What is a Sink? What is a Source? What are the Differences in Output?

Teach Me Please! Ms. Ori
Teach Me Please! Ms. Ori

In this column, the differences between sink logic outputs and source logic outputs are introduced in an easy-to-understand manner with examples of each connection.

  • Mr. Vex

    Mmm... Ms. Ori, I need your help!

  • Ms. Ori

    What's wrong?

  • Mr. Vex

    A customer told me that they want to use brushless motors in equipment for export abroad, and asked if we have one for source logic. When I was talking with the customer, I really couldn't understand what they were talking about because they were talking about sink and source.
    I was so confused and couldn't "sink" straight.

  • Ms. Ori

    Hey! Vex, you've grown up enough to be able to make jokes like that.
    By the way, did they mean I/O circuits?

  • Mr. Vex

    We were talking about higher-level PLC (programmable controller), so I assume they were I/O circuits…

  • Ms. Ori

    I see. There are two types of logic, sink logic and source logic, depending on the different input/output specifications.
    Viewed from the control output device,
    Sink logic is a state that draws in current when turned ON
    Source logic is a state where current flows out when it is turned ON
    .
    You have to match the input side with the output side, that is why the customer was concerned and discussed with you.

  • Mr. Vex

    Ohh, the input/output specifications differ depending on the case. But what exactly is the difference between sink logic and source logic?

Differences between sink logic and source logic ① Wiring Method

  • Ms. Ori

    Sink logic and source logic actually use different control output devices, which can be roughly divided into the following two types, each with different wiring.

    [Figure 1] Sink logic... NPN transistor
    [Figure 2] Source logic... PNP transistor

    [Figure 1] Sink logic connection example
    [Figure 1] Sink logic connection example

    The input circuit is connected between the power supply's positive side (Vcc) and the NPN transistor.
    The signal is turned ON when current flows into the control output device.

    [Figure 2] Source logic connection example
    [Figure 2] Source logic connection example

    The input circuit is connected between the PNP transistor and the power supply GND (0 V).
    The signal is turned ON when current flows out of the control output device.

  • Mr. Vex

    I see! The wiring method need to be changed to match each transistor.

Differences between sink logic and source logic ② Safety

  • Ms. Ori

    In addition to the different wiring methods, there are also differences in safety.
    If an abnormality somehow occurs in a customer's equipment and the I/O signal lines experience a ground fault or 0 V line short, that would be dangerous. [Figure 3]
    In that respect, one reason that source logic is considered a safer connection method is that even if a ground fault or short occurs in the signal lines, the input circuit will not be turned ON because it isn't connected directly to the power supply’s positive side (Vcc). [Figure 4]

    [Figure 3] When a short occurs in sink logic
    [Figure 3] When a short occurs in sink logic
    [Figure 4] When a short occurs in source logic
    [Figure 4] When a short occurs in source logic
  • Mr. Vex

    Now I get it!

  • Ms. Ori

    You probably didn't know about this, but in the past, the products was mainly used for the domestic market, so they only needed to be compatible with sink logic. Nowadays, however, many customers are exporting their equipment overseas, so we can accommodate both types.

  • Mr. Vex

    That's incredible! Products are also becoming global!
    So, I should introduce source logic compatible products to our customers who can use source logic.

  • Ms. Ori

    Exactly! So, Vex, which brushless motor would be the best choice this time?

  • Mr. Vex

    Let me see. The BLE2 or BMU Series!

  • Ms. Ori

    Well done! These products are designed to work with both sink logic and source logic! By the way, did you ask them about the driving conditions?

  • Mr. Vex

    They said that output power is about 400 W and it is used up to 4000 rpm. They also wanted something that was easy to operate and connect, so I think I'll introduce the BMU Series.

  • Ms. Ori

    Mr. Vex, you're really learning your stuff!

  • Mr. Vex

    Yes! Thank you! I feel even more confident based on what I have learned this time! I'm off to introduce to the customer ASAP!

Key points this time

What are sink logic and source logic?

  • Sink Logic: State that draws in current when turned ON
  • Source logic: State in which current flows out when turned ON

(as viewed from the control output device)

Benefits of source logic

Since the input circuit is not directly connected to the positive side (Vcc) of the power supply, the input circuit will not turn on even if the signal line has a ground fault or short circuit. Therefore, it is considered one of the safest connection methods.

Points to consider when selecting a motor

If there is a possibility of using either, a product that supports both sink logic and source logic is recommended.

  • April 15, 2022  Updated with the latest information.

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Teach Me Please! Ms. Ori