Nuts and Volts - March 2008

Q&A

■ WITH RUSSELL KINCAID

In this column, I answer questions about all aspects of electronics, including computer hardware, software, circuits, electronic theory, troubleshooting, and anything else of interest to the hobbyist. Feel free to participate with your questions, comments, or suggestions.

Send all questions and comments to:

Q&A@nutsvolts.com

WHAT’S UP: Join us as we delve into the basics of electronics as applied to every day problems, like:

✓Electric Car Battery Charger

●✓How to Monitor Series Cells

✓Audio Amplifiers

X- 10 PROBLEM

QThe X- 10 appliance module is great for in-house control. The problem I have is with the new compact fluorescent lights. When you turn off the X- 10, there is enough leakage current that the CFL will flash once or twice a second. Not a complete lighting of the whole tube, but enough that it is distracting and a waste of energy. (True for units with or without the sense line cut.)

A couple of tests show that a small (4W) incandescent night light in parallel with the CFL takes care of the leakage current and the CFL won’t flicker, and there is not enough current so the incandescent doesn’t light. (Still wasting energy, but oh well.) Great! But ... when you turn the CFL on, the night light also comes on — another unneeded light, expense, and waste of energy.

What I need is a circuit that is small, cheap, and safe that I could put under the socket of a table lamp with a CFL, or in a dummy plug to be plugged in alongside the lamp into the X- 10 that

■ FIGURE 1

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March 2008

would take care of the leakage current when the X- 10 is off so the CFL doesn’t flicker, but doesn’t “turn on” (light, heat, etc.) when the X- 10 is turned on.

— V. Alan Mode

AYour X- 10 unit must be defective or poorly designed. At any rate, since the four watt lamp works, try a 0.1 μF, 600V capacitor (Mouser part #75- MKT1813410635 should work) in parallel with the compact fluorescent lamp. The capacitor will not use any power and will help correct your power factor.

ELECTRIC CAR BATTERY CHARGER

QI am building an electric, plug-in car using 12 deep cycle 12 volt batteries for the 144 volts to drive the motor. Chargers for 144 volts are expensive so I would like to build my own if possible, that will run off 120 VAC so that I can recharge anywhere there is an outlet. Any ideas for a circuit would be welcome along with

cautions about how not to damage the batteries during charging and how to do it fairly efficiently. There are some “bad boy” chargers I have seen that are simply a transformer and rectifying bridge. Should these be used, for example?

— David Abineri

AThe transformer/rectifier system works okay, you just need some way to limit the current. Most battery chargers have a circuit breaker if the current is too high, but I used a one ohm, 100 watt resistor to limit the current when the battery is in a low charge state (see Figure 1). The transformer is 1:1 when the primary windings are connected in parallel and the secondary windings are connected in series. The peak voltage is 165 volts which is needed to fully charge the 144 volt battery. The tolerance of components and line voltage is such that I would not recommend leaving the charger on the battery for more than a few days. So, there is a switch to disconnect the one ohm resistor, but leaving a 200Ω in place to provide a trickle charge. The switch is double pole, single throw with the contacts connected in parallel. You can leave the trickle charge on indefinitely.