I get emails from Electronic Goldmine concerning their latest sales items. I have noticed a lot of electronic parts of a certain category in their sales.
They may have a large number of parts from manufacturers that are switching over to newer technologies. I have noticed a large number of relays in their sales, specialty relays such as ones used for switching power in AC powered equipment, or for controlling high currents in a 12 volt auto system. Then there are reed relays, where very small currents are switched. Most of all these relays are old style, with through hole for mounting to a circuit board, or with pins to plug into a socket. My assumption is that these relays are being replaced by whole modules which have all functions in a single replaceable module, such as car lighting or accessories. Or through hole parts are being replaced by surface mount parts. Another relay replacement is the SSR or solid state relay. The SSR’s input is isolated from the ‘contacts’ typically by an optoisolator.
One part they’ve had a lot of recently is power MOSFETs. These are through hole, most likely replaced by a surface mounted part. The MOSFET can be used like a relay. The prices can be very cheap when on sale.
Standard relays have the advantage of complete electrical isolation between the coil and the contacts, because the actuation is done electromagnetically. But the disadvantage is they are slow, taking milliseconds to actuate. Another disadvantage is the standard relay requires power to hold the contacts in the actuated position (there are latching relays that avoid this). This power can be substantial, tens to hundreds of milliwatts, which can be more than the power of the controlling circuit. So eliminating the relay can save power and increase efficiency. In contrast, a MOSFET’s gate draws no power when the MOSFET is on or off. It only takes some current to charge or discharge the gate capacitance when switching from one state to the other. That is less power than a standard relay.
As I’ve said before, many if not most of the through hole parts will continue to be available from suppliers, but not at the low prices you can find at surplus electronics companies. That’s one reason I try to get them in quantities to last awhile.