What kind of schematic is this?

That didn’t go as planned!

Yesterday, I posted a challenge question featuring a reverse polarity protection circuit featuring relay (solenoid) and a diode.

• I first encountered the circuit in small 1960s aircraft applications. It’s a reliable way to prevent an external battery from being connect in reverse. Like jumping a car battery, this is not something you want to get backwards.
• The circuit has the advantage of simplicity and reliability as it places the high-current stress on the relay. ?
• My mistake was not including a flyback protection diode, as there may be situations where the commutating diode would be destroyed by the relay.

As an electronics generalist, I often switch back and forth between industrial controls and the embedded world. This can be a problem, as I occasionally drag concepts from one domain to the other. For example, in the last week on the DigiKey TechForum I’ve explored the 西门子 LOGO! PLC, NPN transistors, 瑞萨电子 RA4M1 applications. Also, I’m currently working on a Modbus application with a PLC talking to a microcontroller.

That’s enough to confuse anybody.

Did I cross a line and present a poor circuit diagram?

Maybe, but I'll let you decide after I provide some background information.

Let's start with ladder diagram. Not the PLC representation, but ancient relay-based diagrams such as this start stop, and jog circuit. Generally, it follows a right to left and top to bottom flow with most components on aligned horizontally.

However, this is not a PLC. We are allowed to make vertical connection between rungs such as the CR1 contact. If you are curious, you can learn more about the loop and the yellow highlighted steps here: Tutorial for the Start, Stop, and Jog Controller. ?

Are diodes allowed in relay-based ladder logic?

Certainly not in the PLC representation. Yet, we may find them in relay-based diagrams. The classic example is the flyback diode such as the white clip-in TCS diode in this large 24 VDC 施耐德电气 contactor. For many diagrams we are content to use a M1 designation for the contactor.

There are times when we must call out the diode. I would argue that this is essential for PLC applications with sensitive semiconductor-based outputs. In this situation, we absolutely want to show the diode as it essential to long-term circuit reliability.

Should a relay coil be represented by a circle or slashed rectangle?

Oh no, it feels like I've stepped into another holy war!

Perhaps this is another case where technicians and engineers seem to diverge. In front of me I have:

• Four textbooks that use the traditional circles.
• A European PLC datasheet that uses the rectangle.
• KiCad that uses the rectangle.
• There are also see recommendations from IEEE, IEC, ANSI to use the rectangle

Parting thoughts

Would it have made a difference if I used a rectangle?

How would you have drawn the challenge question?

Thank you to everyone who took the time to respond.

Sincerely,

Aaron