Sinking vs Sourcing Outputs

[sgsims]

I am a PLC novice and I am confused about choosing an output card for DC outputs. Unlike input devices which are clearly designed as sinking or sourcing all of the output devices I see (Solenoid Valves, motor starters) do not specify wether they are sinking or sourcing. Does the sink / source not really apply to outputs? I need to choose a DC output card that is either sinking or sourcing. Does it really matter, can I make any DC output device work on either type of board? Or should I just choose a relay type uutput board.

Any advice would be appreciated!

 

[jacekd]

For passive elements (bulbs, coils) it does not matter if driving them output is sink or source. They only need to be connected properly.
However if you use some electronic devices with sink or source inputs then matching them with proper outputs type is a must (if you don't want to play with pull up, push down resistors).

 

[BobB]

If you are driving contactors or solenoids directly, I would forget about using transistor cards if I were you and use relays.

I normally use all DC output cards of the sinking type. This means I then drive LED type indicator lights direct from the card. The common on the lights is positive and the sinking output switches low to place a voltage across the light. Similarly with external relays.

I only use relays with flywheel diodes to protect the output card. The relays have to be wired the right way with respect to the direction of the diode or the transistor output will be destroyed.

 

[chief weegum]

If you will drive contactor with that card, be sure then your contactor doesn't takes high current. You card will burn (fuse). Dry contact should be a solution for you.

 

[rsdoran]

I am a PLC novice and I am confused about choosing an output card for DC outputs. Unlike input devices which are clearly designed as sinking or sourcing all of the output devices I see (Solenoid Valves, motor starters) do not specify wether they are sinking or sourcing. Does the sink / source not really apply to outputs? I need to choose a DC output card that is either sinking or sourcing. Does it really matter, can I make any DC output device work on either type of board? Or should I just choose a relay type uutput board.

Any advice would be appreciated!

As I see this the question is "Does Sinking and Sourcing apply to DC outputs". The answer is YES, it can. Sinking/Sourcing is used in reference to connecting solid state devices, its basically a method to understand which way to connect the positive and common.

Solenoid valves, relays, contactors are inductive devices, not solid state, so the "electron or conventional flow" will not matter. There may be times when it is necessary to connect a plc output to a SOLID STATE input, in this case "polarity" (sink/source) may have to be observed or more done with the wiring to properly connect the circuit.

As an example you can look at the Automationdirect DL05 manual (Chapter 2). The DL05 offers sinking and sourcing inputs but just sinking outputs but as you will see you can, by use of a pull-up resistor, still use it with a sinking input device.
http://web5.automationdirect.com/static/manuals/d0user/ch2.pdf

For a specific brand read the manual to determine what the plc has or can use, this is good to know prior to ordering.

 

[tom_stalcup]

As an add on, I've found that it can get pretty confusing for the maintenance people troubleshooting items(like solenoid valves) when you use sinking outputs..... They like to see 24 volts when the solenoid is on, and 0 volts when the solenoid is off. Anything else means specific training on that piece of equipment, and a few random phone calls for me a 4 in the morning.

 

[CaseyK]

About ten years ago, I went to using relay outputs exclusively. When you have a card go bad, any replacement will work, without having to rewire, etc.

Since relay outputs work on everything, and generally have much higher current ratings, they seem ideal to me.

Granted, a customer request, or customer stock does have some bearing.

But it has worked pretty good for me and my larger customers.

 

[BobB]

About ten years ago, I went to using relay outputs exclusively. When you have a card go bad, any replacement will work, without having to rewire, etc.

That's about when I gave them away and started using transistor outputs. Got sick and tired of replacing relays in PLCs in the middle of the night. The PLC's that I was mostly using then were Omron C20 units. They had plug in relays, not soldered in as in modern PLCs. It is a bigger pain to replace one now.

I find it much easier to use and replace, if necessary, DIN rail mount relays with flywheel diodes. These are usually raetd at 10 amps also. Multichip diode indicating lamps (don't like the Schneider, Sprechur & Schuh/Allen Bradley high intensity types), are also a must from the transistor cards. The 24VAC/VDC types have built in diodes as well for rectification and protection.

Have never lost a transistor ouptut since adopting this method. Have replaced a few relays though.

 

[STEVEBOT]

Same here had four Pld Siemens Brick pLcs as we call them (S5 101U) and if a relay output went it meant replacing the entire brick if you did not have a spare output to reasign to

 

[Eric Nelson]

That's about when I gave them away and started using transistor outputs.

Yup, me too. If I NEED a relay output, I use an external relay.

I used a ton of the C/K series PLC back in the day, Bob. Those output relays were a breeze to change, as long as the last person in there remembered to put the relay puller back in its 'home'...



-Eric

 

[tom_stalcup]

We use mostly transistor output cards here... except when they are hooked up to an optical relay card(that runs our older servodrives). Occasionally, the transister outputs would active the optical relays, and a servodrive would get a cycle signal for no apparent reason... I replaced the transistor card with a relay card, and we haven't had any "ghost" cycling since then.

 

[elevmike]

Relay card v. Transistors:

Personally I perfer the relays. We've had a lot of problems with leakage current causing led indicators to glow, and had a hard time explaining that "dim" = off & "bright" = on.

However when it comes to "brick" PLCs it might make better sense to use transistor ouputs, cause' thoes transistors are going to way outlast them relays (if their treated right).