How to select a relay for main power input

Question

I will be using momentary switch to control the main power input in my design. Hence to latch the power I need to select a relay. On what basis should I select a relay since I find different types of relay like solid state relay, mechanical relay and on. I dont know the criteria to select a relay. can someone help me in same. The input requirement for my design is 24V and 7A.

Answer 1

For the mechanical vs. solid state argument, there are certain applications where SSRs are indicated. They have the advantages of:

• Generally lower drive currents (20mA or less)
• No inductive coil to suppress
• Faster switching times with no contact bounce
• Vibration/shock resistance

But they have significant quirks that you will need to design around:

• May require current-limiting resistor on the input
• Lower tolerance for load overcurrent
• Inductive loads must be diode suppressed
• Usually require a heatsink for high current (7A, probably, depending on the relay)
• Limited selection of contact arrangements

Unless you have a good reason to pick a SSR, a standard electromechanical power relay will be much easier to design into your system.

Answer 2

Then you need a relay with

• a coil voltage to match that of your pushbutton circuit.
• a contact rating ≥7 A DC or AC depending on your application. This is important. DC is tougher on contacts than AC. On AC the arc naturally extinguishes on each zero-cross of the AC. On DC there is no zero-cross.

A second contact on your relay would be useful for the latching circuit. Have you thought about how you are going to switch this off?

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. (a) Standard latching relay circuit with on and off buttons. (b) A stepper relay will change state on each button press. A small relay can drive a larger relay for a high current load.

A ratchet latching relay is shown below.

Figure 2. The pawl mechanism is advanced on every impulse of the relay coil. Image source: HomoFaciens.