Last month we showed you how to use resistors to reduce voltage. While it's a viable solution for many cases it isn't for others. As noted before, resistors work for devices whose load remains relatively constant: lights and some motors for example.
Where resistors fall short is with devices whose load varies: radios, gauges, and many electronic items. For those we're going to employ the components that make things like portable radios and computers possible: transistors.
When viewed from the printed...
When viewed from the printed side, most 78-series transistors match the following pin-out: one is the voltage input, two is the ground, and three is the voltage output.
What follows are ways to use three common variants of the transistor. Two are voltage regulators, basically infant versions of the ones in modern alternators. One limits voltage to a fixed output; another requires a touch more preparation and can reduce your car's voltage to any figure as little as 1.25 V. The third transistor is a type of amplifier that boosts any of those regulators' current capacity.
This may sound intimidating if you've never worked with electronics but let us reassure you that what we're about to do is exceedingly easy-the simplest example requires only three wires. Just as the case last month, you can even copy our examples. If there's anything tricky about them it's that they require soldered connections. We won't go there, as that's general information already covered in countless books and online tutorials and videos.
The 78-series transistor is hands-down the simplest in the regulator family. The two numbers following the number 78 designate the regulator's output: 05 for 5 V (great for USB ports), 06 for 6 V (great for gauges), and up from there in increments of 07, 08, 09, 10, 12, 15, 20, and even 24. Two exceptions violate this protocol: 7833 is 3.3 V (great for huge arrays of parallel-wired super-bright LEDs) and 7847 is 4.7 V.
This diagram represents how...
This diagram represents how a 78-series transistor works in a system. Feed the vehicle's power to the first lead (input) and connect the second lead (ground) to the vehicle body or chassis. The third lead (output) maintains the transistor's stated voltage.
As their numbering implies, these regulators' values are fixed, meaning their output remains constant regardless of input voltage, provided the input exceeds the output by a few volts and remains less than 30 or so volts. In some cases these regulators can handle 1 1/2 amps, certainly enough to operate gauges singly and probably in multiples. As a bonus, these regulators shut down if overheated or short circuited.
Adjustable Voltage Regulators
The 78-series regulators are great but can't address any voltage outside their specified range. Regulators in the LM family (LM138, LM317, and LM338, to name a few) can be "tuned" to reduce voltage all the way down to 1.25. Depending on the part number, some can handle as many as 5 amps.
Naturally, a little complexity accompanies this versatility. Two resistors govern the regulators' output. Here's their formula:
V = 1.25(1 + [R2 ÷ R1])
In most cases 78-series regulators...
In most cases 78-series regulators can be wired directly. They do, however, require a heat sink to shed the heat they generate. Nearly any large flat metal surface that isn't heat sensitive will work so long as it's exposed to air and the mounting surface wears a coat of silicone paste.
The value of any single resistor isn't important but the quotient of the two is. If we set the first resistor's value at 220 ohms and the second at 1,000 ohms the transistor regulates our input voltage to 6.93 V (theoretically, at least; as explained last month resistor values are usually within a range of their specified resistance but not exact). Here's how it looks on paper (remember to start inside the parentheses and work your way outward):
6.16 = 1.25(1 + [2,200 ÷ 560])
Here's the formula again, this time with a 1,000-ohm resistor for R1 and an 82-ohm resistor for R2.
1.35 = 1.25(1 + [82 ÷ 1,000])
Devout traditionalists will understand the significance of that last number: it's the same voltage as the obsolete battery in a Sun tachometer sending unit. Such a voltage reducer lets the sending unit wire directly to the vehicle's electrical system thereby eliminating the pesky battery altogether. And it's small enough to fit in the old battery socket. See the graph elsewhere in this story for other resistor combinations.
Take special note: the adjustable...
Take special note: the adjustable regulator's pin out doesn't match that of the fixed-output regulator. Know this or expect to fry a regulator. When viewed from the printed side and read from left to right the legs represent one (adjuster), two (output), and three (input).
The hardest thing about an...
The hardest thing about an adjustable regulator isn't the regulator or the resistors; it's its schematic. Just think of the first resistor connecting to the output leg, the second to the ground, and the adjuster leg ties in where the resistors meet each other.
A 78-series transistor's case...
A 78-series transistor's case or heat sink can touch the vehicle chassis but not so with the adjustable ones; their cases correspond with the input leg, meaning they're electrically "hot". Use a dedicated heat sink (this one's from Radio Shack) and keep it away from ground. Better yet, isolate the case with an insulator kit.