# Fuse rating for power supply



## JackM (Jul 29, 2008)

My guess is there's a simple formula, like P=IE, but I sure don't know what it is.

Rather than have a bare power transformer sitting around, I cannibalized a metal box for it and added on/off switch and fuse holder. But I don't know what fuse to use. 120V AC in, 18VAC 10 amp out. Too low and it'll keep popping. Too high and it won't provide any protection. What's the secret formula? (Transformer manufacturer's website provides no information at all.)

jack


----------



## Greg Elmassian (Jan 3, 2008)

Secret formula: fuses normally blow at 200% of rating. 

I assume you are using automobile-style blade fuses. 

Best way is to experiment and find the lowest value fuse that you don't blow often. 

Depends on your load, you might try measuring it. 

Also, there's more... do you want your fuse to blow before the booster shuts down? Doesn't your booster have it's own circuit breaker? 

More info please. 

Regards, Greg


----------



## SteveC (Jan 2, 2008)

Jack

Based on your question and your statements regarding the addition of the metal enclosure and ON/OFF switch. It might be a good idea to explain just how you went about accomplishing the latter (i.e. enclosure & switch, also the power cord), because there's a proper way and a 'not-so-safe' way of doing things. And I don't think anyone here wants you to encounter any nasty surprises.


----------



## Dean Whipple (Jan 2, 2008)

*http://4largescale.com/trains/P28.htm*
I found these Polyswitch's a while back, I got them from * ALL Electronics* and use them when ever I need a fuse, I wire them in parallel them to obtain the necessary rating, I use them in my battery powered engines, I use them on the "Aristo TE" instead of the 10 amp fuse, they're cheap, and you will never have to buy another fuse, plus they offer superior circuit	protection...they work so fast you might think there not working...check out my "*How-To*" page....


----------



## Greg Elmassian (Jan 3, 2008)

For motors and wiring, I think they are ok, although I don't like the loss when you operate one near it's rating. 

But I'd like to see some documentation on how fast they trip, I looked up the 10 amp automotive one, and it was guaranteed to trip within 15 seconds at 50 amps... no thank you for my electronics.... that's too long in my opinion... gradual overloads ok, but short circuits, and electronic components... 



*http://www.tycoelec...div>

Tyco is a representative company, well respected..


Regards, Greg*


----------



## TonyWalsham (Jan 2, 2008)

Greg, 
Dean uses multiple lower rated ones in parallel. 
That way they trip much faster than a single one does. 
Instead of a single 6 amp Polyswitch to protect my 6 amp ESC's I use two 3 amp ones in parallel. Never had a failure since I started doing that. 
I guess because every one is slightly different from another, with a sudden short, one will trip first and then the second one an instant after. 
Together they can handle a gradual overload just fine.


----------



## Greg Elmassian (Jan 3, 2008)

I'd like to see the manufacturer's data sheets. 

I understand the idea of putting them in parallel, but it does not make all the current go through a single unit, paralleling them to handle more current is also on their site. 

Facts gentlemen... please.... I do think people are overlooking what I said about electronics being damaged, vs. a motor or drivers that can take some abuse. 

Remember we are talking about a DCC system power supply here, not a decoder, or motor driver, or a DC loco. 

Regards, Greg


----------



## Dean Whipple (Jan 2, 2008)

Greg, 
I've been using them for 6-7 years at least, both on battery units and on (2) of the Aristo Train Engineer 10amp (I got tired of replacing 10 amp auto fuses) walk around remote control units, and they do trip _instantly_ (nano seconds) .....and they reset just as quick....there cheap buy some and try them they work great!!


----------



## Axel Tillmann (Jan 10, 2008)

First off all I have to agree with the Polyfuse concept. We have converted all of our frog protection in our switches to polyfuses. I think Littlefuse (the maker) deserves a Nobel price for it.









Second in this application I would suggest two fuses, one on the primary side and one on the secondary side. If you find within your range a fast tripping one and a slow tripping one I would use the slow tripping one on the primary side rated at 150V and trigger at 1.5A and on the secondary side a Polyfuse rated at 25 or better yet 30V fast tripping at 10A. This would cover you for all eventualities.

If the specs of the Polyfuses are a little too "overwhelming" then give them a call. They have an excellent engineer advisory that discusses your application and give you the proper recommendation.


----------



## JackM (Jul 29, 2008)

In answer to the questions about my unit... I purchased the NCE 10amp system, including their transformer. Can't have it sitting loose, so I gutted a regulated DC power supply that I've had for ten years and never used for anything - nice enclosure.

Fuse and switch are on the hot wire - the thin blade. NCE instructions are surprisingly silent on how to plug this thing into the wall, and no mention of fusing it that I can find. I still need to cover up the hole where the heat sink was on the original unit. I haven't been able to find out anything more than that - manufacturer (Altran) has no info about it on their website.

Thanks for your advice.


----------



## JackM (Jul 29, 2008)

I should've mentioned that I used the fuseholder and switch that were already in place in the enclosure. Short glass-type fuse. I haven't gotten very far in the NCE manual yet, so I really don't know if there's a circuit breaker elsewhere in the system. None that I can see on the various units, nor any instructions about how to plug this transformer into the wall. (Isn't this the "booster"?) 

jack


----------



## Dan Pierce (Jan 2, 2008)

For the transformer input fuse, I would go with a 2 amp. 

Reasoning is the output is 18 volts at 10 amps which is 180 watts, plus you need to add in the transformer loss of at least 5 per cent. 

This would be slightly more than the 1.5 amps Axel quoted, and the next common fuse size is 2 amps. 

I would fuse the secondary (output) at the rated load max of 10 amps. 

Greg's comments on power should be adhered to as an electronic device can go up in smoke internally faster than a fuse/circuit breaker/polyfuse can react. 

Never use a slow blow fuse if you want to really protect electronics!!! 

I prefer the fast blow (wire springs attached to the lead in the fuse) and even these are not fast enough in some cases to protect electronics, on the transformer primary (input).


----------



## George Schreyer (Jan 16, 2009)

Fuses come in a variety of configurations that are intended for specific usages. They all protect circuits, but they do it following different paths. 

Fuses will typically sustain 135% of their rating indefinitely. At currents above 135%, they wil blow eventually, higher currents cause them to open sooner. At 200%, they are designed to blow "immediately" but that could still take a second or more. In all cases, fuses are slower than electronics so that they protect against collateral damage only. 

If the application anticipates regular overloads (DCC boosters and such), then the electronics must work to protect themselves. Fuses are not fast enough. The fuses are there to protect against failures within the equipment from propagating. 

Most semiconductors have a thermal time constant of the semiconductor itself of about 50uS, much faster than fuses. Their packages have thermal time constants of milliseconds and heat sink time constants of seconds. The fuses will protect the electronics if the overload is beyond the capability of the heat sinks to dissipate. If the devices cannot handle it, they will blow long before a fuse does. 

Slo-blow fuses are intended for the AC power input to electronic devices that have significant in-rush current. It is normal for some electronics (like power supplies for moderate to high power equipment) to draw a big rush of current at turn on as the filter capacitors charge. The slo-blow fuse is intended to ingore short surges, but still open when the sustained current is high enough. The intention is to prevent a fire. 

Fast blow fuses, including automotive types, are designed to blow as quickly as possible, but they are still pretty slow. These are intended to protect against faults that have NOT blown the electronics yet. They are intended to protect electronics against overheating due to sustained over-current. 

Poly-switches are intended to protect electronics where overloads are anticipated but physical accessibility is limited. They self reset but trip slower and have more residual voltage drop than one-time fuses. They will protect a DCC decoder or RX from a sustained overload that the decoder can handle for a couple of seconds but would be fatal if it continued.


----------



## Hans Larsson (Apr 19, 2009)

Posted By Greg Elmassian on 22 May 2010 10:51 PM 
I'd like to see the manufacturer's data sheets. 


The manufacturer's data sheets are available at the manufacturer's home page.
http://www.circuitprotection.com/literature.asp

Look almost halfway down at *PolySwitch Resettable Devices*.

They are a little on the slow side, I think.


----------



## Greg Elmassian (Jan 3, 2008)

Thanks Hans, but I DID already provide a direct link to a polyswitch datasheet (see my previous post) and it says what I said it says. 

Anyone else give me a link to a data sheet that says faster (nanoseconds)?

From a physics point of view, melting a solid into a liquid is normally slower than vaporizing a small fusible link. 


Facts please. 


Greg


----------



## SteveC (Jan 2, 2008)

Posted By JackM on 23 May 2010 06:26 AM 
_{snip...}_ Fuse and switch are on the hot wire - the thin blade. NCE instructions are surprisingly silent on how to plug this thing into the wall, and no mention of fusing it that I can find. _{snip...}_
Jack

The only suggestions I've got is to change the power cord to a three wire grounded type, attaching the green wire to the cabinet, and if the transformer cabinet is going to be outdoors then make sure the electrical circuit is GFCI (Ground Fault Circuit Interrupt) protected.


----------



## Greg Elmassian (Jan 3, 2008)

and a fuse on the 110v side? Or a resettable circuit breaker? 

just some more ideas... 

Regards, Greg


----------



## Tom Bray (Jan 20, 2009)

I see that this thread has been dead for a while but I thought I should comment anyway. 

Just a few of notes about fuses. First, fuses have what is called a time - current curve which describes how long it will take to blow at any given current rating. The curves are different for slow blow versus fast blow. A failure such as shorting out the track out will usually cause a lot of current to be drawn and the fuse blows pretty quickly, regardless of the type. 

Also fuses have a damage curve which show that, for example a 10A fuse, if you draw 10A for an extended period, the fuse gets damaged and may either blow unexpectedly or could take longer because the metal has migrated. 

I set fuses, for the most part, based on the wiring being used. For example, if using 14AWG wire and it is longer than a few inches, I'll use the standard 15A fuse (just like for house wiring). Not long ago I violated that with my lighting circuit - one of the lamp post shorted out the 24AWG wires that went to the light - the fuse didn't blow but there was a lot of smoke until the wires melted. No damage except for the burned wires. 

If you are going to fuse the transformer on the primary side, for a 10A transformer I would pick something no less 3A, maybe 4 - 5A if I didn't have concerns about the transformer. If the transformer is rated for 10A output, that translates into 240W at 120V which is a solid 2A. The transformer will supply far more current than that, the voltage will drop, potentially down to 0 volts. If transformer does this for very long, it will heat up internally and melt the insulation, causing a short internally which is the end of the transformer beyond using it as a door stop. 

The other thing to be aware of is that your DCC controller has built in circuit protection that should trip much quicker than any fuse will. You may want to talk to your booster vendor and find out what they recommend for a fuse on the input of the controller to protect the controller from burning up if there is a problem with that. 

In general I am not a big fan of fuses because I have one engine that loves to jump the track (Eggliner) and it usually shorts out the track, blowing fuses. Circuit breakers in the long run are much easier to maintain. 

Tom


----------



## Greg Elmassian (Jan 3, 2008)

Simple calculation, the input voltage is about 10 times the output, so the output current is about 10 times the input.... 

10 amps on the output will draw about 1 amp on the input...... 4-5 amps on the input is way too high for protection if you are trying to get the fuse to blow over 10 amps on the output... 1.5 amps on the input would be reasonable... 

BUT!!! this is a transformer... so you need a slo-blow, since you can get a startup surge (of course depends on the load that is tied to the output) 

I agree heartily with your "sizing" of the fuses AND the output wiring... putting a 15 amp fuse in a circuit that can only draw 10 amps because of wire size will just let stuff melt down during the short. 

Unfortunately, many people blow fuses and just keep putting larger ones in, when there may be some other problem. 

Regards, Greg


----------

