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Voltage present when outputs are off

Kevin 10 years ago in Expansion modules / XA2, XS2 • updated by Gustav Widén (System support) 10 years ago • 1
We have 24v system that has a siren connected to an output on the XA2 module. This siren takes 24V and draws 30mA. With the output turned off the sounder is emitting a faint siren, im assuming this is voltage and current present for the error checking?

I have measured the voltage and current being drawn with the output off and its 1.3mA @ 5.8v. Is there any reason why we couldn't simply connect asuitable resistor in parallel with the siren to stop this sound being generated?

Answered
Gustav Widén (System support) 10 years ago
What you are seeing (or hearing) the effect of is the leakage current through the high-side switch.
I guess that you are using one of the DOUT:s that share pins witht the COUT (DOUT-G to DOUT-L), these have a leakage current of about 1 mA. The latest version of the instruction book shows <1 mA leakage, but we used to state up to 1.8 mA leakage on these outputs.

http://www.parker.com/literature/Electronic%20Controls%20Division/Literature%20files/iqan-xa2_instruction_book.pdf

If you do not have any load connected to the DOUT, you will see an even higher voltage at the output, close to the supply voltage.
Your idea to put a resistor in parallel with the siren could work, but you will have to select one with a resistance that is low enough compared to the siren so that it takes most of the leakage current. At the same time, it has to be dimensioned to handle the power when you activate the DOUT. And the siren might not be getting enough current when the DOUT is active.

A alternative suggestion would be to see if you can use any of the dedicated DOUT:s, DOUT-A to DOUT-F, as these have a lower leakage current.

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