# Circuit diagram/symbols questions

#### Vetanoob

Joined Jan 25, 2022
4
Hi everyone.

This is my first post here.

A little background, I used to work on purely mechanical stuff but now my new job requires me to brush up on my electrical knowledge which I had not touched since I last attended school more than 7+ years ago. I have a hard time reading circuit diagrams because I can't decipher anything else but the very basics.

I know it may seem like entry-level knowledge for some of you, so I really appreciate if you could help me out with some questions related to the following diagrams:

Question 1: How does the main switch work when the power is turned on? What exactly causes the two switches to snap to the 'NO' position?

Question 2: What does "CMJ" stand for? Is it just some terminology made up by the manufacturer? or is it some form of industry standard? What does the 1 and 2 stand for?

A million thanks for anyone who can help me!

#### boostbuck

Joined Oct 5, 2017
381
The switch has three contacts - COM (common) on the left, and NO (normally open) on the upper right, and NC (normally closed) on the lower right. In the diagram, only NO has been labelled. There are TWO switches shown, and the vertical line between them indicates they are ganged to operate together, normally indicating they are in the same physical housing. They are mechanically sprung to move rapidly between either NO or NC. In this case, likely because your thumb has pushed a lever to force a change.

CMJ - I don't know. I think probably specific to the manufacturer.

1 and 2 are to number the wires in the connector block for reference purposes - probably the physical connector has labels on it.

Last edited:

#### BobTPH

Joined Jun 5, 2013
7,554
What exactly causes the two switches to snap to the 'NO' position?
That is a mechanical question, which is not typically part of a schematic.
Question 2: What does "CMJ" stand for?
It a component designator, used only to uniquely name the components in the schematic. There are common ones like R for resistor, and C for capacitor, but you can make up any combination of letters and a number. J usually means jack, so I would say it refers to some specific type of jack.

Bob

#### AnalogKid

Joined Aug 1, 2013
10,585
View attachment 258712
Question 1: What exactly causes the two switches to snap to the 'NO' position?
Your fingers. And that is not two switches, it is one switch with two poles.

Question 2: What does "CMJ" stand for? Is it just some terminology made up by the manufacturer? or is it some form of industry standard?
That is intended to show both halves of a connector pair. It might be bad drafting technique, where J1 id the right half part, and CM is the mating part, with a space missing between them.

What does the 1 and 2 stand for?
Those are pin numbers.

ak

#### Vetanoob

Joined Jan 25, 2022
4
Thanks guys.

So now I get the gist of it.

So "NO" isn't actually meaning 'no' but stands for a "Normally Open" switch until someone physically flips it to a closed position.

So the "CMJ2" is probably a manufacturer's designation for a connection terminal and i'll have to look it up myself.

Thanks!

I'll post my next question in a follow up post.

#### Vetanoob

Joined Jan 25, 2022
4

Question:
Am I right by saying that this diagram basically shows the transmission of electrical power to all of the above mentioned motors and stuff?

#### AnalogKid

Joined Aug 1, 2013
10,585
While NC next to a switch or relay contact set means Normally Closed, NC next to a connector probably means Not Connected.

ak

#### boostbuck

Joined Oct 5, 2017
381
The diagram shows the layout of the wiring, rather than power as such. It shows how the various motors and whatnot are connected to the 'Hydraulic Motor Relay PCB'. Some detail of the function within the PCB would show the distribution of power.

#### GetDeviceInfo

Joined Jun 7, 2009
2,174
View attachment 258715

Question:
Am I right by saying that this diagram basically shows the transmission of electrical power to all of the above mentioned motors and stuff?

It shows pictorially, the interconnections of the PCB, but does not show or imply transmission of power to any specific item. A schematic is the method of communicating that information.

#### Vetanoob

Joined Jan 25, 2022
4
I see. thanks guys.

If i have more questions i'll follow up here for now.

if not then it'll be on a different thread.

#### Tonyr1084

Joined Sep 24, 2015
7,454
Here are three switches showing their insides. Each switch is an SPDT (Single Pole Double Throw) type switch. It can be configured in many different ways. One would be the normal operation of switching between 2 & 3 or 2 & 1. The first switch is shown connecting pin 2 to pin 3. The second switch is shows pins 2 & 1 in contact with each other. The third switch shows a switch with "Center Off", meaning pin 2 is not connected to anything.

Each switch has a spring where you toggle the switch one way or the other and spring back to the center (third switch). Those are known as "Momentary" type switches. It could be loaded that no matter what the switch always comes back to the center position as shown in the third illustration. OR it could be spring loaded in just one direction and spring back to center. But if you push it the other way it switches and maintains that position. The first two switches are SPDT. The third is SPDT "Center Off" - or On-Off-On. If it's a momentary switch one of the two "On's" would have a circle around it. I don't have an illustration for such.

A switch known as a DPDT (Double Pole Double Throw) will have two sets of these contacts internally, just three more pins. Pins 1 & 4 would be one side, pins 2 & 5 would be the common (center) connection and 3 & 6 would be the other side. There can be many configurations of switches.

Here are three more switches: They are called "Slide Switches". Exactly how it's numbered may depend on the manufacturer. But in the case of these illustrations you can see the the bar going from side to side amongst the three pins suggest that the switch can be used to turn on a single circuit or two circuits at once. If you slide the switch far enough you break the connections between all three pins. In the switch below you can connect pins 1 & 2 or 1, 2 & 3 or 2 & 3. I didn't draw the switch in the off position, but just imagine sliding the bar just a little further so that the contact surface is only contacting a single pin, either pin 1 or 3.