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.

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.

Fixed-Voltage Regulators
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.

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])

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.