Using Solid State Switches (Instead of Mechanical Relays)0

By David Navone

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 depend­able? Well, for starters, the coil in a typical automotive relay has a DC re­sistance 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 com­bined resistance increases the trigger current.

Other reasons for seriously re­considering 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.

A “New” Transistor Switch?

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 applica­tion 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 2N6387 NPN Silicon Darlington Power Amplifier and its complement, the 2N6667 PNP Silicon Darlington Power Amplifier. A Darlington transistor is actually con­structed of two transistors in a single package, but for this article we will only be concerned with the effective NPN or PNP features of the transistors.

The 2N6387 is also an ECG263 replacement part and is available at electronic parts houses across the nation. The 2N6667 is an ECG264 replacement part. Both transistors are packaged in the TO‑220 configuration AND THE TAB IS ALSO THE COLLEC­TOR. This means that extra care must be taken when mounting the transis­tors so that the tabs do not touch the chassis of the car! Insulated heat sinks are available at electronic part houses. Be careful.

Where to Purchase Transistors

Another good source for these transistors would be Mouser Electron­ics. Their phone number is 800‑346­6873 and the correct part numbers are 511‑2N6387 and 570‑2N6667. The single quantity pricing is around $1.00 and $2.00 respectively. If you can’t find the 2N6387, Radio Shack sells their MJE-3055 in the T0-220 package. This part will work, but not quite as well in all applications.

Theoretically, these Darlington transistors can handle up to 10 Amps through their Collector‑Emitter junctions. Since they have a gain of over 1000, this means that the base current, through the 1 K‑Ohm resistor, would be something around 10 mA maximum. WOW!!!

The circuit schematics are drawn next to their pictorial illustrations. There are three different applications depicted. The two resistor, positive trigger circuit will provide +12 Volt DC to a load that is grounded.
The resistors can be any wattage from ¼-Watt on up.

The one resistor, positive trigger circuit will provide a ground to a load that is already connected to a source of + 12 VDC. This circuit may seem a little backwards, but it saves a resistor and still gets the job done. (This configuration is Richard Clark’s favorite.)

And now for the complementary circuit using the PNP 2N6667. This transistor will connect +12 Volts DC from the Emitter on through to the Collec­tor when the Base swings low. On the other hand, when the trigger at the Base gets +12 Volt DC, the transistor will NO longer conduct and the switch will be “off.”

This nifty PNP circuit can also be used as an inverter to convert +12 Volt DC at the trigger into a low at the Collector and a grounded trigger into +12 Volt DC at the Collector. Mechanical heavy current relays are really wasted in simple inverter circuits. Use a transistor.

The maximum voltage drop across the semi‑conductor junction will be around .6 Volt and should present little problem in control applications. Always fuse connections made to the + 12 Volt supply with an appropri­ately sized fuse. Heat sink if neces­sary, however when switching 1 Amp or less, no heat sink need be used. And don’t forget that the Collector is also the tab!

Diagram 1

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Diagram 2

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Diagram 3

A2409 transistorswitch3