How To Replace This Fuse?

PaulyWally · Oct 3, 2019

Hi All,

I've blown a fuse in my O-Scope. It's an AGC sized fuse, 0.5A, with a 1-ohm resistor inside. I've attached a pic. O-Scope has no power and the fuse is reading around 400K ohms.

I'm having difficulty locating a replacement. I'm wondering if I can replace it with a standard AGC 0.5A fuse, and also solder a 1-ohm 1/4 watt resistor in-line? Or a better idea?

I have confirmed that the scope works with a new standard ACG fuse. But I prefer not to run it like that since (I assume) it is meant to have this 1-ohm resistor fusey thing in there.

TIA!

Pommie · Oct 3, 2019

Never come across this before, will be interested in the replies. Does however, look fake.

Mike.

PaulyWally · Oct 3, 2019

Pommie said:
Does however, look fake.

Probably because I blew up the picture. I've never seen this either. And haven't found much info online, or a part source to purchase another.

Ylli · Oct 3, 2019

Use the standard SB fuse. The internal resistor is actually part of the slow-blow mechanism of the fuse. Different manufacturers do it differently.

Pommie · Oct 3, 2019

Seen the spring in slow blow fuses but have never seen a resistor. Can you elaborate?

Mike.

tvtech · Oct 3, 2019

Bullshite lol. Resistor in a fuse.... hahaha
What's next? You can't dream this stuff up.

tvtech · Oct 3, 2019

Fuse says: I wanna blow.
Resistor says: No. Not yet.

Huh?

tvtech · Oct 3, 2019

That's dumb. Whoever designed that fuse plus series resistor...is bat shite crazy. I mean common....

No.more.comments.from.me.

Ylli · Oct 3, 2019

The resistor is attached at the spring end with a low temperature solder. Current flowing through the resistor causes the resistor to generate heat. When the resistor transfers enough heat to the low temperature solder, it melts and the spring pulls the circuit open. Over the last 50 years in electronics, I have seen these on occasion.

In normal operation, up to that 0.5 amp rating, the resistor will drop at a maximum 0.5 volts. Hardly significant in a 120 or 240 VAC supply.

Normally when they slo-blow, you will see the resistor separated from the spring. A severe overload can cause the resistor to open without creating enough heat to melt the solder.

PaulyWally · Oct 3, 2019

Ylli said:
The resistor is attached at the spring end with a low temperature solder. Current flowing through the resistor causes the resistor to generate heat. When the resistor transfers enough heat to the low temperature solder, it melts and the spring pulls the circuit open. Over the last 50 years in electronics, I have seen these on occasion.

In normal operation, up to that 0.5 amp rating, the resistor will drop at a maximum 0.5 volts. Hardly significant in a 120 or 240 VAC supply.

Normally when they slo-blow, you will see the resistor separated from the spring. A severe overload can cause the resistor to open without creating enough heat to melt the solder.

Good info. Thanks! Any suggestions on how to replace this?

Ylli · Oct 3, 2019

Just use the AGC fuse you refered to in your first post. Forget about the resistor.

tvtech · Oct 3, 2019

Ylli said:
The resistor is attached at the spring end with a low temperature solder. Current flowing through the resistor causes the resistor to generate heat. When the resistor transfers enough heat to the low temperature solder, it melts and the spring pulls the circuit open. Over the last 50 years in electronics, I have seen these on occasion.

In normal operation, up to that 0.5 amp rating, the resistor will drop at a maximum 0.5 volts. Hardly significant in a 120 or 240 VAC supply.

Normally when they slo-blow, you will see the resistor separated from the spring. A severe overload can cause the resistor to open without creating enough heat to melt the solder.

OK. If you say so
Glad I never came across this before. If you learn to do things this way good luck to you.

Really good luck

tv

Ylli · Oct 3, 2019

Don't know if they make them this way anymore, but here is another post describing the 'fuse with internal resistor'. https://www.electronicspoint.com/forums/threads/resistor-inside-a-fuse.275907/

dr pepper · Oct 3, 2019

I've seen resistors in fuses before too, including devices that looks like r's but without colour stripes.
I didnt know they were heaters though, thought it was some other reason.

gophert · Oct 3, 2019

Look up, "fusistor".

This is a less common permutation but fusistors, originally made by BelFuse have been around for 50+ years. Zenith TVs (pre-solid state) used them. They were originally sandblocks with crimpable lugs, then plastic capsules with blades that plugged into a socket.

be80be · Oct 3, 2019

I just made one what do you think lol

gary350 · Oct 4, 2019

Get creative. If fuse fits into a clip in fuse block this will work. If fuse fits into a fuse body this will not work. Solder resistor, new fuse, short wire in series. Solder this in parallel with bad fuse.

Reloadron · Oct 4, 2019

They are not all that unusual. Just replace it with a same dimension slow blow fuse having the same current and voltage ratings found on the end caps.

Ylli said:
The resistor is attached at the spring end with a low temperature solder. Current flowing through the resistor causes the resistor to generate heat. When the resistor transfers enough heat to the low temperature solder, it melts and the spring pulls the circuit open. Over the last 50 years in electronics, I have seen these on occasion.

In normal operation, up to that 0.5 amp rating, the resistor will drop at a maximum 0.5 volts. Hardly significant in a 120 or 240 VAC supply.

Normally when they slo-blow, you will see the resistor separated from the spring. A severe overload can cause the resistor to open without creating enough heat to melt the solder.

That's about it. While not quite as common as those with just a spring / alloy mechanism I have run across them on a few occasions. If you look at a 0.5 amp Littlefuse Series 313 the fuse resistance is shown as cold 1.2600 Ohm so for what you have the resistor seems about right.

Ron

ChrisP58 · Oct 4, 2019

gary350 said:
Get creative. If fuse fits into a clip in fuse block this will work. If fuse fits into a fuse body this will not work. Solder resistor, new fuse, short wire in series. Solder this in parallel with bad fuse.View attachment 120869

I don't think so. The resistor isn't there to just supply some resistance to the circuit that the fuse is part of, or else the resistor would be in that circuitry and not be inside of the fuse.

The resistor acts as a heating element. When the current through the resistor rises up to the melting point of the special low temperature solder, then the fuse opens.

You proposal wont work unless you duplicate the thermal behavior and mechanics of the original fuse.

A few points to consider:

The glass body contains heat for a greater temperature rise, whereas the external resistor has moving air past it to lower the temperature rise. It will probably still blow, but not at the intended current.
The melting point of the solder must be the same as the original
Even if you duplicated the solder to the original, the fact that your resistor is external (not constrained) means that when it did blow, the resistor (and possibly the rest of the patched on assembly) would then be floating around where it probably shouldn't be.

gophert · Oct 4, 2019

I am guessing there is also some spring tension on the spring to make sure the circuit disconnects (and stays disconnected) once the solder melts.

How To Replace This Fuse?

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