Testing fuses in circuit: With power and no power

[kisk]

I have a surface mounted fuse that I have tested and it seems open (OL on my DMM), however when I have the device powered on (LCD monitor) the resistance shows 0.0. What is the idea behind this and why am I getting this reading if the fuse is blown? Why wouldn't it be OL with both readings?

[Kiriakos GR]

Re: Testing fuses in circuit: With power and no power

Your second way is not considered as correct practice about making a correct measurement.
Therefore any answer on that would be plain guesswork.

[kisk]

Re: Testing fuses in circuit: With power and no power

Thank you. So always test fuses either out of circuit, or in circuit with no (and discharged) power?

Another fuse question: I was testing this fuse with the continuity setting on my DMM and it showed OL. When I switch to ohms (auto) the fuse reading jumps up about 30 M-ohms each poll and the reading evens out around 1.6 k-ohms. Can I assume there is a short somewhere or are some circuits designed like this? Would I have to desolder the fuse to get a true reading?

BTW, I just got this new DMM which I'm using.

[tron]

Re: Testing fuses in circuit: With power and no power

No, you do not have to extract
the fuse from the circuit board,
to get a true(shorted) reading

[tom66]

Re: Testing fuses in circuit: With power and no power

When the circuit is energised, you are basically putting high voltage across meter terminals expecting around 200mV. This is the same reason behind getting negative resistance readings sometimes on powered circuits.

[kisk]

Re: Testing fuses in circuit: With power and no power

No, you do not have to extract
the fuse from the circuit board,
to get a true(shorted) reading

This is what I expected, but why is the end-to-end point of the fuse reading 1.6k Ohms. This is what I would expect reading off of a resistor, not a fuse. ?

, and thanks for the info tom66.

[budm]

Re: Testing fuses in circuit: With power and no power

You are reading the circuit resistance, for example the power filter cap may have bleeder resistor (for example, 1K) across the cap (+ leg and the - (gnd) leg of the cap) or other branch circuits resistance that power supply is feeding before the fuse, then the fuse is connected to the positive leg of the cap, another end of the fuse will go to the load which has resistance (lets say 1K). So when the fuse is open, you are reading the resistance of the circuit, 2K.
Just draw out the diagram then you see why you are reading resistance.

[Radio Fox]

Re: Testing fuses in circuit: With power and no power

For future reference, never use the continuity or resistance (Ohms) setting on your meter to probe a circuit while it's powered up. You could damage your meter.

[Longbow]

Re: Testing fuses in circuit: With power and no power

For future reference, never use the continuity or resistance (Ohms) setting on your meter to probe a circuit while it's powered up. You could damage your meter.

Well that might have been true back in the days of the Simpson 260. Lots of meters got smoked when trying to measure the "resistance" of a battery. These days electronic meters will self protect because even the best of us occasionally forget to change functions.

As far as measuring a fuse - this is extremely easy. 1). with the power off, the fuse (if it IS a fuse) will measure nearly ZERO ohms. Meter on ohms function. 2). With the power on, you will measure the same voltage at either end of the fuse. (volts function). Pick one. Neither method requires the part to be removed.

SMD fuses do exist, but the pcb will usually be marked "F01" or similar. There are also fuse resistors, which are low value resistors and not fuses.

Since your fuse is open, the next step is to measure the resistance (power off) to ground to test for possible short circuits that might have caused the fuse to blow. Schematics?