I have a control card where two pins can be shorted with a push button to trigger an action (5V can be measured between the two pins.)

  • If I briefly short the pins with a jumper, the action is triggered as expected
  • If I short the pins with a current-meter: nothing happens. By that I mean that I'm trying to measure the current between the two pins. I would expect the current meter to short the circuit, trigger a current and measure it.

My question: is that because the current-meter has too big of an internal resistance, or is it something else ? I reckon the approach is pretty naive but it got me curious now.

Background of why I'm doing this: I intend to remotely control the action using an Arduino. My idea is to use an optocoupler that acts as a push button (and keep both circuits isolated.) Then I wondered if I would need a resistor to avoid frying the optocoupler's transistor so I tried to measure a current. Apparently it doesn't work.

Also, should I be worried that my optocoupler will also have too big of a resistance to make the circuit work?

EDIT: I feel very stupid, but I think my multimeter is simply broken :/ It does measure voltages, but setting it to current even fails to measure a current through a simple resistor.

  • \$\begingroup\$ How did you use the current meter? What kind of meter is it? Is it a multimeter, set to current measurement mode, leads moved to correct current measurement socket and fuse is know to be good? \$\endgroup\$
    – Justme
    Jul 3 '21 at 7:29
  • \$\begingroup\$ Multimeter, set to current measurement. I started with highest values, then lowered the calibration. I guess I could check that the multimeter works. At least it works for voltage measurement \$\endgroup\$
    – aherve
    Jul 3 '21 at 7:33
  • \$\begingroup\$ What current the did the multimeter show when you did the measurement? \$\endgroup\$
    – HandyHowie
    Jul 3 '21 at 7:41
  • \$\begingroup\$ Multimeter always shows 0 \$\endgroup\$
    – aherve
    Jul 3 '21 at 7:42
  • \$\begingroup\$ Thanks @Justme you were right: the fuse of the multimeter is indeed the issue here \$\endgroup\$
    – aherve
    Jul 3 '21 at 7:49

Well, I feel stupid but I think my multimeter was simply broken :/ Trying to measure a current through a resistor also failed. The fact that it still measures voltages (with proper settings of course) got me confused.

  • \$\begingroup\$ If it is only the fuse, it does not mean the multimeter is broken. \$\endgroup\$
    – Justme
    Jul 3 '21 at 7:56
  • \$\begingroup\$ True. I'll change the fuse and try again :) \$\endgroup\$
    – aherve
    Jul 3 '21 at 8:00

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