Encoder Output voltages

[gsjhand]

Hello Everybody
I have a question regarding encoder voltages. I have an encoder with input voltage 24 volts. Output of this encoder is connected to two devices PLC and Optoisolator. Output of Optoisolator is further connected to two different PLCs.
If I leave encoder output unconnected and check voltage level at output A with respect to 0Volts, using oscilloscope I get 24 Volts pulses which seems to be normal as input is 24 volts. As soon as I connect this output to any device (PLC or Isolator) voltage level drops to 12 volts. Is this normal? If I connect that PLC to output of isolator then output level of isolator remains 24 volts. This makes me to believe that this is the property of encoder which drops the voltage. But why voltage doesn’t drop when I connect my oscilloscope as this is also a device? I am guessing this is related to input impedences of the devices but not sure and would like to know more about encoders.

 

[Skogsgurra]

Yes, the outputs of some encoders are open collector with a pull-up resistor. The resistor is often in the 10 kohms range. So, a 1 Megohm scope input doesn't load the output down. But a PLC input does.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sreid]

The PLC Inputs could be set for "PNP [i.e., pull-down resistor]. Then if the Encoder pull-up resistor and the PLC input pull-down resisror are the same value, the High voltage would be half voltage or 12 V [and marginal at best for the PLC input].

 

[damiens]

What's the manufacturer of your encoder? You should try the datasheet on manufacturers website. Most manufacturers have all their product tech data available online now. We use Kubler & Accu-Coder. They have lots of information available on .pdfs.

http://www.clrwtr.com/Kubler-Encoders.htm

http://www.ctiautomation.net/Accu-Coder-Encoders.htm

 

[roydm]

One thing I have learned about encoders the Open collector types have a high frequency limit. The TTL output variety have more driving power so they will operate at higher speed.