Electrical – Safest way to limit current output of a signal generator


I need to feed a TTL-like signal (0-5V) from a signal generator to another equipment (herein 'load'), whose manual states that the input current should be about 10 mA, and 30 mA maximum. As I work with 5 V, I'm thinking I should use a coaxial cable to get the output from the generator, then to connect a 500 Ohms resistance in series with the load, which should give me 10 mA of current.

I would like to know if this is correct, and if there's some safer way to do this, as the equipment I'm trying to feed this signal to is borrowed from another laboratory.

Updated to add the link to the equipment manual (page 34, section 3.13.1) http://www.biomech.hacettepe.edu.tr/manuals/FASTCAM_SA3_HW_Manual_Rev105en.pdf

Best Answer

It might be helpful if you would describe the "load" in more detail. I would expect that an input in a piece of test equipment is rather high impedance. I guess that the current values stated in your question are properties of the device and not absolute maximum ratings, i.e. the input impedance is such that a current of 10 mA typical, and 30 mA maximum, when a TTL signal is applied.

In any case, a 500 Ohms series resistor won't damage the device. However, if the input is an actual TTL input (i.e. it does not just require TTL-style voltage levels), a 500 Ohms series resistor will prevent the signal generator from pulling the input to Low level.