Control Basics: The Difference Between Sink and Source Logic
Two Seats Electric Car,Handicapped Tricycle,Eec Electric Car,Disabled Tricycle YUMBOMOBILITY LTD , https://www.yumbomobility.com
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For those new to wiring I/O systems for motion control, it can feel daunting the first time around. If things aren’t hooked up properly, you might encounter anything from a motor not responding as expected to irreparable equipment damage. Even after years in the field, I still feel that initial jolt of nerves before hitting the START button during a demo. After all, Murphy's Law always seems to be lurking.
The challenge begins when engineers or manufacturers use varying terminology for wiring. How do you ensure you’re speaking the same language? For instance, is sourcing logic the same as PNP logic? Or is a “sink†actually a “source� Over the years, we’ve heard it all while providing technical support for motion control applications.
Often, tech support teams will guide you toward a wiring diagram and emphasize following it precisely. But what exactly do sink logic and source logic mean? Let’s start by breaking down some key terms.
### Basic Terminology
**Electronic Circuit (Digital)**
An electronic circuit comprises components like resistors, transistors, capacitors, inductors, and diodes. These elements connect via conductive wires or traces on a printed circuit board (PCB). It requires both voltage and ground, with ground acting as a reference point to measure voltage potential. Digital circuits utilize direct current (DC) voltage and operate on discrete values (on/off). A DC power source moves from positive to negative.
**Input/Output (I/O)**
I/O refers to inputs and outputs, meaning anything that performs an action based on input. For example, a keyboard serves as input, and a monitor as output. In this context, I/O describes the communication between two devices (like a PLC and a stepper driver) using binary logic.
**Electrical Load**
An electrical load represents a component or section of a circuit consuming electric power. This contrasts with a power source, such as a battery or generator, which generates power. Examples include light bulbs and motors. In this case, we’re discussing an input circuit.
**Logic Circuit**
A logic circuit functions as an electric circuit (I/O) where the output depends on the input. It can include one or more binary inputs (on/off) and a single binary output. Components like switches, relays, diodes, and transistors form part of a logic circuit.
### Sink Logic vs Source Logic
Sink and source logic circuits are frequently linked to PLC I/O signals and apply exclusively to DC circuits. They differ based on the types of components used and how current flows.
- **Logic is defined by the type of components in the circuit.**
- **Logic determines the direction of current flow in the circuit.**
- **Whatever logic you use for the output, the opposite applies to the input.**
**Sink Logic**
In sink logic, an NPN transistor creates a path to ground for the electrical load. An NPN transistor circuit operates only when paired with a PNP transistor circuit. Therefore, a sink logic circuit requires connection to a source logic circuit.
Figure 1 illustrates a sinking digital output connected to a sourcing digital input. The input circuit connects between the positive power supply (Vcc) and the NPN transistor.
**Source Logic**
In source logic, a PNP transistor provides a path to voltage for the electrical load. A PNP transistor circuit works only when paired with an NPN transistor circuit. Thus, a source logic circuit needs to connect to a sink logic circuit.
Figure 2 shows a sourcing digital output linked to a sinking digital input. The input circuit connects between the PNP transistor and the power supply ground (0V).
A useful mnemonic for remembering sink vs source logic is to think of a source logic circuit as the voltage source (providing a path to the source), while a sink logic circuit sinks toward the ground (providing a path to ground).
### Tip: Compare Wiring Diagrams Side by Side
When connecting I/O between a PLC and a servo or stepper driver, I found it helpful to print out the wiring diagrams from both the PLC and the driver. Placing them side by side allows you to visualize the current flow from the voltage source to the load and back to ground.
### Trace the Current Flow
Much of my remote support involved tracing current flow over the phone. To avoid damaging customers’ PLCs, I’d print the wiring diagrams and trace the current from the voltage source, through the load, and back to ground. Supporting remotely taught me the importance of understanding exactly which side of the I/O the customer referred to.
For a PLC sourcing output to trigger a sinking input on the driver, we must ensure adequate voltage and current. Current must enter a positive terminal from the PLC side, pass through the output circuit, reach the input circuit (the electrical load), and exit through another terminal back to the power supply ground to complete the circuit. Each I/O signal typically provides two terminals—one for incoming current and one for outgoing. Sometimes, terminals group together under a shared label called “common,†which could represent either the voltage source or ground.
### Tip: Don’t Forget Power Requirements
Pay close attention to voltage and current requirements for both inputs and outputs. If the output demands current-limiting resistors, use Ohm’s Law to calculate external resistance, but don’t overlook internal resistance. You must meet both voltage and current requirements of the input.
### Which Is Safer?
When determining the correct wiring method, it’s essential to consider the type of logic or transistor. Additionally, there’s a safety distinction. If something goes wrong—like an I/O signal line experiencing earth leakage or a ground (0V) short circuit—it could pose risks.
However, if source logic is used, the input circuit doesn’t directly connect to the positive power side (Vcc), so earth leakage or a short circuit won’t activate the input. This makes source logic considered safer.
### Summary
“Sink†and “source†refer to the flow of direct current in an electric circuit.
- A sinking input/output circuit provides a path to ground for the electrical load.
- A sourcing input/output provides the voltage source for the electrical load.
Logic depends on the type of components in the circuit:
- A sourcing input/output circuit requires a PNP transistor.
- A sinking input/output circuit requires an NPN transistor.
A simple electronic circuit includes one digital input connected to a digital output. To power the circuit, you need a voltage source, ground, and a load.
- A sourcing input/output circuit supplies the necessary voltage.
- A sinking input/output circuit supplies the necessary ground.
- The digital I/O provides the electrical load required for the circuit.
### For Flexibility, Use Products Offering Both Sink and Source Logic
Certain products in the market offer both sink and source logic for flexible connections. Bidirectional diodes wired in parallel enable this functionality, while photo couplers minimize wiring damage. Choose these products if flexibility is a priority or if you plan to repurpose them later.
Here’s what the actual wiring diagrams look like for the BLE2 Series brushless motor drivers. One diagram shows sink logic outputs, and the other shows source logic outputs. The PLC is on the left, and the motor driver is on the right. INx designations are inputs, and OUTx designations are outputs.
Take a look at the first input, “IN-COM0†(Inputs Common). On the top diagram, it’s connected to 24VDC, and the input has a path to ground. On the bottom diagram, “IN-COM0†is connected to 0V, and the input has a path to the voltage source. The bidirectional diodes in the input circuits allow this flexibility.
We hope this helps! Most of our newer drivers offer both sink and source logic. If you need assistance finding them, reach out to our helpful technical support team.
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