# How to select a relay for main power input

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.

• A little help is it AC or DC?
– Gil
Commented Jun 3, 2021 at 17:25

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.

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.

• I have considered a dual coil relay from TE for switching it off. I have attached a image of how I am planning to design a circuit. I have considered TE bistable relay. 1. When the momentary switch is pressed for first time the set coil gets energized and output is latched. The output will remain as long as reset coil is energized. 2. When the same momentary switch is pressed for second time the reset coil gets energized and output is unlatched. Is my understanding correct. Also can you please provide your ideas on how to turn it off Commented Jun 3, 2021 at 17:29
• I have added image in my first question since I was not able add picture in this comment Commented Jun 3, 2021 at 17:36
• See the update. Always put the information in the question - not buried in the comments. Commented Jun 3, 2021 at 18:24