Using Solid State Switches Instead of Mechanical Relays

Mechanical 12 Volt DC automotive relays are used throughout the car audio industry to turn on amplifiers, lock doors, roll ‘up windows, etc. However, there is another way to activate such loads, and that way is to use solid state switches. Why would anyone want to replace a mechanical relay that is so depend ­able Well, for starters, the coil in
Using Solid State Switches Instead of Mechanical Relays - schematic

a typical automotive relay has a DC re ­sistance of around 83 Ohms. This would yield a current of over 140 mA with a 12 Volt source. Although this 140 mA may not seem like a large amount of current, there are many car audio components, as well as sensitive vehicle accessories, that cannot supply even this much current. And when multiple relays are connected to the same trigger output, the com ­bined resistance increases the trigger current. Other reasons for seriously re ­considering the use of mechanical relays would have to include cost, space, installation labor, and the annoying click, click, clicks. The cost of a good quality SPDT automotive 12 Volt relay is typically around $2. 25 each at the dealer level and $6. 00 or more at the retail level. Add to this the cost of the terminal connectors (relays are at least four terminal devices), and the fact that relays take up at least three cubic inches each and you might be interested in at least trying a new  method. And now for the new  method. Actually transistors have been around for over 40 years and there is really not much of anything that is new  about them. It is only their applica ­tion in car audio installations that might be considered as new.  Rather than get into a discussion of how transistors work, what they do, etc. , let`s just jump to one particular application ‘ switching energy on and off. Our choice for a good transistor on / off switch is the...

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