EEVblog #376 – Multimeter Fuse Diode Followup

How diode bridge protection across a shunt resistor in a multimeter works, and why the fuse blows before the diode does on a well designed multimeter like the Fluke.
A followup to the previous video on Multimeter input protection

Fluke 27 Service Manual

Bussman Fuse Datasheet
LittleFuse Datasheet
DF02 Diode Bridge Datasheet
1N4007 Datasheet

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    • Jay

      Well done, Dave. I think you showed very well the importance of studying and understanding information in datasheets, even for simple parts like fuses and diodes.

      When I was first learning electronics, I heard about a basic “rule” to avoid incompetent designs: “Never depend on a fuse to protect a transistor.” This is not bad general advice, but as the Fluke design shows, sometimes you need protection and you can use a fuse as part of that.

    • Mark

      What would happen if you measure an power supply with a constant current of 1.5A?

      • Berni

        The multimeter shows out of range for 1 second until the fuse blows.

        Then if the current source is a real life one its output will drop to 0A because its not capable of providing enugh voltage.

        If you have an ideal current source its voltage will shoot up to a few kV at what point something inside the multimeter will arc over and set the multimeter on fire. Luckuly these current sources arent real.

    • Panu

      I just wanted to point out that the manufacturer is “littelfuse” not “little”. I came across this couple years ago when finding those smd fuses.

    • allan

      Thanks for pointing that out, Dave. I would never have thought to look at the dynamic aspect of fuses or of diodes.

      Not sure what the diode
      bridge has to do with R20, since they aren’t connected.

    • Paul

      I believe that one of the L1 functions is to reduse the current rise rate DI/DT in order to reduce the voltage spike during the diode turn on time.
      That spike can be problematic with high voltage regular silicon diodes like the 1N4007.

    • Dave ,
      As Usual a Excellent Video To Show Us The Multimeter Protection .Forever Yours Fan

      Amarbir From India

    • Worf

      If anyone wants to know – the 10A range has a 5 milliohm sense resistor. 10A is 50mV through that. even 100A is only 500mV – barely above the threshold of ONE diode!

      On the milliamp range, 1A would induce a 5V across the sense resistor, well above the 3.5V needed by the diode protection.

      That’s why there’s no protection for the 10A range.

    • CRi

      It seems that you linked the wrong video, #374 instead of #373 😉

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