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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

It sounds like your problem is an impedance mismatch in the speaker system. Maximum power transfer occurs when the source impedance (output of your amplifier) matches the load impedance (speaker). The gauge of the speaker cable may contribute to the problem. The smaller the gauge the higher the IR loss in the wire. And NO... Monster Cable is NOT significantly better and definitely NOT worth the extra cost.

A better solution is an additional amplifier for those ‘other room’ speakers... defining another ZONE. That amplifier should be fed by a low level output from your receiver My guess is that the previous homeowner had a system with an amplifier per zone and a low level signal distribution system to feed the amplifiers. This can get a bit complicated in design but may translate into a more versatile system.

Douglas Hall, CPBE
Vilas, NC

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



Answers

Most smart chargers are designed to not put out current if the voltage on the load is too low. This protects the charger against short circuits and any load that isn’t a short but would overheat the charger.

Batteries that have sat that long may or may not be rescuable. The best thing to try is a “dumb” charger with external current limiting. For a car battery, the external current limiter can be a sealed beam or H4 halogen headlamp bulb; the charger should be 6 amp or bigger. For motorcycle batteries, the same rig but with a tail lamp bulb.

Once the battery has some voltage on it, you can switch over to the smart charger. If the battery voltage is above the smart charger’s go/no go threshold, it will charge the battery. A field expedient to the dumb charger and bulb limiter is to use another battery of the same voltage which has charge in it, plus the bulb limiter, across the discharged battery to bring its voltage up. A rescuable battery will have the bulb glow brightly and then gradually dim as the dead battery voltage rises.

Jonathan Wexler
Los Angeles, CA

Lead acid (PbA) batteries have a high self discharge rate. They will go dead just sitting. They also have a short calendar life, more than 3 years old you can expect to have problems. A smart charger will see the voltage is to low and abort charging. The longer the cell voltage remains below 1.5V the more damage is done shortening their life and capacity. To extend PbA battery life requires a float charger like the 'Battery Tender'.

From the 1.5 to 2 years of storage mentioned I'd say you now have paper weights. A slow trickle charge done with a bench power supply at a low current to about 2V per cell, may bring them up. But how useful they'll be remains to be seen. Then you can hook up your smart charger and see what happens. Don't leave the charging unattended, you can stop it over night by disconnecting one terminal. Some power supplies can be back fed from the battery when turned off. You have to watch out with batteries of unknown condition. Fires can happen. This applies to PbA as well as lithium based batteries. An alarm is useless when there's no one around to take action.

Randy Carter
Kent, WA


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



Answers

Electrolytic capacitors are polarized and every half cycle of the powerline their polarity will be reversed. Depending on the values and types, they may get hot, or more exciting, blow out their pressure relief attended by a puff of smoke and fumes. In any case, their life is rapidly shortened if reversed. Non polarized caps are available but it all depends on the values needed. Most other capacitors are non-polpoarized and they should work for you.

Len Powell
Finksburg, MD

Mr. Gotts seeks information on employing the reactive property of a capacitor to reduce AC line voltage to 24 volts.

The short answer is “Don’t do it.”

Been there. Done that. Didn’t know any better. In my case, I had a small circuit comprising one vacuum tube having a 12-volt filament drawing 0.15 amperes. Dropping the voltage from 120 volts required a series impedance of 720 ohms. Like you, it occurred to me that the reactive impedance of a capacitor might provide the needed voltage drop, eliminating a large (and hot) series resistor. A capacitor of 3.7 uF at 60 Hz provided the necessary 720-ohm impedance.

The technique worked and nothing blew up. I was lucky. Seventy subsequent years of experience, however, lead me to consider the reasons NOT to use this technique:

  • Most importantly, no isolation from the AC line is obtained, so that the possibility of shock hazard always exists. Furthermore, because the reactance of a capacitor varies inversely as the frequency of the applied voltage, and lacking any line-to-load isolation, it follows that any high-frequency line-voltage transients will be coupled through to the load without attenuation.
  • The technique does not, in general, work where the characteristics of the load are unknown. If the load is resistive and the current drawn is constant, — as in my vacuum-tube example — the technique works. If the load varies, however, then the voltage across the load will vary accordingly.
  • In any application with this technique, phase shift of the load voltage occurs relative to the AC line voltage. This phase shift is unimportant to a resistive load. If, however, the load is inductive or capacitive, this phase shift may produce wild swings in load voltage due to oscillation. Heater control circuits usually employ relays, and relay coils are inductive loads.

One would never use an electrolytic capacitor for this job. Film-dielectric non-polarized motor-start and motor-run capacitors are available with operating voltage ratings suitable for the job. But the off-the-shelf tolerances of such devices is relatively large, running to 6% for motor-run capacitors and 10% for motor-start capacitors. For a 24-volt resistive load supplied through an off-the-shelf motor-run capacitor, the load voltage may be anything from 22 to 25 volts.

A small transformer is less expensive than a motor-start or -run capacitor, it provides safety isolation from the AC line, and the load voltage won’t oscillate.

Peter A. Goodwin
Rockport, MA

That is not at all practical. Use a 24 volt transformer from a sprinkler timer or thermostat.

Richard Cox
Thousand Oaks, CA

Most aluminum electrolytic capacitors are not suited to having large amounts of AC voltage on them. Also they have to be used in pairs, to handle both polarities of voltage. And yes, you may have them overheat and "rapidly disassemble."

Also the heater cable will not be isolated from the AC line, which may be hazardous under fault conditions. You didn't say how much current you needed at 24VAC, but I assume it might be more than an ampere. My first choice would be a 120:24V transformer. You'd get decent efficiency and isolated power.

Jonathan Wexler
Los Angeles, CA


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



Answers

As a first thought a current sensor, watching the motor’s current draw, comes to mind. A current sensor is simply a single winding coil that one motor wire passes through. It is a basic transformer and the coil develops a voltage relative to the motor current. They are available commercially or can be salvaged from a junk box transformer.

OK, but that seems to be more bother than it’s worth since all you really need to know is if the fan is running not it’s actual current draw. So now it looks like an air flow switch is the best choice. Don’t go off the deep end here, they are quite simple. Many commercial airflow switches are nothing more than a lightweight paddle connected to a micro switch actuator arm. This is placed in the airflow, the air lifts the paddle and the switch operates. Just be sure the paddle falls freely without airflow and that the airflow raises the paddle high enough that it doesn’t dance or flutter on the air stream.

A common SPDT micro switch offers many options for alarm or indication connections without the fussiness of measuring the current sensor voltage and the circuitry required. Since the micro switch is isolated it could be connected to a line power, low voltage or even an alarm system! Low cost possibilities and reliable operation are unlimited.

No power, clogged duct work or fan assembly, bound up motor, a squirrel in the squirrel cage... all result in no airflow! Not having specific information about your fan or it’s installation, it would seem easy to cobble this together with common, and easy to find items. Hope this solves your problem.

Len Powell
Finksburg, MD

Judging by the current, the fan motor is probably a shaded pole motor. Bad bearings may not alter the current very much, until they freeze up, at which point, you'd have the motor's locked rotor current. I assume you'd like to know about motor trouble before it fails. Since you are able to tell by ear, why not use a cheap intercom to monitor the fan?

Jonathan Wexler
Los Angeles, 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


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


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



Answers

For prototypes I use Press-n-Peel. You laser print the pattern to the PNP then use a clothes iron, or better, a laminating machine to bond it to the copper side of the board. Peel the PNP off the board and etch it. Clean the residue off the board then drill as needed and assemble it.

This from my post on forum.nutsvoltscom Re: Making PCBs at Home Post by Lenp » Wed Sep 02, 2020 3:43 pm

  1. Photocopy, or laser print the artwork onto overhead projector film unless you already have reverse artwork.
  2. If you use the the projector film, turn it over as needed to get a reverse image on the blue Press N Peel transfer film.
  3. You may have to tweak the copier’s toner for a dark image, and it might be a just a bit off scale, if that matters much.
  4. The circuit board should be bigger than the PCB pattern to help fix any misalignment problems.
  5. Clean the circuit board with something like a Scotch Brite pad, and wash well with alcohol to get rid of any oils or finger prints.
  6. Hold the board by it’s edges and align the PNP pattern on the board properly, taping one edge to keep it in place
  7. If you have a pouch laminator, run the board and film, with the taped edge leading,through using a pouch carrier.
  8. Without a laminator, you can iron it on with a clothes iron. Put a few sheets of paper on top of the film to keep it from slipping.
  9. The toner on the film will get sticky when hot, and bond to the board. Allow the board to cool, or rinse in cold water.
  10.  If the board is missing a bit of a trace, choose to do it over, or repair the damage with a Sharpie marker.
  11.  Acetone will remove the pcb pattern from the board so it can be reused if necessary, but clean it again.
  12.  Etch in any available etchant. (I prefer amonium persulfate since it is less mess.)
  13.  After etching, I leave the PNP pattern on until the drilling is done, then clean it with acetone but you can clean it first if desired

Overhead film is available online and maybe the big box office stores still carry it (who uses overhead projection any more?). PNP Blue film comes with an instruction sheet and it is available online, and from several electronic suppliers that also supply the copper board and etchants. Look online, it’s there as well as several videos showing the process.

This PNP process is great for a not too complex board prototype, but not for quantities. You can have a board from artwork to drill in an hour, correct your mistakes and go again without waiting for a board house turnaround and cost.

Good luck!

Len Powell
Finksburg, MD

I quit doing that when I spilled a gallon bottle of Ferric Chloride in the garage. A friend of mine was recently making some, but the materials are getting hard to get. Besides, ordering small quantities from China is way cheaper than buying the board and chemicals.

Richard Cox
Thousand Oaks, CA

The two most common methods I'm aware of are toner transfer and Photosensitive Dry Film. Most people seem to have much better luck with one or the other. I was never able to get toner transfer to work so I use UV film. The supplies for either are available at the usual sources (Mouser, Digikey, Jameco, Amazon, etc). There are a number of articles and videos on the web which describe the process better than I can.

Basically, a UV sensitive film is applied to the PCB, then a transparency is laser printed to overlay it. The areas blocked by the design are those which will be etched away (the print is a negative). The board is dipped in a developer for a couple minutes before it's washed with water. There are a number of echants, but the most common in hobby use is probably ferric chloride. If you use it, please check handling and disposal methods.

You may want to look at PCB prototyping services. Several advertise in Nuts & Volts. For $25-30 you can get 5 boards, 6 mil spacing, drilled, vias, masked, and screened, at your door in a week or so. Honestly, this is probably the way to go, unless you need faster turnaround. Or just wanna (a valid reason in itself!). It is very important to closely check the design rules.

Another possible option is CAD/CAM milling. However, I'm not aware of affordable equipment which has enough precision. I haven't looked in a while, so this may have changed.

Jay C
Louisiana

In the past, I designed and produced many circuit boards using several methods. By far, the best way to produce circuit boards is to just send the files to a company on the web and have them make them because if your time is worth anything, it is cheaper. However, if you insist on doing it yourself, you can etch them or mill out the unwanted copper, leaving the circuits that you want.

Milling works much better because circuit design software can produce files which can be converted to a program that runs on an othermill home router. It’s likely that other home routers can also be used, but the two people that I’ve seen do it used othermills. These machines are very popular at Makerspaces and school shop classes, etc. Adafruit uses this method to produce all of their one of a kind circuit boards for production testing their products.

This process starts with getting and learning KiCad software, which is free. YouTube has many videos on learning KiCad and Lady Ada often shows how she designs products for Adafruit using KiCad on her “Desk of Lady Ada” YouTube series. Watching this series from the beginning is suggested for anyone learning electronics.

After the KiCad design is done, it’s a matter of contacting the owner of a usable milling machine or sending the files to a board maker. The Hackaday guide: Why Etch A PCB when You Can Mill is suggested reading.

   If you are dead set on etching your own boards, you can get several kinds of etchant from Amazon as well as some blank boards. However, a search of the web will likely show several other sources.

Forget any Ferric Chloride based etchant as it’s nasty stuff and works much slower than the better product Ammonium Persulfate, which stays clear enough to see how the process is coming along. You’ll also need a way to time the exposure of the boards and the right kind of lamp. I would suggest making the final board pattern on clear film that can then be taped over the copper clad board and exposed.

The other way to do it is to draw the pattern directly onto the circuit board with a “resist pen,” but it’s very hard to do an accurate job that way.

The transparent film can be printed on certain printers if the correct film stock is used, however getting the size of the image correct to produce the correct size printout might be tricky. We always drew everything 4 times bigger on graph paper and then had a photo shop reduce it 4:1 and print it on clear film.

Good Luck

Dale Freye
Grand Haven, MI

With the rise of the cheap board manufactures that can provide you a professional board for just a few dollars, this is something of a lost art. I still make my own boards most of the time. The time I have to bring my ideas to life is pretty limited and when I have a good idea I want execute on it right away.

When it comes to making my boards, I would call myself a jack of all trades and master of none. I use a few different methods depending on the complexity of the board and how polished I want the final product to be.

My go to method is photoresistive film and that produces the best output for me. Autodesk Eagle is my tool of choice for laying out my board, printing that output on laser printer transparencies and using a cheap laminator from the local big box store, a cheap light box and photoresistive film from eBay and I can lay down my artwork in about 30 minutes. There are plenty of Internet resources on how to do this online and it works great.

Etching is where the real issues are with making your own boards. It’s messy and dangerous. after all you are dealing with chemicals strong enough to erode metals. I used ferric chloride for years, and it gets the job done, but it’s slow and extremely messy. If you ever spill any, plan on the brown stain left behind to be there forever.

For the past 5 years or so I’ve been using muriatic acid (Hydrochloric Acid) and peroxide for etching my boards in a 50/50 mix. It is super effective and I can run to the local hardware store to grab a gallon, but it is has quite a few real hazards.

First and foremost it gives off chlorine gas so is has to be done outside and away from others and you must always use eye and breathing protection.

Secondly, anything within reach of the fumes made of ferrous metal will rust almost instantly. Both ferric chloride and muriatic acid like to be warm and agitated to work best. I made a simple servo driven device controlled by an arduino to rock the container back and forth and that speeds up the process greatly.

Drilling is next in line and again I have a few methods I use. My primary is just a simple micro drill press and hours of doing my best to hit the center of the pads. Drilling after etching allows you to use the raised copper pad to guide the drill bit.

Lately I have been working on a good technique for using my cheap 3018 CNC router to do this more effortlessly. By etching some alignment marks on two corners of the board I can pin the board down to the waste board on the CNC and let it do all the hard work. So far it is hit and miss but when it works it does a great job.

inally there is making it look good with some solder mask. While it is mostly for aesthetics for me, it does have a real purpose to keep your copper traces from oxidizing. I nearly always use Dynamask film.

Since I usually do photoresist film on my boards, and Dynamask works just like it, I have the tools out and on the bench anyway. Occasionally I use the liquid mask that you can order on Amazon or eBay, and it works, but it just doesn’t product the quality I get out of Dynamask.

So there it is, when you add all the time up and include cleanup afterwords, for a simple board you have used up the better part of a day. Is it worth it? No it’s not, considering the low cost of ordering professional quality boards now.

Will I keep doing it? Absolutely. While it is a ton of work and mess there is just something about it I really enjoy.

After I do my prototype on my hand made board and work out the details, I will get the real deal from a PCB service but the ability to test out an idea on a board the same day the idea hits you is awesome.

David Carter
Frankfort, KY


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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