Nuts and Volts - June 2014

24 June 2014

schools throughout the US for 'fire insulation.' Now, it's a big problem.

I'm with you — err on the side of caution until someone figures out what's going on. Certainly, the manufacturers aren't publicizing this information.

Bryan Bergeron

Potential Burnout

I'm writing about "The Decade Box Revisited," by Frank Muratore in the April 2014 issue. This circuit is similar to one published in Nuts & Volts by Fred Blechman about 10 or 12 years ago.

However, I believe Mr. Muratore's circuit arrangement is a little better than Fred's was.

Frank mentions the possibility of burnout if a 10-turn potentiometer is used. This is also possible, though not as likely in the circuit printed. The specified pots — according to the 2013 Parts Express catalog — are rated at 1/4 watt.

The power rating is much lower when the pot is set to a low resistance, and can result in pot burnout (I know from experience). I have two suggestions to reduce the chance of burnout. First, connect a 1/2 watt resistor — 10% of the resistance of the pot — in series with each pot.

Second, consider the use of higher power pots, especially for 100 ohms and 1,000 ohms. Pots with a two watt rating are available at a higher cost.

Bill Stiles

Definitely a 'retro' article. An experiment really. I had a similar box 30 years ago.

In any event, your catches are right on target. Great mods.

Bryan Bergeron

Points to Consider

There are some discrepancies in the April 2014 Ba TESLA article:

1. C1 is listed in the Parts List as a high voltage cap, but C4 is referenced in the write-up (p. 26).

2. A five foot CFL is referred to on p. 27. Where could I purchase that?

3. The use of the term "winding" to describe one turn around the form is not common in electronics (see p. 22, et al.).

4. The Parts List would be much more useful if manufacturer's part numbers were listed.

5. The use of the term "metric" on p. 23 to mean parameter is out of place in DIY electronics.

6. It would be a great help if the schematic had parts values on it.

7. Use of the power rectifier BR1 at RF frequencies might better be served by four lower capacitance high-speed switching diodes. Likewise, use of an electrolytic capacitor at RF (C3) is not necessarily going to produce good results unless a low inductance capacitor is paralleled with it.

8. The external DC input referenced on p. 24 doesn't seem to be on the schematic as such. There is a TP there, but that's not usually considered to be an auxiliary input connection — especially since there's no polarity indicated nor another connection for the other power supply connection.

9. What is a "rectified input jack" referenced on p. 24?

10. How is the "programming port" used?

Phillip Milks

I have addressed each of Phillip’s points below:

1. The C4 reference on p. 26 is a typo and should read "C1." C1 Is listed as a "1 µF metalized polypropylene capacitor" in the parts and does not have a voltage designation. The article points out that it must withstand the high voltages developed. A good source for this type of capacitor is from the circuit boards of obsolete CRT monitors. It is possible to use multiple 250V or 800V caps.

2. This is obviously a typo and should read "five foot FL (FLUORESCENT)":

3. I'm not sure how the term "WINDING" does not properly describe something that is "WOUND" on some type of form.

4. I agree.

5. I am not sure how this is a discrepancy.

6. I am not sure how this is a discrepancy.

7. Any type of diode rectifier circuit that can be quickly charged/discharged will work. Feel free to use any type of diode/capacitor combination you have on hand for this circuit.

8. This is a typo and should have read "external AC source." It should have been deleted from the article since I did not include an external power jack in the schematic.

9. See previous answer.

10. The programming port is the three-terminal audio jack that connects Serial In, Serial Out, and GND to the PIC for programming it in place (audio jacks are the most common type interface for PICAXE). This connection to the programmer is not necessary if you program the PIC outside the circuit. Most micros use an FTDI converter for USB to RS- 232. The DATA(+) and DATA(-) become Rx/Tx which connect to the serial pins on the MCU. The Tx connects to the SeriaI In and the Rx connects to the Serial Out pin.