Teardown: Relay DIN rail base

Here is a tear-down of one automation product. It is a relay base made by Phoenix Contact. As you can see is is a pretty normal looking relay product you see on automation systems. It mounts to DIN rail.

As you can see in the picture the whole product is designed for 48V DC operation. That 48V voltage level is used in telecommunications systems, on some data center applications and some automation applications.

When looking at the other side what is interesting is that the relay that sits on this base had 24V coil in it. This meas that there must be something more than just wires on this base. So let’s take a look inside.

It seems that inside the device there are several diodes, resistors and one LED. The diodes seem to make sure that relay only operates at right input signal polarity and protect against inductive kick-back from relay coil. One resistor is for LED current limiting. Two other resistors are current limiting resistors that make that 24V relay coil to work at 48V, so they together have the same resistance as the relay coil has. This means the resistors take 24V and relay coil 24V from the incoming 48V. There are two resistors because one small resistor could not properly handle all the needed power dissipation.

Interesting note is that this product has traditional screw connections for the incoming wires. Inside the device the connections to components are made using same technology as used in Insulation-displacement connector.