Are Siemens wrong with their sinking/sourcing definition?

Hi,

I read this on the Siemens website:
https://support.automation.siemens....objaction=csview&extranet=standard&viewreg=WW

(Click the Sinking / Sourcing FAQ question on that page)

To my understanding they have Sourcing and Sinking wrong when it comes to the Digital Inputs.

I've always thought of 'sourcing' as getting the 24V and 'sinking' as opening a channel to ground.

Am I wrong or has Siemens got their definition muddled up.

I've done a search on Google which seems to suggest that I'm right but I just want to make sure that I'm not missing something here....

Many thanks

;-)

The description on the Siemens Website looks correct to me.

What is troubling you with their explanation?

Stu....

sthompson said:

The description on the Siemens Website looks correct to me.

What is troubling you with their explanation?

Stu....

Click to expand...

Hi Stu,

I always thought that 'sourcing' was having the switch connected to the 24V rail and then to the input. So the switch comes on and you are 'sourcing' the voltage from the switch (so to speak...).

And I always thought of 'sinking' as having 24V on the switch and then when you switch it you pull the voltage to ground and sink it.

The Siemens drawing shows the opposite and, actually, more stuff on the internet is agreeing with the Siemens drawing so I think I've had it wrong. :-(

uptown47 said:

I always thought that 'sourcing' was having the switch connected to the 24V rail and then to the input. So the switch comes on and you are 'sourcing' the voltage from the switch (so to speak...).

Click to expand...

No the "sourcing" relates to the input, not anything else. It is the input that sources the 24V to the switch.

uptown47 said:

And I always thought of 'sinking' as having 24V on the switch and then when you switch it you pull the voltage to ground and sink it.

Click to expand...

Not sure what you mean. Arent you describing a short-circuit ? That would definitely be one way to switch between 24V and 0V
Again, the "sinking" pertains to the input, not the switch.

edit: The word "sinking" have too many meanings, which I guess is the cause of the confusion.
The "sinking" is like a kitchen-sink. The water is drained into the sink, like the electrical current goes into the input.
The "sinking" is not like something sinking like a ship, which is your analogy with "pull voltage to ground".

uptown47 said:

Hi Stu,

I always thought that 'sourcing' was having the switch connected to the 24V rail and then to the input. So the switch comes on and you are 'sourcing' the voltage from the switch (so to speak...).

And I always thought of 'sinking' as having 24V on the switch and then when you switch it you pull the voltage to ground and sink it.

The Siemens drawing shows the opposite and, actually, more stuff on the internet is agreeing with the Siemens drawing so I think I've had it wrong. :-(

Click to expand...

It depends which end of the System you are looking from.

If you think about it, the Input/Output and Device have to be opposite pairs. So, if you are going to supply 24VDC (Source) via a Switch, or other Device to the Input Card, it must provide a path to -24VDC or the Common. Therefore, the Input Card is Sinking. If you are switching the Input to Common(Sinking), then the Input Card must supply 24VDC (Sourcing).

The same idea applies to an Output Card. If we have a number of Relays with their - Terminal connected to -24VDC, then to operate them from an Output, we have to supply +24VDC (Sourcing) to the Relay Coil. If the +24VDC is being used as the Common, then the Output must pull the Relay Coil down to -24VDC (Sinking).

These days many Input Cards will do both, depending on whether you connect +24VDC or -24VDC to the Card Common(s). I haven't seen any Output Cards that will do both, but that doesn't mean they don't exist.

I hope that helps, it is getting pretty late here.

Stu....

Jesper, Stu,

Thanks for the clarity of your answers. I get it now. My mistake was thinking of the terms from the point of the switch as oppose to from the point of the input.

I get it now!

Thanks again

;-)

No problem. I am glad we could help.

Stu....