# Electronics Question re: Determining Fuse rating



## greco (Jul 15, 2007)

I just finished building a low voltage AC power supply.
The reason for this is to try some simple electronics experiments using a "safer" source of VAC
It is very basic, but well made (I'm quite proud of doing a decent job on this)...LOL

I am thinking of adding a fuse, as I have several fuse holders that are the same that are typically used on
amps. I have left enough wire in the enclosure to add the fuse holder.

The step down transformer is from a clock radio and is centre tapped (~12 VAC - ~6 VAC- ~12 VAC)
I no longer have the clock radio to get any idea of how many watts it was rated for (IF that spec was even listed somewhere on it)

Given my understanding, the turns ratio is about 10 : 1 (120 VAC primary divided by 12 VAC secondary is also equal to Number of turns primary divided by Number of turns secondary) ..Correct?

I am using Vp/Vs = Np/Ns = Is/Ip (no losses are being accounted for)
Current secondary divided by Current primary should also be equal to Volts primary divided by Volts secondary.

The current draw under the load of a 560 ohm power resistor (5 watts) seems be about 20-24 mA.
But what load should I be using to help me get calculate what the VA rating for the transformer is? 
There are no markings on the transformer (re: VA (watts) rating)..only a part number or something...an internet search of this number got me nowhere.


I know that a slow-blow fuse is recommended...but how do I know what max current rating to put in?
I am aware that the fuse is certainly overkill, as the transformer is small and inexpensive...but it would be nice to protect the transformer, just to save having to replace it if I overload it during an experiment.

I know this might seem like Grade 9 electronics to some...but I want to understand this stuff and apply it.
I have tried to read about it, but I am missing something...somewhere.

Many thanks for your comments/suggestions.

Cheers

Dave


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## keeperofthegood (Apr 30, 2008)

Dave, in my experience I have found that math simply fails for this. Big old transformers, line losses, power factors and so on make this math a headache. The easier math and what I would do is go to the library and get a watt metering device. Here at least you can get them. Plug in, play and see what you are drawing at the most. Put a volt meter across your line at the same time too (unless those watt meters from the library have that as a function as well heck if it tells you the current being used even simpler!! and they are compensated meters because the government intended them to be used for people to see how bad their old washers and fridges were) and get the voltage across the amp. Then put that into this:


Watts = Current * Voltage, no ^2 losses here, as I said, meter should already read compensated. If you get Wattage from the meter, and Voltage then you know your working Current by (unless again it simply gives you the current, bonus if it does):

Current = Wattage / Voltage

Running current should be 80% lower than what is capable to be supplied. So, in a typical house 15 amp plug, the highest Current draw should be 12 amps. Hold the amp to the same standard. So, if the Current you were drawing was 12 amps you'd put in a 15 amp fuse.

However, not the case LOL working all that math backwards you end up with:

Fuse = ( Wattage/Voltage ) / 0.8

So, if you found you were topping at 5 watts at the most while jamming, and your line voltage you measured to be 125 volts then

(5 / 125) = 0.04 amps
/ 0.8 = 0.05 amp fuse

that is a 1/20th amp fuse which works out to be a wee thing looks like a glass resistor.

So my approach is: get a read on what you are pulling when in use, and your line voltage and go from there


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## greco (Jul 15, 2007)

Thanks Keeps



keeperofthegood said:


> Dave, in my experience I have found that math simply fails for this. Big old transformers, line losses, power factors and so on make this math a headache. The easier math and what I would do is go to the library and get a watt metering device. Here at least you can get them.


I have never heard of these devices being available at a library...interesting



keeperofthegood said:


> *Plug in, play* and see what you are drawing at the most.
> So, if you found you were topping at 5 watts at the most while *jamming.....*


 This is not an amp...just a basic power supply for VAC electronics experimenting. It would be difficult to know what I am drawing (as a maximum) as it would depend on the components, etc in the experiment(s)



keeperofthegood said:


> (5 / 125) = 0.04 amps/ 0.8 = 0.05 amp fuse
> that is a 1/20th amp fuse which works out to be a wee thing looks like a glass resistor.


What are the smallest rated glass tube fuses available for the type of fuse holders that are typically used in amps....any idea? I'm betting that 0.05 (or anything near that low of a rating) amp fuse is likely not available for this type of holder 
(if I need a fuse of about that rating).

The "wee thing looks like a glass resistor" sounds like an interesting little fuse. 

*Conclusion*.......Maybe I should just drop the whole idea of putting in a fuse. 

Cheers

Dave


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## keeperofthegood (Apr 30, 2008)

Ahh Dave, hadn't caught that. Long day, hope all is well with you 

In glass buss fuses both cartridge as well as solder tails I have found down to 1/16 amps such as on this data sheet http://www1.cooperbussmann.com/pdf/50eaa38c-9df3-4fe7-870d-76d2502e3a87.pdf though I am unsure of their general availability and there are several glass fuses of the guitar amp style so if you have a fuse name code that would be the place to start.


Ack it is also late. Those ones could be AGC fuses too, and here, mouser

http://ca.mouser.com/Circuit-Protection/Fuses/_/N-5g3l?Keyword=agc+fuse&FS=True&Ns=Current%20Rating|0

Yes, if the price doesn't send you screaming you're a better man than me because I just kinda vomited into my own mouth when I read the pricing on them :C


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## greco (Jul 15, 2007)

Hi Keeps:

Thanks for doing all the research. You are right, those fuses are much too expensive to consider. 
They are like "boutique" fuses...probably the demand for them is quite limited.

BTW, I looked at my present clock radio and it is consumes/is rated for 4 watts, so your guess of 5 watts was very reasonable.

Hope all is well you also. I read the thread about your wound...hope it is healing quickly.

Cheers

Dave


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## keeperofthegood (Apr 30, 2008)

Hey thank you Dave. Yes, the swelling and bruising is looking better and the general OUCH factor is actually feeling better today  Puckered skin means a neat-o scar when it heals but I do like neat-o scars 

You could look at making a switch instead? Something that turns your supply off is too much current flows instead of a fuse. I can ask about for some circuits that do that if it interests you. Probably an op-amp (for voltage comparing) and a jfet constant voltage reference and a transistor to act as the switch plus parts would do it.


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## greco (Jul 15, 2007)

keeperofthegood said:


> You could look at making a switch instead? Something that turns your supply off is too much current flows instead of a fuse. *I can ask about for some circuits that do that if it interests you. *Probably an op-amp (for voltage comparing) and a jfet constant voltage reference and a transistor to act as the switch plus parts would do it.


Thanks for the offer Keeps, much appreciated, but that seems like "a long run for a short slide" sort of thing.
I'll just keep it simple for now. We have a local surplus place and I will look at their fuses....you just never know.

Cheers

Dave


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## keeperofthegood (Apr 30, 2008)

Hahaha no problems Dave.

I have become a convert to Big Bang Theory of late. Everyone in my family compares me to Sheldon the most. But when they redesigned Pennies entertainment centre everyone in the family turned and looked square at me :bow:


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## jb welder (Sep 14, 2010)

Fuse the secondary. 
As KOTG pointed out, fusing the primary requires a tiny fuse that is much too expensive (probably because of the precision required of the fuse). The clock radio will not have had such a tiny fuse, but CSA would require something to prevent meltdown/fire, so I expect there is a thermal fuse in the primary winding. However, you want to prevent having to replace the transformer. We can assume the 4 watt consumption of the clock radio was not the max. capability of the transformer (good thing you found that rating, it is otherwise impossible to determine). 
So, if the transformer is good for 5 watts, in a perfect world there would be no losses and the secondary would be good for 5 watts. But in the real world there will be losses so lets say the secondary is good for 4 watts. At 12VAC output, that would mean a .3A fuse, so try a 1/4 amp. Run it into a load so you are drawing close to 250mA, like a 50 ohm 5watt. Does the transformer get hot? If not, you might even try a 1/2 amp, load it so it draws near 500mA (25 ohm 6 watt) and see how hot the transformer gets. I say this because it's possible the VA rating of the transformer was quite a bit more than the 4 watts listed as consumption. With the 1/2 amp on the secondary, you will blow the fuse at around 6 watts consumption at the secondary. Keep track of what the voltage is doing under load and adjust calculations accordingly.
Also, you said "is centre tapped (~12 VAC - ~6 VAC- ~12 VAC)". Not sure what you mean, what I have said is for 12V output voltage (loaded), if it makes sense you can adjust to whatever output voltage you are using.


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## J-75 (Jul 29, 2010)

FYI, the 'tiny' fuses are available as BUSS nanofuse. They're about 4 bucks each, and solder-in, axial leads, look like a 1/4 watt resistor.


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## jb welder (Sep 14, 2010)

J-75 said:


> FYI, the 'tiny' fuses are available as BUSS nanofuse. They're about 4 bucks each, and solder-in, axial leads, look like a 1/4 watt resistor.


Good point, also known as pico fuses. Down side is they are not "quick change". Another option may be PTC (resettable) fuses, which reset themselves after cooling. Quite inexpensive.


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## keeperofthegood (Apr 30, 2008)

I did mention those fuses too. Dave was asking about standard fuses but that is an expensive option. If it was me I would do as I mentioned and build a circuit breaker. In doing some "experimenting" popping a fuse is part of the "oh doh shouldn't aughta a done that" experience 



greco said:


> I just finished building a low voltage AC power supply.
> The reason for this is to try some simple electronics experiments using a "safer" source of VAC
> It is very basic, but well made (I'm quite proud of doing a decent job on this)...LOL
> 
> ...


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## J-75 (Jul 29, 2010)

jb welder said:


> Good point, also known as pico fuses. Down side is they are not "quick change". Another option may be PTC (resettable) fuses, which reset themselves after cooling. Quite inexpensive.


jb's just being polite - he's 100% correct about "pico fuses", THAT'S their ONLY name. My crappy memory mis-fired when I called them nanofuse. Well, I was only wrong by three orders of magnitude!


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