# Electrical/Electronic Fuse Spec's

• posted on June 8, 2011, 11:33 pm
I need an education in fuse sizes.
I want to put a fuse ahead of a 120vac transformer putting out 6 or 12vdc to a component rated at 15 amps.
At the store, I was going to buy a 120v 15 amp fuse, but all the fuses for AC were listed at 250v.
What do I put in front of this transformer?
--
croy

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• posted on June 8, 2011, 11:56 pm
wrote:

The voltage rating of the fuse is a maximum only. You can use a 600 volt fuse on a 6 volt circuit, but you can't use a 150 volt fuse on a 600 volt circuit.
If you want to fuse the low voltage side, you want to fuse it at or above the load current, but below the transformer current rating. If you want to fuse and protect the primary, you will want to fuse it at about 1.5 amps.
I say "about" because I don't know exactly what transformer you have - and what the pawer (in Va) rating is.. You take the va rating, and devide by input voltage for the input fuse, or the output voltage for the output fuse.
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• posted on June 8, 2011, 11:57 pm
On 06/08/2011 07:33 PM, croy wrote:

fuses don't limit voltage, they limit current. so 250V fuses are appropriate (what that means is, they will reliably interrupt 250V, so they should be fine for 120V.)
Now the question is, how much current does the xformer draw when loaded to 15A at 12V? It ain't 15A. It probably isn't 1.5A either (that would imply that it is 100% efficient - remember, P = IV) probably something around 2A or 2.5A would be appropriate
good luck
nate
--
replace "roosters" with "cox" to reply.
http://members.cox.net/njnagel
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• posted on June 9, 2011, 12:28 am

You also don't say what device it is, makes all the difference. Incandescent lighting has extreme start up currents. Capacitor input equipment Also.
Transformers matter also. Some tend to use power by itself. Also, transients, depending at which point it's switched on in the cycle matter.
That being said you would need at least a 2ASB minimum. 3 A might also work. Ju st guessing minimums.
250 volt is the one to use on the input or output being AC. DC fusing is slighty different because of arching.
Greg
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• posted on June 9, 2011, 12:31 am

Let's see if this one posts.
WHAT ARE YOU POWERING! AND ARE YOU SWITCHING IT ON AND OFF???????
Greg
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• posted on June 9, 2011, 7:55 pm
On Wed, 8 Jun 2011 19:54:57 -0400, "Stormin Mormon"

I was going to put the fuse between the line source and the transformer, as a general safety feature.
Transformer is:
Pri: 120v; 60hz     Sec: 6.3-0-6.3v
I'm planning to use the center tap (6.3v).
--
croy