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.
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?
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.