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Author Topic: Which fuse to use?  (Read 6015 times)
N8FNR
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« on: September 11, 2012, 05:45:59 PM »

I recently bought a switching gasafet ARR preamp for 70cm to use with a Arrow dual band handheld antenna to hear the downlink from the FM satellites. http://www.advancedreceiver.com/page7.html

The preamp draws 100ma at 12v and the wire I am using is 16 gauge and about 1 meter long.

I would like to fuse the positive line because I really want to protect the preamp and wonder what is the ideal fuse rating I should use?

BTW I went to Radio Shack today and the smallest rated fuse they had was a quick blow 1/4 amp for 250v.

Would it be ideal to use a blade type car fuse rated for 1 amp for my application as that seems the lowest rated commonly available one?

Any suggestions on this will be greatly appreciated.

Zack
N8FNR
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W8JX
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« Reply #1 on: September 11, 2012, 06:37:46 PM »

Well remember that a 1 amp fuse does not blow at 1 amp. That is its rated load. Also 16 gauge wire is way overkill here as 20 or 22ga would do. As far as size of fuse, car type fuse are a bit crude and slow blowing. I would think a 1/2 amp fuse here would be more than enough.
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K1CJS
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« Reply #2 on: September 12, 2012, 04:17:01 AM »

That 1/4 amp quick blow would do you fine.  But does the manufacturer recommend a fuse in the power lead?  Usually, power supplies for items like that are fused already in the power supply unit itself.

In any event, remember the old adage:  The circuit you're trying to protect with a fuse will usually let go first--protecting the fuse from blowing.  It may not make much sense, but it is all too often true.
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W8JX
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« Reply #3 on: September 12, 2012, 05:29:44 AM »

That 1/4 amp quick blow would do you fine. 

I considered that but I also factored in any possible startup surge draw for amp too and why I suggested 1/2 amp FB fuse.
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WB2WIK
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« Reply #4 on: September 12, 2012, 09:15:58 AM »

How will a fuse protect the preamp? Wink

Normally in a situation like you describe a fuse would be located at the power source (not at the preamp) and mostly protects the wire from burning up in the event of a short circuit somewhere along it, or inside the preamp.

A GaAsFET device such as used in the ARR preamps don't fail because they draw too much line current.
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W8JX
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« Reply #5 on: September 12, 2012, 09:59:04 AM »

How will a fuse protect the preamp? Wink

Normally in a situation like you describe a fuse would be located at the power source (not at the preamp) and mostly protects the wire from burning up in the event of a short circuit somewhere along it, or inside the preamp.

A GaAsFET device such as used in the ARR preamps don't fail because they draw too much line current.

A proper sized fuse can also prevent power surges from coming into amp.
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WX7G
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« Reply #6 on: September 12, 2012, 10:04:34 AM »

A fuse will not protect the preamp. The fuse is there to protect the power source and wiring if and when the preamp fails shorted.
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WB2WIK
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« Reply #7 on: September 12, 2012, 10:40:08 AM »

How will a fuse protect the preamp? Wink

Normally in a situation like you describe a fuse would be located at the power source (not at the preamp) and mostly protects the wire from burning up in the event of a short circuit somewhere along it, or inside the preamp.

A GaAsFET device such as used in the ARR preamps don't fail because they draw too much line current.

A proper sized fuse can also prevent power surges from coming into amp.

How will it do that?  Usually a source-induced surge is a voltage spike, and that's what pops GaAsFETs.  Without a voltage spike, there can't be a current spike.
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KD0REQ
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« Reply #8 on: September 12, 2012, 10:58:17 AM »

look again at W7XG's post.

the purpose of fusing power lines is to protect the wiring from overcurrent and fire.

fusing the power leads at 1/4 or 1/2 or 10 amps is not going to protect the FET.

if you want to protect an internal circuit like, say, keeping 1600 volts plate power out of a 300 volt display line that also runs the screen, you need to do several things:

1) have a fuse that blows before the sensitive parts of the circuit does

2) failing that, have some sort of a trigger that snarfs up the overvoltage long enough and hard enough to take out the fuse.

in the instance I'm thinking of, you use large zener diodes to kick in at a 15% overvoltage, and that blows a 50 mA fuse, hopefully before a dozen TTL chips ending in display drivers fry.

in the case of the low currents being presented to the GaAsFET, I don't think this will work.

so next step is to think about a shunt.  put a small resistor in series with the FET input.  have some sort of avalanche diode ahead of it, so if you are lucky, the diode conducts past the higher gate-puncturing voltage.

values will depend on how well you read the spec sheets and do algebra.  alternatively, you could come up with several scenarios, buy extra parts, breadboard the circuit several times, and generate some failure states.  if any awshyt pulses in the breatdoards are successfully diverted (possibly at the loss of the protection," then you have proven your case.

-0-

the simple solution, however, would be to design the preamp to run on lesser voltage, and put a regulator ahead of the circuit to shunt off surges.
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WB2WIK
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« Reply #9 on: September 12, 2012, 03:00:08 PM »

I think perhaps the point has been missed entirely by some.

GaAsFETs used in the ARR preamps are extremely sensitive to ESD, transients and all sorts of stuff that takes them out.  The "by far" leading cause of preamp failure with those is operator error, like transmitting into the preamp.  Doing that for even a microsecond can destroy the FET, and there isn't any kind of fuse that will protect it from that.

Best thing is "don't do that."

Lightning transients are another leading cause of such preamp failures.  A fuse won't help protect against that, either. Wink

Best way to protect such preamps is to not use them near transmitters, or leave them connected to antennas when you're not actually using them.
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W8JI
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« Reply #10 on: September 18, 2012, 05:01:40 PM »

As an equipment designer of about 30 years or more, I agree with Steve and others. Fuses rarely protect against equipment failures, and they certainly do not eliminate damaging surges.

While there are rare exceptions, such as a fuse that prevents sustained modest overload, fuses are almost always there to prevent catastropic damage AFTER something fails. They mostly prevent melt downs, not failures.


Fuses are slow, even fast fuses. Even with a 200% overload they can take 30 seconds or more to open.

If you burn out the preamp, it almost certainly will be from RF or static on the feedlines. There is very little that can be done other than good wiring practices. A 1 amp fast fuse or a bit less will be safe enough to prevent fires or power supply damage. With a GAsFET preamp, not much will protect the preamp.

73 Tom
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KH6DC
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« Reply #11 on: October 01, 2012, 04:41:36 PM »

500mA fast blow would work fine
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73 and Aloha,
de Delwyn, KH6DC
AA4PB
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« Reply #12 on: October 02, 2012, 10:28:28 AM »

Some time ago I developed a product that has a FET switch in the output. I placed a small quick-acting pico fuse on the PC board to protect the FET in the event the user shorted the output. In every case, the FET shorted before the fuse opened to protect it. In addition, I discovered that the FET was less expensive than the pico fuse. So, we eliminated the pico fuse and installed the FET in a socket.

A fuse in the DC power line is not going to eliminate surges UNLESS you also have a surge protection diode across the line to clamp the maximum voltage. What the fuse can do in the event of a short inside the preamp is to protect the power supply, power wiring, and the PC board runs between the power connection and the shorted device.
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