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Assistance Against Cyber Attack 2020 Issue-5

What materials make a good phased array antenna (i.e., efficient transmission and reception and the shape of the individual components)? What frequencies go through earth and seawater above 10 GHz also?

#05203
William Zimmerman
via email


In-Line speaker Amp 2020 Issue-5

I recently moved into a home that has in-ceiling speakers. I have them connected to an A/V receiver and in one room they work great. In the other room, the sound is very muted. There’s a volume control in that room which I’ve replaced and checked. I’m looking for some kind of amplifier that I can purchase or build that can just increase the volume level on that pair of speakers (there’s a pair leaving the receiver which goes into the volume control and then splits into four speakers). I have checked obvious issues and swapped the A/B pairs just to make sure my receiver hasn’t failed.

#05202
Michaeljon Miller
Trabuco Canyon, CA



Answers

Because you swapped the A/B speaker leads and got the same audio results, the culprit might be speaker-impedance mismatch. Check the output impedance of your A/V receiver and of the low-volume speakers. The receiver manual should specify an impedance, which in most cases comes to, 4, 8, or 16 ohms. If not in the manual, check for a label at the outputs. 

Also, find the impedance of your speakers in the manufacturer's information or on a speaker label.  You want the same impedance at both ends.  A mismatch can cause reduced volume and even distortion. If you want to measure impedance, here's a link to a helpful article: https://www.wikihow.com/Measure-Speaker-Impedance. If all else fails, look for an impedance-matching transformer. More information here: https://www.electronics-tutorials.ws/transformer/audio-transformer.html.

Jon Titus
Herriman, UT


Battery Woes 2020 Issue-4

I have some brand new lead-acid batteries that have never been used. They have been stored in my garage for a while (1-1/2 to 2 yrs). My smart charger errors and won’t charge them. Why is this and is there anything that can be done to revive them?

#04205
Reva Pino
Charlotte, NC


Capacitor P&Cs 2020 Issue-4

What are the pros and cons for using electrolytic capacitors in a voltage divider circuit to provide about 24 volts AC to a heater cable from the 120 volt AC line?

Is there a possibility of having a capacitor explode from overheating? If so, could that be prevented by stringing several capacitors in parallel to provide for additional heat dissipation?

#04204
Robert Gotts
Madison, IN


Fan Indicator 2020 Issue-4

Good day to all you experts!  I have a plywood basement floor that is suspended like any other floor in the house (bentonite soil in my area requires this construction). The actual dirt ground is about two feet below the wood floor, covered by a rubber tarp.

To prevent a build-up of mold and stale air, this space has a 6” duct vent fan that turns on via a humidity sensor rheostat. The supply side duct is on one side of my basement and the evacuation duct is on the other.

In the past, I could hear this fan running, so I knew when the bearings were wearing out. It was an easy job to buy a new duct fan and replace it. We just had our basement finished, putting drywall around the perimeter wall. Now I can no longer hear this fan when it kicks on.

Does anybody have a suggestion for some sort of sensor that detects when the fan is turned on by the humidity sensor but drawing too large of a current supply, so on the verge of bearing failure? Ideally, I would like some sort of an indicator light that I can make part of the access panel that is over the fan. Even an AC ammeter movement would be adequate.

At the location of the fan, I have both the switched 120 VAC power supply and a constant 120 VAC available if needed. I don’t have the specifications on this exact fan available, but a quick search online found several that had operating currents of 0.35-0.40 amps. I know the start-up amps would be a little higher but not too much because the motor is small and has very little inertia to overcome. Thank you for any suggestions!

#04202
Bill Young
Denver, CO


Electronic Candle Circuit 2020 Issue-3

I’m looking for a simple circuit for a 24 hour electronic candle that uses very little power. The candle would drive a single LED. It would run for x hours (say five), then turn off; 24 hours after it  has first activated, it would automatically turn back on for the predefined time.

I've found several ideas, but most of them surrounded the 555 chip which has a very limited time frame.

#03205
Scott Lapp
Simi Valley, CA



Answers

It would be almost trivial to write a small program for a microcontroller to do this. I actually built almost the same thing to turn on a window fan in the evening and off in the morning. An 8 pin PIC with a 32. 768 Khz crystal. A single button reset the processor, which then would time for 24 hours, then activate the fan. Easy enough to add additional times. No display or time setting needed.

Richard Cox
Thousand Oaks, CA

Check out the Custom Silicon Solutions CSS555C Micropower Timer. With a little programming and perhaps a small additional capacitor you will be able to get the delay times your looking for. Also see the article https://www.nutsvolts.com/magazine/article/february2016_CSS555TimerICs

Kurt Stefans
Valparaiso, IN

I built a similar circuit to control a window fan. I wanted it to turn on and off at a certain time each day. I used an 8 pin PIC clocked by a 32.768 KHz crystal. The circuit was installed in the fan's remote controller.

Timer 1 was clocked such that it created a rollover interrupt every second. The software then counted the seconds and incremented a minutes and hour counter. Pressing a button connected to the reset pin set all the counters to zero. Then when the seconds, minutes and hours counters were zero every 24 hours it output a signal to the remote control button to turn the fan on or off.

What you want would turn the candle on when the hours and minutes are zero, then turn it off when the hours are five and the minutes are zero. Contact me at [email protected] for details.

Richard Cox
Thousand Oaks, CA


Bench Power Supply 2020 Issue-5

I would like to build an inexpensive AC power supply for my workbench. I want something much smaller and lighter than a variac, 0-30 VAC, and maybe one or two amps would be fine. Can anyone point to a good schematic or even a well-written circuit description?

#05201
Jeff Bowles
Columbus, OH


Home Circuit Boards 2020 Issue-3

Does anyone still make oneoff circuit boards at home? What methods are being used by hobbyists and where do you get supplies?

#03204
Alvaro Collazo
Gulfport, MS


Model Train Controller 2020 Issue-3

My son has recently become a model train enthusiast and asked me how to control multiple trains on the same track. Is it possible and how would one go about building a controller?

#03202
Paul Sills
Grand Rapids, MI



Answers

To do what you want you will need to use the system called DCC. To see an explanation of how it works, see Practical Electronics, Jan 2021 where they show (on page28) how the voltage going to the tracks is encoded with digital control information as well as being the power source for all the trains and other equipment controlled by the system.

I suggest reading the magazine at a book sellers place while enjoying a drink because the magazine costs $12. I would not suggest building the circuit when similar factory built units are available, because the blank circuit board is 12 Pounds each plus VAT and shipping from GB.

Amazon lists a DCC controller called NCE PROCAB #5240010 which looks like a large TV remote with a display that can control your whole layout. This equipment is not cheap.

A review of the Bachmann HO scale "ChargerSC-44" diesel locomotive with AmTrack Cascades 1400 paint as shown in Model Railroad News, May 2021 is pretty favorable. It is part #67904 MSRP $469 from Bachmann Trains. 800 356 3910 https://bachmanntrains.com Also check out Walthers 800 487 2467 https://www.walthers.com for some less expensive models.

I hope this will be useful and get you started in the right direction.

Dale Freye
via internet

You and your son are in luck, controlling multiple trains on the same track is a problem already solved with Digital Command Control (DCC). The beauty of using DCC is that the National Model Railroad Association (NMRA) has a series of electrical standards that define the signal between the transmitter (known as the command station) and the receiver (known as the decoder). The NMRA  published the first of the DCC standards (see https://www.nmra.org/index-nmra-standards-and-recommended-practice) 20 years ago, so these standards are well established and stable.

There are multiple manufactures of DCC systems. Each system has a command station to create the signal and a booster to amplify the signal that is sent out over the rails. The manufacturers differ in the manner they input the user signal into the command station, via a throttle (also called a cab), since this in not covered by the NMRA standard. 

A decoder is placed in each locomotive. There are also multiple manufactures of decoders. Compliance with the NMRA standard allows any manufacturer’s decoder to correctly interpret any manufacturer’s DCC signal. Should you wish to purchase a “starter set”, my advice is to avoid over-researching, since given the time-honored DCC standards, there are no “bad” systems on the market. Rather, find a neighbor, friend, or local model railroad group that is willing to assist, then purchase whatever DCC system they are using.

Being a Nuts and Volts reader, you have other options to explore. Type “DYI DCC” your browser’s search. You will find numerous variations using an Arduino or Raspberry Pi to provide the signal while an old computer running a Java-based cross-platform program, JMRI (see  https://www.jmri.org), provides all interface. If your son’s locomotives do not have a manufacture installed decoder, he will be able to learn advanced soldering skills.

In summary – Jump into Digital Command Control and get much more out of the hobby than you and your son originally expected.

Dick Schwanke
Model Train Controller

There is a model train system called DCC where electronics in the train engine are controlled by a wireless remote control. Multiple trains can be controlled for speed, lights, horns, smoke etc. Visit a good hobby shop that has a train section or search online.

Richard Cox
Thousand Oaks, CA

What you're probably looking for is DCC (Digital Command Control) technology. I use the Digitrax system for my layout. There are also a few DIY projects out there: https://dccwiki.com/DCC_Projects.

The simplest explanation for this is each loco has a unique decoder embedded inside. The tracks are always energized and commands are sent along the tracks from the controller, causing the recipient loco to execute that command. Good luck!

Michael Picco
Placerville, CA


Transformer 2020 Issue-3

How do I calculate the number of turns for both the primary and the secondary windings of a transformer?

#03203
Opeyemi
Ibadan, Oyo



Answers

The main problem with transformer design is the determination of the minimum number of turns for the primary. With too few turns, the transformer will overheat. Too many turns is only a problem if the turns will not fit within the “E-I” core windows.

We also need to determine what size of wire to use in the windings. With the aid of information extracted from Reference 1, we can design a power transformer having a 50 degree C temperature rise above ambient.

Figure 1


Use Equation 1 Figure 1 to estimate the size of the silicon steel laminated core required, given the wattage of your application. For example, a 12V 2.6A = 31.2W transformer requires a core with a cross section of one square inch (sqrt (31.2))/5.58 = 1.0. If you already have a core, multiply the height and width of the center leg to compute the cross section area as in Figure 2. Use Equation 1a to calculate the wattage rating of your core. The area will be A’ in Equation 3 if you measure in inches; A if measured in cm.

Figure 2


Equation 2 from Reference 1 was solved for Number of turns Np as Equation 3 The right-hand side of Equation 3 has a conversion factor applied to accept A’ in square inches instead of A in square centimeters. For our example as a 120 VAC 60 Hz transformer, set Ep=120, F=60, and A’=1.0. Consult Figure 3 for the recommended flux density B in gauss for the wattage of 31. Set B = 13,000. Equation 3: Np = 120(10e8) / (28.65)(60)(1.0)(13,000) = 537 turns.

Figure 3


Note that Equation 4 for Ns number of secondary turns has a 1.05 multiplier to account for winding resistance loss.

Continuing with our example, Ns = (1.05)(12/120)(537) = 56 turns.

The next problem is to determine what size wire to use for the primary and secondary windings. The size of wire depends on the wattage of the transformer. Small low-watt transformers dissipate heat more readily; thus, can get by with smaller wire size. Our 12V 2.6A = 31.2W example requires 597 circular mils of wire area per amp of current according to Figure 3. We need 597 * 2.6 = 1552 circular mils for the secondary wire. Looking up Reference 2, we find that 18 AWG wire has 1,624 circular mils of area; thus, 18 AWG for the primary. Since our primary/secondary turns ratio is 10/1, the primary current will be one tenth the secondary current (1/10)*2.6A = 0.26A. As with the secondary, we need 597 circular mils per amp. We need 597 * 0.26 = 155 circular mils for the primary. Consulting Reference 2, we find that AWG 28 has 160 circular mils for our primary winding.

Though I have not encountered any difficulty fitting the windings within the window space, you can estimate how much space will be required by consulting Reference 3 for turns per inch of a layer, and Reference 2 for wire diameter to estimate layer height. Use 90% of the turns per inch, 110% of wire diameter for layer height. Allow for thickness of the form on which the windings reside, allow for ends of the form, and insulation between the primary and secondary. Small diameter wire may require thin paper between each layer.

A complete re-wind of a transformer is only applicable to “E-I” cores that can be disassembled. Welded or epoxied cores cannot be disassembled. However, a few turns of a high-current secondary wire may be threaded through the window. According to YouTube videos, the secondary of a microwave oven transformer may be hacksawed and replaced with a few turns of heavy gauge wire.

Rather than a complete re-wind, consider re-using an existing primary by applying a custom secondary.  Count the turns unwound from a known voltage secondary. Calculate turns per volt to aid in determination of the custom secondary turns.

  1. Reference Data for Radio Engineers, 4th Ed,1956, ITT, pp 275.
  2. Copper wire gauge table, www.ibiblio.org/kuphaldt/electricCircuits/Ref/REF_3.html#cuwire.tbl.
  3. AWG gauge vs. turns per inch, www.ibiblio.org/kuphaldt/electricCircuits/Ref/11099.png.

Dennis Crunkilton
Abilene, TX

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