Convert AC I/O to DC I/O

Tim Ganz · Jul 9, 2019

Have a small project to convert a machine we have with a 5000 PLC from ac inputs and outputs to dc inputs and outputs. It’s 25 inputs and 10 outputs.

The inputs are 11 limit switch and the rest are push buttons.

The outputs are 3 relays, 2 motor starters and the rest coils on the air valve block.

Changing this to dc should I use sink or source dc card for the inputs and should I use sink or source card for the outputs?

Trying to get my order together and the control logic PLC in here you can order sink or source inputs cards and sink or source outputs cards.

Which is best? Or which is most common?
.

James Mcquade · Jul 9, 2019

Tim,

I would do the following

(1) use sinking inputs, more common and uses less power if I am not mistaken.
(2) use source outputs, more of a standard and requires less power if I am correct.
(3) change wiring to blue (dc) for inputs and outputs.
(4) dc common, I have seen this both ways blue with white strip or white with blue stripe.
(5) change the cad drawings to reflect any color and wiring changes.
(6) use reverse bios diodes for the outputs to protect the outputs.
(7) make sure the outputs can power the motor starters, use interface relays if necessary if you are not changing out the contactors.

hope this helps,
james

JaxGTO · Jul 9, 2019

You will need to replace the solenoid valve coils as well. A good quality coil will have the diode in it. Same with the motor starters.
Also depending on the output card some have the diode built in.

Tim Ganz · Jul 9, 2019

Yes we have new starters and a new valve block that is 24 volts dc as well as a couple pilot lights we will add but I was just not sure if I needed sinking or sourcing on the input or output cards. I know either can be made to work but I am not sure what is best or most common.

Steve Bailey · Jul 9, 2019

Which is best is a subjective decision. Ask five different people and you're likely to get six different answers.
Sourcing outputs are the more common choice. Inputs are more closely split between sourcing and sinking.
Sourcing outputs and sinking inputs are a bit more straightforward for troubleshooting because when you are checking things with a voltmeter, the ON state is indicated by a positive voltage with respect to DC common.

sparkie · Jul 10, 2019

Sinking uses less power in general. Electrons have more mobility than holes.

You can use sinking OR sourcing. Of course you know this. To directly answer your question:

It is going to make a lot more sense to match the convention of AC IO.

Use sinking inputs.
Use sourcing outputs.

No need to go into much more detail here. AC outputs have a neutral at the device and switch voltage out to the device. Sourcing outputs match this convention.

AC inputs have a neutral at the card and voltage is switched to the input. Sinking inputs match this convention.

https://cdn.automationdirect.com/static/specs/sinksource.pdf

parky · Jul 10, 2019

I would use the same as already configured, or you will have major wiring mods, and if different colours for cable have been used for supply & common will need to be changed or confusion to other engineers may ensue. There is little to gain in changing source or sink except possible noise, however there is little chance of that, most systems I have come across have been source for both inputs and outputs.

sparkie · Jul 10, 2019

parky said:
I would use the same as already configured, or you will have major wiring mods, and if different colours for cable have been used for supply & common will need to be changed or confusion to other engineers may ensue. There is little to gain in changing source or sink except possible noise, however there is little chance of that, most systems I have come across have been source for both inputs and outputs.

Most DC systems I have found in the US tend to match the convention used for AC IO as it just makes sense. This is by no means ubiquitous, of course, but I find it similar to the motor starter debate. Do the overloads break the hot or the neutral? Almost always in the US the overloads are wired to break the neutral.

Tom Jenkins · Jul 10, 2019

steve bailey said:
which is best is a subjective decision. Ask five different people and you're likely to get six different answers.
Sourcing outputs are the more common choice. Inputs are more closely split between sourcing and sinking.
Sourcing outputs and sinking inputs are a bit more straightforward for troubleshooting because when you are checking things with a voltmeter, the on state is indicated by a positive voltage with respect to dc common.

+1

Jeff23spl · Jul 10, 2019

sparkie said:
but I find it similar to the motor starter debate. Do the overloads break the hot or the neutral? Almost always in the US the overloads are wired to break the neutral.

Is it still the standard in US ? i was assuming it was an old practice? We are no longer doing it at our shop and few others around and cut the live instead...

We also use positive signal from sensors (Sink) and have a common negative at the plc input card. We do use source 24vdc+ at the output with again a common negative return....
I know using NPN output sinking to ground like automotive industry brings many advantages for driver signals but as stated, it makes troubleshooting harder...

sparkie · Jul 10, 2019

Jeff23spl said:
Is it still the standard in US ? i was assuming it was an old practice? We are no longer doing it at our shop and few others around and cut the live instead...

We also use positive signal from sensors (Sink) and have a common negative at the plc input card. We do use source 24vdc+ at the output with again a common negative return....
I know using NPN output sinking to ground like automotive industry brings many advantages for driver signals but as stated, it makes troubleshooting harder...

I have not ran into any new equipment that wasn't, or built any that wasn't. Pretty much all of the old equipment does it. Now, it may be changing, but I haven't seen it yet. Speak about the motor overload of course.

Convert AC I/O to DC I/O

Tim Ganz

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James Mcquade

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JaxGTO

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Tim Ganz

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sparkie

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Jeff23spl

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sparkie

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