What is the correct way to test a fuse in a circuit with voltage applied?

Thread Starter

zsnow

Joined Jan 24, 2007
5
Do measure across the fuse? (I know if you measure a voltage the fuse is bad.) or
Do you go from one side to ground and then the other side to ground? (if the fuse is bad you will measure voltage only on one side.)

Thanks for your info. Zsnow
 

beenthere

Joined Apr 20, 2004
15,819
Hi,

If it's in a clip with both sides exposed, a meter will read the same voltage at either end. Any voltage difference indicates a bad fuse.
 

Murod

Joined Dec 24, 2005
30
To check if the fuse is broken or not while the voltage is applied, we can measure the voltage bitween one fuse's end to ground, and one other end to ground. The voltage should be equal, if not then the fuse is broken or the connection is bad. You can also measure the voltage between fuse's end-to-end voltage. If zero the the fuse is good, if not zero then the fuse is bad/broken.
 

Thread Starter

zsnow

Joined Jan 24, 2007
5
I Know the two ways to check a fuse in a circut. One of the two ways is supposed to be more correct than the other, at least for a written exam purpose.

Thanks, zsnow
 

fanie

Joined Jan 20, 2007
63
two ways to check a fuse
??????????????

A fuse is either short circuit meaning it's max current rate hasn't been exceeded,

Or it will be open circuit as something caused the fuse 'wire' to melt.
 

n9352527

Joined Oct 14, 2005
1,198
I usually measure the voltage from the fuse terminals to ground. The reason was because I tend to use an analogue meter for troubleshooting instead of a digital one and measuring the voltage across the fuse without finding out which terminal is the hot side might load the meter the wrong way around and send the needle banging on the left peg.

That's just me :D
 

Murod

Joined Dec 24, 2005
30
??????????????

A fuse is either short circuit meaning it's max current rate hasn't been exceeded,

Or it will be open circuit as something caused the fuse 'wire' to melt.
That's true, but the case is that we need to check it on site, while the circuit is powered. So we found two methods. There's no best one method within them, as they have a same fundamental theory.

Regards,

Hasan Murod.
 

beenthere

Joined Apr 20, 2004
15,819
Hi,

If the circuit/equipment is in normal operation, why would you want to check the fuse? If there is a malfunction, you always pull the fuse and check with a meter. In-circuit checks, even with power off, can let a cap discharge through your meter.

Exam questions, IMHO, are written by people who have much more theory than practise. They also tend to be nagging control freaks who cannot appreciate any problem-solving approach but their own.

By the way - a 1 1/4" length of aluminium control shaft makes a handy fuse substitute if you're looking for a problem. It's for the smoke test. After applying power for a couple of seconds, trun it off, and look for the source of the smoke, which should coincide with the failed component.
 

mrmeval

Joined Jun 30, 2006
833
The smoke test can also smoke traces or wires up to the failed component depending on the amount fo current the power supply will source. In some cases the component can decide it's kindling or an explosive and make a real mess of the board. If the wiring insulation is made of less than totally inert material it sometimes likes to have a bonfire, something akin to "Burning Man" but with polymers.

You might guess how I know this.. :)
 

JoeJester

Joined Apr 26, 2005
4,367
I've tested fuses in many manners.

Across the fuse, 0V indicates a good fuse and Source voltage indicates a bad fuse.

From fuse to ground (or return), Voltage on the source side and voltage on the load side indicates a good fuse. Voltage on the source side and no voltage on the load side indicates a bad fuse.

Pulled the fuse and did a resistance measurement ... 0 ohms good, high ohms bad.

Visual inspections can fool you with some fuses. Not every failure is a complete burnout of the fusable material.

I guess I use which ever method is best for the situation. Accessibility to the fuse certainly is a factor.

I've worked on equipment where the fuse was a distance from me, about 8 or 10 feet, but the connections were on a terminal board in front of me.

The correct way to measure a fuse is the one where you don't kill yourself. Other than that, it's whatever way you choose.
 

thingmaker3

Joined May 16, 2005
5,084
:D
Step one: Adjust R1 until fuse blows
Step two: switch in metr to meaure curret which blew F1
Step three: compare measured value to rated value to determine if fuse would have worked properly.

fusetest.jpg
 

fanie

Joined Jan 20, 2007
63
Sorry Murod, I was like the English say - here but not all there. I assume the 6mm screw is not the right replacement then... ?

I have a simple method to deal with fuses. Replace with a new one - if the appliance works, break the old fuse ( one's always tempted to store them just 'in case'... of what? ).

If the appliance is still not working, the fuse may have blowed again, or there is another fault - either indicate a worse underlying condition other than just the fuse - but just for the hell of it make sure you attempted the right 4A replacement fuse and not the 400mA one...

Your next step of action would be to make a decision, probably the most difficult, the one of fix or fuks - if it's worh it or the wife is very sentimental over it bla bla bla and it's actually paid for you may want to fix it or have it fixed. If it's the stingy neighbour's 278 year old TV yet again blowing fuses just before the clouds come up, better tell him this time he's really done it - so get him to scrap it, just remember get the rubber feet for your new project's enclosure.
 

Thread Starter

zsnow

Joined Jan 24, 2007
5
HI,

Thanks for all the input on my question. While it may sound like a dumb question it was on an electronics technician’s test I took a few months back. I will be retaking the test again this Monday.

I read in one of the training manuals I have on VOMs that you measure the fuse in parallel. I guess this means measure across the fuse?

As to powering down the equipment to remove the fuse for testing. I work on industrial type of equipment to power it down and then back up takes more than a few minutes. They want the repairs completed as quickly as possible. Of course before components are replaced we do power down and lock out the equipment.

Thanks, Zsnow
 
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