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USING SOLID STATE SWITCHES
(INSTEAD OF MECHANICAL RELAYS)
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!
Check the Diagrams Below:
(click the picture for larger image)
Diagram 1
Diagram 2
Diagram 3
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