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Old April 20th, 2005, 06:49 AM   #1
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TimothyMoulder is offline
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PNP-PNP connection

This is a variation on the usual NPN-PNP conversion theme that I did not find while searching the site.

I have a PLC output that is PNP (sourcing), and a labeller input that is also PNP (sourcing). I want to connect these two without using a relay, since this labeller is very susceptible to bouncing.

I had considered a pulldown resistor:

PLCout -----------Lin
R 1K

But I was unsure how this would work. If the PLC puts out 24 VDC, and the Labeller input is floating at about .07VDC, could I fry the labeller input?

Anyway, I'm using a relay with capacitor debounce, but I'd still like to know if this arrangement could have worked. I'm out of time to try and experiment, so do one of you know?


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Old April 20th, 2005, 01:04 PM   #2
panic mode

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if the labeler input is rated to accept 24V signals you would not fry anything since digital inputs are relatively high impedence
(~ 3kOhm or so on PLCs) and there is no power source in the circuit until the 1k resistor is connected. using pull up or pull down resistor creates circuit for open collector output (PLC transistor outputs are normally open collector). this internal impedance of the inputs is load for the open collector outputs. if the load is tied to "wrong end" (same side of the power supply), there is no short circuit because the "switch" (output transistor) and load (labeler or PLC input) are in parallel - and with no power source (imagine wiring pushbutton in parallel with relay coil and with no power source).
this 1k resistor would be in series with the parallel circuit we just mentioned. this way the current would be limited to 24V/1kOhm=24mA in worst case (output on is "true short").
but when switch is open (output is not on), this 1k would be still in series with the current limiting resistor of the labeler input. the bigger the value of this "1k" resistor, the less current flows into input and eventually at certain value of the pull up/down resistor input will not see it as signal on.
the lower the value of the resistor, the more current will flow through it when output is on and the resitor power rating need to be bigger. although this works, it is not exactly the same as PNP/NPN converter because signal is inverted (output on is seen as no signal by the input and the other way around).
1k resistor in 24V circuit should be rated 1W.
depending on duty of the signal 1/2 watt could be used as well but it is better to be safe than sorry. at least resistors are dirt cheap...
electromechanical contacts (like in relays) have tendency to bounce but you could replace it with solid state relay (phoenix contact, allen bradley or anybody else...) or you can make you own with an optocoupler and resistor.

Last edited by panic mode; April 20th, 2005 at 01:07 PM.
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