Leakage Current

[Bob O]

How do you verify that the leakage current on a solid state PLC output won’t adversely affect your field device? Example: ML1500 with min. 1.5 mA off state leakage current. Switching 24 VDC Operating a pneumatic valve with power consumption of 1. W Do I use Ohm’s law and calculate P=E*I so in this case it would be 24VDC*.0015A=.036W which seem insignificant. Any pointer are appreciated, Bob O

[Togadude]

Having a "real" load (coil, lamp, etc.) on an output shouldnt affect much. The only times I have had problems with leakage current is when used with solid state devices (SS relays, inputs to other PLC's). I had to use load resistors or switch to a mechanical relay to remedy it.

[Bob O]

Thanks for the reply. I think I need to experiment with this to better understand it. If anyone has any experiments for testing this on the bench for both the inputs and outputs I would like to hear them. Thanks

[benbrad]

Check the resistance between the input of the device you're switching and the ground associated with it (inputs are optically isolated from plc power). The ideal value based on a 24v system with 1.5mA of leakage would be less than 16kohms (V/I=R). If you find it has a higher resistance e.g. 20kohms, then the item will see an input even when signalled to be off. In this instance the addition of a resistor across the load would be required. For the above example of a 20kohms input impedance, use Ohms law and work it backwards i.e. 1/Rtotal = 1/R1 + 1/R2 for resistors in parallel 1/resistance required - 1/20k = 1/required load resistor 1/15k - 1/20k = 1/0.016666 1/0.016666 = 60k This is very common in the electronics industry as most devices have high input impedances but also have similar leakage currents to that of plcs. The way to test outputs to find the amount of leakage current, would be to disconnect the load and connect an ammeter in its place, as if it were the load. Make certain that the output is off before you start as a dead short could cause damage to the output module (there should be in-built protection but......). Using a bench power supply with adjustable current limit for any experiments would be the best thing to do. I use a Thurlby Thander with 2 x 32v 2A outputs as I can then have isolated circuits or reconfigure for higher voltage or higher current applications. Disclaimer - DONT DO THIS UNLESS YOU KNOW WHAT YOU ARE DOING. I cannot be held responsible if you put 24v@massive amps through a simple transistor and fry the thing. It's your fault! Hope this helps, Ben Edited December 13, 2005 by benbrad

[Bob O]

Ben, Thank you for the explanation. Bob