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Author Topic: Need basic info on fuses  (Read 605 times)
N8JRH
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Posts: 30




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« on: December 01, 2008, 11:14:25 AM »


I recently bought a 1960s-era AN/GRR-5 military HF receiver.  The seller was a military vehicle collector and never powered it on.

The unit can run off four different voltages and has a fuse for each input.  A few of the fuses are unreadble and I'd like to replace them before I fire it up.

I noticed that Radio Shack sells glass fuses, but they're only rated for 125V or 250V.  There's no mention of AC or DC.  My basic electronics never progressed much past E=I*R.

Can someone give me some basic info so I can buy the right fuses?  Specifically, I need:

110VAC 1A
24VDC 3A
12VDC 4A
6VDC 8A

Thanks,
Joe, KD8DBJ
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WA3SKN
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Posts: 5559




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« Reply #1 on: December 01, 2008, 11:25:48 AM »

"they're only rated for 125V or 250V"
That is the maximum safe voltage to use them with.  They will work at lower voltage ratings. Do match the current ratings.
73s.

-Mike.
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WA6HDZ
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Posts: 52




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« Reply #2 on: December 01, 2008, 11:27:39 AM »

Fuses are rated for the carrying current first. There are also "slow-blow" and "fast blow" models for specialized uses.

You can use a fuse with a higher voltage rating in a low voltage circuit. Generally the DC voltage rating of the fuse is lower than the AC since the arc created when a fuse blows on DC is not quenched by the reversal of voltage that happens on AC. But your DC voltages are not high enough to fuss much so the 125 or 250 VAC fuses will work fine.  The common standard for pure DC fuses in automobiles is 32 VDC and you can buy those at most auto parts stores, just be sure you get ones that fit in the holders.
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W4LWZ
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Posts: 23




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« Reply #3 on: December 01, 2008, 11:32:54 AM »

Essentially, fuses are rated by both their current capacity and voltage rating.  Most glass cartridge fuses are rated at 32, 125, or 250 volts and various current capacities.  There are also quick acting fuses that will blow when they exceed their current rating, and slow blow  fuses that will take an overload for a short period of time.  Slow blow fuses are typically used where there is an input surge during start up, such as motors or power supplies with a high inrush current.  
Fuses are just links that melt when they reach a certain temperature and open the circuit.  The heat is caused by the current passing thru the fuse.  The voltage also plays a part because while heat increases according to the square of the current it also increases directly proportional to the voltage.  Thus, its not a good idea to use a 32 volt fuse in a 125 volt circuit, even though the current rating is the same.  
Folr your applications I would use 32 volt fuses for the lower voltages and 250 volt fuses for the 120 and 240 volt supplies.
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N5LRZ
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« Reply #4 on: December 01, 2008, 11:52:44 AM »

There are several kinds of fuses or what function as fuses.  The fuses most common are of two types as noted above: slow blow and fast blow.  As the name implies one will blow faster than the other.  

For General use, unless otherwise observed via a blown fuse, I always get fast blow fuses for no other reason other than I feel safer useing a fast blow fuse.  

Also as stated above fuses are rated for both voltage and amps.  The voltage on the fuse is the safe working voltage as stated above.  The amps stated on the fuse is the amount of current that will be permitted to flow before the fuse blows.

Use a fuse that provides enough working voltage (with a little common sense margin for safety of course).  Use a fuse or should I say, do not use a fuse that is rated greater than the original fuse.  Using a fuse rated greater than the original fuse will result in damage to the radio when the fuse does not blow when it should have blown.  Using a fuse that is significantly lower in current rateing  will result in the fuse blowing premturely resulting in your needlessly constantly changing fuses.

Other than using a fuse that can handle the voltage you need to use a fuse that has a current rateing that blows when the fuse should blow--not prematurely and not late.    See the specifications of the radio for the appropriate fuse or the fuse that comes closest to the original specifications as possible.

IF the fuse is a specialty fuse in so far as value or style you can probably find it on a web merchant page somewhere.

Other sources of fuses are as follows.  You mentioned Radio Shack.  Home improvement stores (Home Depot) and auto supply stores also sell fuses as well.  

ALSO before you say a fuse is good you should check the fuse with a VOM to make sure that the fuse wire in the fuse did not break where it was attached.  Every now and then you will find a fuse that looks good to the eye but the VOM shows that the fuse was blown 'somewhere'.
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N5LRZ
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« Reply #5 on: December 01, 2008, 11:54:49 AM »

PS

Almost forgot...Also check the fuse holders themselves.  Make sure that they are not damaged and that the contacts of the fuse holders are not damaged.  Replace fuse holders with new fuse holders as needed.
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KA1MDA
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Posts: 543




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« Reply #6 on: December 01, 2008, 02:06:04 PM »

Keep in mind that fuses take a LONG time to blow. If I remember my Littlefuse catalog correctly (and I'm doing this from memory), a FAST BLOW fuse will blow only after passing its rated current for something like 60 seconds. A SLOW BLOW fuse takes around 120 seconds at rated current to blow. If the device you're using doesn't have any motors in it, go with the FAST BLOW type. Also, (I don't know if they even still sell them or not,avoid the Radio Shack fuses which have the elements that look like a zig-zag or sawtooth shape. We had so many nuisance failure problems with those in instrumentation where I used to work (industrial environment) we ended up going through the entire plant and throwing out any fuse with a zig-zag shaped element and replaceing it with BUSS fuses with the straight element. Never had another nuisance trip again!

Tom, KA1MDA
www.ka1mda.org
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KB1LKR
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Posts: 1898




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« Reply #7 on: December 01, 2008, 02:31:36 PM »

110VAC 1A
use a 250V or 125V 1A fuse here

24VDC 3A
12VDC 4A
6VDC 8A
use 250V, 125V or 32V fuses here. 3.15A could be substituted for 3A if needed, and 8A 250V fuses may be less common than the lower voltages.

The voltage rating is the maximum voltage the fuse is rated to safely/effectively interrupt, so higher voltages are fine.

You probably want fast blow unless labeled to the contrary (slow blow, time delay, type T, etc.)

Buy from Mouser, Digikey or your favorite electronics supplier.
Buss, Littelfuse, Wickmann or Schurter brands should all be fine; but with "Brand X" your mileage may vary!
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KC0RBX
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Posts: 165




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« Reply #8 on: December 02, 2008, 08:57:53 AM »

All fuses have an inverse time rating, be they fast or slow blow.  Depending on the time/current curve, the quicker the rise in current the quicker they blow.  Some just take longer to blow than others given the same current running through in the same amount of time, if exceeding the fuse's rating.

Use fast blow fuses to protect sensitive electronics and use slow blow fuses to protect wires, motors and transformers.
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AA4PB
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Posts: 13032




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« Reply #9 on: December 02, 2008, 12:06:43 PM »

The voltage also plays a part because while heat increases according to the square of the current it also increases directly proportional to the voltage
------------------------------------------------------
Actually, the voltage across a fuse is very close to zero until it blows so the circuit voltage has no impact on the heating of the fuse element. When it blows then the line voltage then appears across the fuse. The reason you don't want to use a low voltage fuse in a high voltage circuit is that the fuse may arc across the open element when it blows. It is okay (within reason) to use a higher voltage fuse in a lower voltage circuit (like a 250V 3AG fuse in a 12 volt circuit).
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AA4PB
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Posts: 13032




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« Reply #10 on: December 02, 2008, 05:06:53 PM »

Keep in mind that fuses take a LONG time to blow
----------------------------------------------------
It depends on how much current you pass through it. A 1A 3AG Littlefuse takes 1000 seconds to blow at 1.5A and only 0.01 seconds at 8A. Obviously fuses are made to protect against major problems like a short circuit. Don't expect one to protect a solid state component thats drawing a few per-cent too much current.

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