Sinking or Sourcing?

Confused about sinking and sourcing in DC circuits? Let's take a look.

 

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In this article

• What do sink and source mean?
• Where does sink or source matter in automation? 
• When is it more cost effective to choose sinking over sourcing?
• How do you choose which output module to use?

DC circuits

Sinking and sourcing basically refer to the two methods of wiring a discrete DC circuit. 

The choice is where to put the switch.

In a DC circuit, power flows in one direction: from the positive side of the power source, through the load, to the negative side of the power source.

To switch the load, you can place the switch either in front of the load (on the positive leg of the circuit) or behind the load, on the negative leg.

 

 

Sinking

Sinking is switching the load in the negative leg of the circuit, as shown below.

Sourcing

Sourcing is switching the load in the positive leg of the circuit, as shown here.

Where does sink or source matter in automation?

Quick answer: you often have to choose an output module because the device you are wiring requires either a sink or source. For example, if you have a prebuilt electrical cabinet, it may be wired using a common positive or a common negative method. Thus you HAVE to choose the correct discrete output module for the way the panel is wired.

Thankfully, you don’t have to worry about choosing between sinking or sourcing when it comes to alternating current. Since AC swings back and forth between a positive voltage and a negative voltage, it requires a different switching method. It’s direct current (DC) where we have to choose.

A little background on the construction of transistors: there are two types, NPN and PNP. They are built differently at a silicon level and switch currents with different methods.

Back in the day (1950s to 1970s), transistors could sink (NPN) more current than they could source (PNP) for the price, so a sinking or sourcing decision made a difference in cost. In fact, even today many electronic components have asymmetric current drive ability and can switch more current to ground than to the positive rail.

The two types of transistors

The other place sinking and sourcing made a difference was on the electrical panel company’s shop floor. Some companies built their panels to have a common positive rail, and some built them to have a common negative rail. Sometimes the layout of the electrical equipment lent itself to one or the other way, but often it was simply a case of “that’s how we’ve always done it.”

This is where we, the automation folks, come in to sort it all out and make it work. Since we’re talking about DC circuits and how our loads are wired, we have to pick the right kind of discrete output module to switch those loads.

If we’re retrofitting an older non-automated control cabinet, say one that is full of relays, we need to take a look at how it’s wired. We have to think about the cost of rewiring versus just buying a discrete output module that will fit in with the cabinet’s existing common-positive or common-negative wiring.

Sinking example, with ground as common

A modern example of this choice would be an LED quad-color stack light. (Sometimes called a tower light, it’s a stack of red, yellow, green and white colored LEDs in a housing). Does it have a common positive, or a common negative? 

We usually cannot change the stack light wiring. It’s built either way by the manufacturer. We then need to pick a discrete output module that can drive the four lights to the specified rail and thus turn each of the LEDs on as required.

One of the most common quad-color stack lights has a common-anode (positive) lead.

In this case we would wire that lead to the positive power supply rail, wire the other four legs to our discrete output module, and then wire the discrete module’s common to the negative rail of the power supply. This is a sink configuration. We turn on the discrete output, the power is sourced from the positive rail, flows through the LED, is switched through the discrete module, sunk to ground, and the LED is turned on. 

 

LED quad-color stack light, wired to a SNAP sinking DC output module

Mixed wiring

But what if you have a mix? What if, say, your stack light is common-cathode (negative), but your machine electrical panel is wired as common-anode (positive)? Rewiring the whole panel is not an option, and neither is ordering a different stack light. You have to automate what’s there. What to do?

In this case you can look at using an isolated module, where all discrete output channels are totally separate from each other. With this channel-to-channel isolation, each channel can be wired as either sinking or sourcing. 

Opto 22 sinking and sourcing discrete output modules

Opto 22 offers sinking, sourcing, and channel-to-channel isolated discrete output modules in two product families: groov EPIC I/O and the older SNAP I/O. Both families give you reliable, field-tested I/O you can count on, with a lifetime warranty and free product support, manufactured and supported in the U.S.A. 

Sinking and sourcing with groov I/O

For groov EPIC, your best choice may be a flexible channel-to-channel isolated discrete output module like the GRV-ODCI-12 or the GRV-ODCIS-12. You simply configure each channel for sinking or sourcing as needed. 

IMPORTANT: For inductive loads, make sure that there is a commutating diode in the circuit, as shown below.

Wiring for sourcing on groov EPIC isolated discrete output modules

If you just need sourcing, you can choose the GRV-ODCSRC-24, a dedicated sourcing module. 

 

Other sinking options are available, too. If you need both analog and discrete inputs and outputs, take a look at the multipurpose EPIC GRV-MM1001-10. For remote use, the groov RIO universal I/O modules (GRV-R7-MM1001-10 or GRV-R7-MM2001-10) offer similar options. 

On both the EPIC multipurpose module and the RIO universal I/O modules, channels 0–7 may be used for sinking outputs or for other purposes.

IMPORTANT: For inductive loads, make sure that there is a commutating diode in the circuit, as shown below.

Wiring for sinking on groov universal I/O modules

Sinking and sourcing with SNAP I/O

For greatest flexibility with SNAP I/O, you can choose a channel-to-channel isolated discrete output module, like the SNAP-ODC5-I or SNAP-ODC5A-I. Because the channels are isolated, you just configure each channel as sinking or sourcing. 

SNAP sinking modules

For a dedicated sinking module, choose the SNAP-ODC5ASNK or SNAP-ODC5SNK, or the high-density SNAP-ODC-32-SNK. 

IMPORTANT: For inductive loads, make sure that there is a commutating diode in the circuit, as shown below.

Wiring for SNAP sinking modules

SNAP sourcing modules

For a dedicated sourcing module, choose the SNAP-ODC5SRC or the high-density SNAP-ODC-32-SRC.

IMPORTANT: For inductive loads, make sure that there is a commutating diode in the circuit, as shown below.

Wiring for SNAP sourcing modules

 

Conclusion

So there you have it. In automation we often do not have a choice. We have to choose sinking or sourcing either because of the way transistors are made, the way electrical panels are wired, or the availability of the electrical or electronic parts we are given to use.

You can also take a look at our animated demo about sinking and sourcing. 

About Opto 22 products

Opto 22 industrial automation products include the latest groov EPIC edge programmable industrial controllers and groov RIO edge I/O, as well as field-proven legacy products like SNAP PAC and solid state relays (SSRs).

With groov products, you can collect, process, view, and exchange data where it's produced—at the edge of the network—and securely share data among databases, cloud platforms, web services, PLC systems, and other systems and equipment. groov products give you built-in cybersecurity and multiple control programming options including CODESYS, plus Node-RED, Inductive Automation's Ignition Edge, and MQTT communications.

Opto 22 products are ideal for OEMs and machine designers, process control, machine monitoring, power and energy monitoring, predictive maintenance, remote installations like water-wastewater and oil & gas, SCADA, and industrial internet of things (IIoT) applications. 

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