferable. Also I see the OHM rating and after reading on Gregs sight the lower OHM rating would be better.

Seems like a 3A polyswitch is pretty common at 120mA (1.2A) hold current 240mA (2.4A) trip current and down as fast as .75 second time to trip. have a OHM rating of 8-10.5 and are .72 cents for a 1 or .60 for 100. They are 60v rated. Is this what I should be looking at?

http://www.digikey.com/product-detail/en/TRF250-120T-R2-B-0.5/TRF250-120T-R2-B-0.5-ND/1774010

Edited because Petes post came through while I was typing

#11

When paralleling them is it a multiple of the single? What mean is are 2- 1A polyswitches going to provide 2amps of protection and 3 going to provide 3 amps. or in case of a 2A polyswitch would it be 4 amps for 2 and 6 amps for 3. Is it linear like that?

Thanks for the link Pete. Digikey has similar though I cant find that exact one but I think I get the picture now.

#12

Devon, your conversion of milliampres is off by a decimal point. IE, 240ma is .24 amps.

And yes, you are correct in paralleling them, provided that they are matched. If one has a bit more resistance then the other, then you wont quite get double the current by having 2 in parallel

#13

David Maynard said:

Devon, your conversion of milliampres is off by a decimal point. IE, 240ma is .24 amps.

That would make more sense.

#14

I use these and double them up for the USA trains.

http://www.allelectronics.com/make-a-store/item/cpx-100s/resettable-circuit-protector-1a-2.5a/1.html

But they also heve these:

http://www.allelectronics.com/make-a-store/item/rxe-065/resettable-circuit-protector-0.65-1.3-amp/1.html

and these:

http://www.allelectronics.com/make-a-store/item/rsf-304/4a-30v-ptc-resettable-fuse/1.html

and these:

http://www.allelectronics.com/make-a-store/item/rue-250/2.5-5a-resettable-circuit-protector/1.html

#15

Devon Sinsley said:

When paralleling them is it a multiple of the single? What mean is are 2- 1A polyswitches going to provide 2amps of protection and 3 going to provide 3 amps. or in case of a 2A polyswitch would it be 4 amps for 2 and 6 amps for 3. Is it linear like that?

Thanks for the link Pete. Digikey has similar though I cant find that exact one but I think I get the picture now.

Pretty much as you say. When one goes, the other instantly follows.

#16

I’ve gone to 6 amp polyswitches in all my latest installs. First thing in the circuit coming off the battery connection. They are cheap to buy from a electronics supply like Mouser.

#17

Paul Burch said:

I’ve gone to 6 amp polyswitches in all my latest installs. First thing in the circuit coming off the battery connection. They are cheap to buy from a electronics supply like Mouser.

6A? I am not disagreeing with you but that is contrary to what I have been told on a couple of occasions. I imagine this would be highly dependent on what your wanting to protect. Tony at RCS Australia recommended 3 Amp protection straight off the battery to protect his ESC. And the current project which is to build the LED controller ESC all the parts there are rated at 2 Amp.

So As Pete is doing it would make sense for me to use multiple 1A versions in parallel to achieve the required protection. That’s to say Because I am going to buy a bunch in bulk. I can see need 6A protection on some stuff but I can also see that not protecting certain other things.

#18

Devon,

The 6 amp is there to protect the overall current draw. I run Airwire. It has its own protection built in for its outputs.

#19

The problem with understanding these Devon (remember I said it was not simple?) is that there are other factors at work here.

The trip current is the current where they “open”, but the hold current is where they do not open. The issue is that they do not work like computer logic, hold at 3 amps and trip at 3.001 amps…

The way they work internally is current going through them makes heat, enough heat and the material inside melts and they go to a high resistance state (open).

So, if they have been running NEAR the trip current for a while, they can open because of the accumulated heat, also ambient temperature has an effect.

There are also (if you read my web site) higher precision / tighter tolerance / faster acting versions.

A rough rule of thumb is to run double the rating you expect to need continuously. So if you want to be able to handle 3 amps continuously, then a 6 amp one would make sense (Thus Paul’s choice is good)…

Another item, is if you buy the cheap ones, paralleling them does not work nearly as well as getting the right amp rating in the first place, minor differences in resistance will make one trip long before the other in a parallel situation.

Like I said, not simple.

Greg

#20

Greg,

That makes sense, I followed that logic. Certainly they aren’t going to break the bank so having a few sizes isn’t a problem. When you say cheap, and after reading your article on the subject, it doesn’t seem price is the factor so what makes a good one versus a cheap one. In your article you mention the lower resistance or Ohm rating but what is the base line, what IS a good Ohm value or is it a matter of lower is always better?

I mean if I had to pay twice as much for them to get good ones I am out 80 cents. So price isn’t really a factor. I actually tired to find the one you list on digikey and couldn’t find it. So absent of that one I am wondering what a good one is from a numbers standpoint.