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I have accidentally shorted my circuit (5 Volt Vcc) for approximately 1-2 seconds max. While the circuit was short circuit I use the FLUKE as ampermeter in miliAmper mode (400mA max fused) The current was reach above 400mA (I think 480mA and then I quickly disconnect it ) and now I am afraid that the fuse was damaged. I can still see current measurements and I did fuse test and the resistance I got is 8kOhm. It is sound too big for me but FLUKE manual says that the result should be lower than 10Kohm.

link to FLUKE user manual

As far as I understand the Ampermeter should have very low resistance (close to zero) so do you think 8Kohm it is still good? and why FLUKE says below 10Kohm it is good?

Thanks

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  • \$\begingroup\$ As 8kohm is less than 10kohm then it meets what Fluke puts in the manual - if you have the figures correct... \$\endgroup\$ – Solar Mike Sep 11 '18 at 17:16
  • \$\begingroup\$ Is it normal that the fuse ok after short circuit? \$\endgroup\$ – Daniel_ee571 Sep 11 '18 at 18:09
  • \$\begingroup\$ If that range still works then the fuse must be ok \$\endgroup\$ – Solar Mike Sep 11 '18 at 18:15
  • \$\begingroup\$ That does seem odd. But if you followed the procedure, then I think you can trust the result. I have seen them blow. When they blow, they go totally open-circuit. \$\endgroup\$ – mkeith Sep 20 '18 at 2:57
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Your fuse is fine.

Briefly, the way these meters work is that there is a precision resistor (shunt resistor) in series with each of the current terminals. For the 10A range, the resistance is maybe 0.1Ω. However, when your meter can go down to a resolution of 0.1µA, the voltage generated at that resolution is very small (about 10nV), and that's hard to measure. To compensate, larger resistances are used to generate a larger voltage to measure at the lower current ranges.

It's likely that the 8kΩ you are measuring is either the burden resistor for the 400µA range, or perhaps the resistance of a safety circuit to protect the current input when un-powered.

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  • \$\begingroup\$ Thank you everyone. Im glad to hear that my meter is still fine. Wish you all safe work in future \$\endgroup\$ – Daniel_ee571 Sep 11 '18 at 18:49
  • \$\begingroup\$ @W5VO Something I've been wondering about, without going into advanced circuits or expensive high end comparators, what is considered a good amount of voltage for measuring current on a series resistor? I try not to give up more than .1V, and that seems to work for my low current hobby stuff, but I really have no idea how low I could go. \$\endgroup\$ – K H Sep 12 '18 at 1:51
  • \$\begingroup\$ @KH That's such an "it depends" question. Dave Jones, of EEV blog Fame, made a low burden resistance amplifier. You can probably go a bit lower, but it gets harder and harder. \$\endgroup\$ – W5VO Sep 12 '18 at 3:27
  • \$\begingroup\$ @W5VO I thought it might be so I may just have to experiment with my comparators a bit. So much to learn! I'm using them for voltage regulator circuits and so far .1V hasn't been a problem, but I've been curious about this. Next time I have one breadboarded I'll try reducing it until the output starts to suffer. Thanks for the video link. It's cool to see how upside down multimeters work. \$\endgroup\$ – K H Sep 12 '18 at 4:04

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