Fault Finding Electrical Circuits: Handy Tips

Thought we might spend a bit of time this week to look at a few tips that could save you some time during fault finding on circuits.  We will start with two tips on this post, so let’s move onto the first one:

Solenoid Valves

How can you establish whether a solenoid valve is energized or not, without trying to test on the very small connection block these units have? Before answering that, we need to understand how a solenoid valve actually works for it to make sense. Again, I am using my own description and terminology to try and explain it as easy as possible.

The principal of operation is the same as that of an electromagnet. We have a coil wound around a core and when power is applied to the coil, it creates a magnetic field which will magnetise the metal core. This principal is applied to the operation of the solenoid valve. Inside the metal core, you will find what I will call a “plunger”. When the core is magnetised, the plunger will be pulled upwards which means that the valve ports will be either opened or shut.

So, our way of establishing whether it is energized uses this magnetic field. We simply place the tip of a small (non-magnetic) screwdriver on top of the locknut of the solenoid valve. When energised, this magnetic field will attract the screwdriver and if you lift it away very gently, you will feel how it holds it before it comes away. Done! The solenoid is energised

Checking to see if a contact is made

This one is a bit different. Assume you have a control circuit and you have tested up to the point where the circuit goes out to a limit switch in the field. You have found the wire going to it but have no idea where it comes back into the panel as the circuit is not labelled. How do you determine if this limit switch is activated?

The visual check shows you the limit should have switched based on the position of the switch, but your circuit is still not operating. Let’s say the control circuit voltage is 24V DC. Open the limit switch cover and test across the contact of the limit switch. Should you record a voltage, it means the contact is open. Should you see no voltage, it means the contact is closed.

To explain this better, have a look at the sketch


If you test across the contact of Limit Switch L1 and you get a voltage reading, it means we are seeing the +24V Dc on the one side of the contact and the 0V is actually coming through the coil of the relay, hence we read the voltage. If the contact is closed, you will not see this 0V at that point as you will have +24V on both sides of the contact. Make sense?

That’s it for this week so remember: Safety First!