Nuts & Volts - July 2004

Q&A

Voltage, on the other hand, is equal to V = current/power. In logarithmic language, that’s a 20 ratio, not 10. The formula is Volts (dB) = 10^(dB/20)and can be calculated on the PC by using the “x^y” function of the Windows calculator. First, divide the dB value by 20 and save it to memory (MS). Clear the calculator using the Esc key, type in 10, click on x^y, read the memory (MR), and hit Enter. That is your voltage ratio.

The decibel system has another advantage in that the ratios can be added and subtracted without going through a lot of math. For example, let’s say that you want the voltage ratio between 25 and 40 dB or between 40 dB and 34 dB. Just subtract the values — negative numbers are permitted and represent an attenuation rather than amplification. Run your answer through the calculator (or look it up in Table 1) and you will have your power and voltage ratios. In the first example, 40 minus 25 is 15 dB for a voltage gain of 5. 62. In the second, - 6 dB equals a voltage attenuation of exactly half with one-fourth the power.

I have rewired the rotors so that the resistance through each of the 26 possible paths in each rotor is less than 1 Ω. That was, comparatively, easy. The problem is that the intermediate stators are built out of contacts and springs ( 98 parts per stator!) and the resistance through each of the 26 paths in the stators is not uniform — varying from about 1 Ω to about 4 Ω, depending on the particular stator and path.

Can you show me how to build a power supply that has the following characteristics?

LM317

T

IN OU

COM

Figure 1

18V - 35V

0.1

R1 3. 9

Constant-Current Regulator

320mA Out

• If it sees an open circuit (say, more than 100 Ω), it doesn’t panic and produces no output.

voltage is needed to push 320 mA through a 100 Ω load. Using Ohm’s Law, E = IR = 0.32 x 100 = 32 volts. Because the LM317 needs at least 3 volts to operate, the minimum input voltage is 35 volts DC. If you can live with an upper limit of 50 Ω, the minimum input voltage is 18 volts, which is easily obtained from a cheap wall-wart. Be sure to heatsink the IC because it can run hot — up to 10 watts with a shorted output.

• If it sees a resistance of 10 Ω, it cranks out 3 volts at about 0.3 amps.

High-Power Variable Voltage Regulator

• If it sees a resistance between 10 and 100 Ω, it puts out whatever voltage is required to develop 0.3 amps.

Peter Ingerman via Internet

Wide-Range Current Regulator

Q. I’m refurbishing a pair of old Hebern cryptographic machines and I think I need something rather peculiar in the way of a power supply.

The machine (from an electrical point of view) looks like a voltage source that is connected to the “left stator.” Then there is an alternating sequence of rotor-stator-rotor-stator-rotor-stator-rotor-stator-rotor, ending with the “right stator.” There are 26 possible paths through each rotor and each stator. The left rotor is connected to a keyboard; depressing a key “wets” one of the 26 contacts on the left rotor. The power is then transmitted through the rotors and stators to the right rotor, which illuminates one of 26 light bulbs to indicate the enciphered letter. The orientation of the rotors to the stators changes on a letter-by-letter basis.

JULY 2004

A. What you need is a constant current power supply of 0.3 amps or 300 mA. This is easily accomplished using an LM317 adjustable voltage regulator (Figure 1). Resistor R1 determines the output current of the LM317 by setting the current of the internal reference voltage, which is 1.25 volts. If R1 is 1.25 Ω, then the output current will be 1 amp; a 3. 9 Ω, 1/2 watt resistor limits the current to 320 mA.

Now, let’s determine how much

Q. Could you suggest a schematic for a variable power supply to replace the electronics inside my old HeathKit (BE- 5) battery eliminator? I don’t need the 6 volt option, but something with better 12 volt regulation would be nice. I’m thinking of replacing the variable transformer with a pot and a fixed transformer — as long as it fits in the original cabinet, which I have grown to love.