Powering a 18v Priority encoder

[Kvdeurzen]

Hi all,

I'm trying to use a 18V priority encoder to retrieve the status of 8 sensors on 4 pins of a PLC. The problem is that the encoder becomes really hot when I connect the Vdd and Vss. I'm using a CD4532BE encoder. This IC has a maximum supply voltage of 18V which suffices for my PLC pins as they will trigger at 15V. To reduce my 24V supply I'm using a 7818ACT voltage regulator, with on both sides 220nF capacitors between supply and ground.

I measure a resistance over Vdd and Vss of 144.4 ohm, which seems a bit low to me. Can anyone explain whether I'm doing something wrong?

Kind regards,
Kanter

 

[Harald Kapp]

Do you have all inputs connnected to either Vss or Vdd, no floating inputs?
CMOS does not "like" floating (unconnected) inputs. Use at least a pull-up or pull-down resistor (e.g. 100kΩ) to either Vss or Vdd (not both) to ensure a stable input signal level.

 

[Kvdeurzen]

Thank you for your reply. I think you are right. I connected all inputs to ground and it started working immediately.