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Follow up of this question I asked. My DAQ can sink a maximum of 4mA. The instrument it is hooked up to doesn't have a data sheet or anything that tells me how much current the pins supply. How can I measure this without shorting?

Knowing that the pin has 5V, can I simply put a resistor in series from the pin to my DAQ's digital input pin to limit the current?

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    \$\begingroup\$ So let's say they can supply 10A. Then what? It is not a current source, is it? \$\endgroup\$ – Eugene Sh. Sep 1 '16 at 14:33
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    \$\begingroup\$ Why do you want to measure it without shorting it to ground? That's how you will be using it. Just measure it with a multimeter. \$\endgroup\$ – Dave Tweed Sep 1 '16 at 14:45
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    \$\begingroup\$ If it's an input that is "normally 5V" it will be pulled up by a resistor internally. You won't be "shorting to ground" because you are going through that resistor first. \$\endgroup\$ – Majenko Sep 1 '16 at 15:03
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  • DAQs are pretty standard 5V input logic low is also pretty standard <0.8V for HCT Using active low,open collector out from DAQ, connect 1-10K pullup to 5V and connect to 5V logic input. This is 1mA.

Other details, not likely needed.

  • The easiest way is assume fixed output resistance and measure voltage drop or rise from the rails using 1K load to the opposite rail for 4mA range. Then compute offset V/1K to determine internal R.

  • Often current load for logic depends on TTL thresholds of 0.8V and 2V which limits current of 4mA due to 200 Ohm internal resistance of driver. If wanting to indicate on an LED, then 3mA Is feasible with RED,

using Rs=(5V-2V-0.8V)/3mA = 730 Ohms or more.

  • subtract ESR of driver and 15R for 5mm LED and Vf of LED from Vcc to compute RS.
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