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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?

Robert Gotts
Madison, IN

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?

Reva Pino
Charlotte, NC

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!

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.

Scott Lapp
Simi Valley, 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?

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?

Paul Sills
Grand Rapids, MI


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

Relay Diodes Answered 2019 Issue-6

I sometimes see a diode placed across relay contacts backwards. Can someone explain the purpose of this and when it’s necessary and when it’s not and why?

Jürgen Abend
Pleasanton, CA


All but one of the answers here didn't answer the OP question, which was about a diode across the CONTACTS, not the coil. As the one answer that addressed the issue indicated, the reason is to suppress arcing across the relay CONTACTS when the coil is de-energized (particularly if the contacts are connected to anotrher inductive load).

Jay Jaeger
Madison, WI

It is used to protect the components of the circuit from burning out from a release of the induced voltage when the relay is turned off. The energy stored in the magnetic field of the relay coil is now released and the voltage can get quite high and case a spark. If high enough, this can cause damage, but even if it’s a low voltage, it can cause RFI and cause a circuit to misbehave. This diode has a number of names, but I like calling it the flyback diode.

Example: I once built a 8080 based microcomputer and programed it to turn lights on and off in the house when we were on vacation. But, it was always glitching, the program would get lost and hang up. Until I realised I was not using any flyback diode on all the relays. Once these were installed the RFI noise was removed, or at least reduced enough, so that the computer would not lose its place in the program and all worked fine from then on.

Aside: A flyback transformer where this flyback voltage is put to good use, see: https://en.wikipedia.org/wiki/Flyback_transformer

I looked this up to find more for you, check out these web pages: This is a good explanation: https://resources.altium.com/p/using-flyback-diodes-relays-prevents-electrical-noise-your-circuits

Here’s a good article on Wiki: https://en.wikipedia.org/wiki/Flyback_diode

Phil Karras KE3FL

The diode is called a flyback diode. The purpose is to dump the energy from the relay coil when it is disconnected from power. The coil acts as an inductor and when the power is disconnected, as when a transistor turns off, the current can not change instantly. This results in the voltage across the coil spiking to try and keep the same current flowing.

The diode lets the current flow thru it when the power is turned off keeping the inductive kick from destroying the transistor. No current flows thru the diode when the coil is energized because it is reverse biased then.

Some relays have the diode built in and do not require an external one. If you replace the relay and there was no diode across it the old one may have had an internal diode. If your replacement does not have a built in diode you should add one externally.

Larry G Nelson Sr
Webster, MA

When a voltage is removed from the relay coil, the magnetic field collapses, inducing an inverse pulse to flow. This inverse pulse can damage transistors and - more easily - ICs. The diode sorts this pulse so that it doesn’t damage any sensitive components.

If you are controlling a relay with any silicon device - e.g. transistor, IC, etc - you probably need to use a diode (called a snubber diode, by the way). If you are controlling the relay directly, say with a switch, then you don’t really need it.

Derek Tombrello
Shelby, AL

When the energizing contacts of a relay (or any inductor) open, the collapsing magnetic field forces the current to keep flowing. If the applied voltage was large and the coil is large enough, a substantial voltage will build up across the ends of the coil. If the voltage is very large, it will cause damage to the circuit. The diode is to short the voltage and prevents damage.

Lance Corey
Santa Ana, CA

Not across the contacts, but across the coil. It is called freewheeling or flyback diode. It provides a path for the high voltage created by the collapsing magnetic field when current is cut off. Otherwise the high voltage can destroy the driving circuit. Also used across solenoids and sometimes DC motors.

Richard Cox
Thousand Oaks, CA

Mr. Abend asks about diodes placed backwards across relay contacts.  NOTE that this technique is applicable only in DC circuits. 

The diode is called a “snubber” and it serves to protect the relay contacts from overvoltage in inductive circuits. Relay contacts in DC circuits are subject to sparking as they open when they control an inductive load. This opening can occur intentionally, or can occur during contact closure due to contact bounce. Examples of inductive loads include motors, solenoids, or other relay coils.

Current and voltage in an inductive circuit are related by Faraday’s Law, one form of which is V = -L di/dt. It states that the rate-of-change of current (amperes per second) in an inductor is proportional to the voltage applied across the inductor and inversely proportional to its inductance (henries). The minus sign indicates that the inductor resists a change in current by opposing the applied voltage.

Consider a voltage applied across an inductor through a set of relay contacts: At the instant of contact closure, load current is zero. As time advances, and dependent upon the load inductance, the current through the inductor will increase at a constant rate determined by the ratio V/L. At some point, the inductor will saturate — i.e., it cannot be magnetized further — whence the limiting current will then be governed by the winding resistance.

Now consider what happens when the relay contacts open: The magnetic field in the inductor immediately begins to collapse, and as it does so, it will attempt to drive current back into the voltage source. But since the voltage source no longer exists — we have an open circuit — there is nothing (other than the inductor’s winding resistance) to limit the magnitude of the voltage induced, and this kickback voltage will therefore appear across the relay contacts that are in the process of opening — contacts that have, in fact, barely moved from their closed position. The polarity of the kickback voltage is opposite to that of the original voltage that was used to energize the inductor. The kickback voltage creates a spark that degrades the contact surface, and over time, this sparking can cause contact failure.

A backwards diode connected across the relay contacts will switch into conduction when kickback voltage is applied. In this case, the decreasing rate-of-change of inductor current will be governed by the inductor winding resistance (ignoring the forward voltage drop in the diode).

Practical examples: An inductor having an inductance of 1 henry and a winding resistance of 1000 ohms, energized by a 25-volt source. The rate-of-change of increasing current in the inductor will be 25 amperes per second (25 milliamperes per millisecond) and the limiting steady-state current will be 25 milliamperes — which will be attained in about one millisecond.

When the voltage source is removed, as by opening a relay contact protected by a snubber diode, the initial value of the inductor current will be the same as its limiting value earlier — 25 milliamperes. The decrease in inductor current over time (t) is an exponential function of winding resistance (R) and inductance (L) proportional to e (exp -tR/L).

When the switching device is a transistor, the snubber diode serves to protect the transistor from destruction due to the imposition of kickback overvoltage.

Peter A. Goodwin
Rockport, MA

More often the diode is placed across the coil such that it is reverse biased. Its purpose is to absorb the counter EMF when the coil is de-energized . When the relay coil is energized it stores energy in its magnetic field. When the current is turned off that magnetic field reverses and tries to keep current flowing in the circuit. However, since the path is open that energy must dissipate. The energy can produce a rather high-voltage spike. This is how the Kettering ignition system works in early automobiles spark generation.

Since in most small electronics we do not want that high voltage to damage components the diode across the coil shorts out that reverse Electro-Magnetic-Field (EMF) and thus saves delicate components. You may find a diode or other suppressing component across contacts for the same reason. i.e. To prevent a spark which would shorten the contact life or damage other components.

In the automobile ignition system there is usually a capacitor placed across the points in the distributor. The ignition coil produces the spark when the contacts open because the magnetic field collapses sending that energy to the spark plug(s). I hope this helps you understand a little bit more about inductance in coils.

William B Runyon Sr WF4R
Chesapeake, VA

When a relay (or other) coil is de-energised, the stored charge can create a substantial back e.m.f. amounting to several hundred volts. This voltage can damage delicate components (FETs etc.) in the ciruit.

The reverse connected diode will clamp this voltage to 0.6 or 0.7 volts for a silicone diode. This limits any fast going high voltage spike in the circuitry.

Paul Reckless
Aylesford, Kent,

The problem the diode is solving occurs when the current flowing through the relay coil is switched off. This on-to-off transition is relatively fast and the collapsing magnetic field (the field generated to actuate and hold the relay in its switched condition) generates a very high reverse voltage across the relay coil. The relay coil current is usually switched by a switching-transistor.

This reverse voltage can easily exceed the capacity of the switching-transistor’s reverse-breakdown voltage and, thus, destroy the transistor. To solve the problem, the backwards diode provides a forwards path for the collapsing current to cycle back through the relay, and thus the backwards current flow dissipates within the relay.

Greg Gentile
San Francisco, CA

A diode is used on relays to keep back EMF from taking out the device driving the relay when power to said relay is turned off. When power to a coil goes off, the magnetic field collapses. This causes very high voltages and currents on the relay coil. They can go backwards into the drive circuit, damaging the active device. They are used where damage is very likely. Hope this helps.

Robbie J
Green River, WY

The coil of a relay is a fairly large inductor. Inductors store energy in a magnetic field that builds up around the coil when current if flowing through the coil. If the current is turned off suddenly the magnetic field collapses and "induces" voltage in the coil, just like a generator. The stored energy has to go somewhere and the induced voltage tries to keep the current flowing in the same direction.

The voltage will continue to rise with a polarity opposite of what had been applied. That means the end of the coil that had been connected will generate a voltage much higher than, and added to, the supply voltage at the other end of the coil. That voltage can easily rise to a level high enough to damage the circuitry. The diode placed "backward" across the coil provides a path for that current. It effectively shorts out the voltage induced in the coil and allows the stored energy to dissipate in the diode and the wiring without reaching dangerous levels. The diode should be rated somewhat higher than the supply voltage and should be able to handle at least the same amount of current as the relay coil.

When is it necessary? The full answer requires quite a bit of math and physics but the short, simple answer is anytime an inductive device is switched off and on. It applies not only to relay coils, but also to motors, solenoids, and any other type of devices with coils/inductors. In some cases the switching circuit will have protective diodes built in. For instance, the ULN2803 darlington array IC is intended for driving such loads and has the diodes built in. The L293D (D for Diode) stepper motor driver / dual H bridge IC also has protective diodes built in.

William Cooke
Adams, TN

The diode is there to protect any solid state device driving the relay. The current flow through the energized coil creates a magnetic field around the coil. When switched off the collapsing field results in an instantaneous polarity change in voltage across the coil. Without the diode this voltage can become quite large exceeding the voltage rating of the switching device. The changed polarity of the voltage induced into the coil forward biases the diode limiting or clamping the voltage rise to around 0.7 volts.

Sometimes referred to as a snubber diode or snubber circuit. Sometimes an RC circuit is used in this fashion but diodes have become more common. Some 12 V automotive relays include this diode internally, others do not. I would not replace one with a diode with one without. The diode is "backwards" because we do not want it to conduct until the voltage reversal across the coil happens. Automotive relays with the diode need to be connected so the internal diode is reverse biased. Typically shown as a rectangle with no polarity indications.

George W Shaiffer
Colorado Springs, CO

Amplifier For VHF TV 2020 Issue-3

I have a rather complicated TV reception system. I have four antennas all successfully combined into one by using modules to convert the RF signal to VHF siganls. What I am lacking is a constant input for all signals into the combiner.

I would like to take signals from 54-216 MHz with 1-8 dBmV input and put out 18-25 dBmV with as little noise as possible. It would be great if I could clamp all signals to 25 dBmV.

Howard Epstein
Gilford, NH


The Televes Avant X would be an ideal solution for that situation. The Avant X is pretty much an MATV headend in-a-box. It is a filter > processor > amplifier allowing to individually filter RF multiplexes coming from up to four different antennas, automatically adjusts the levels individually for a balanced response, and even allows to frequency-shift when needed if two same RF frequencies are coming in from two different markets.

On top of that it doubles as a launch amplifier with a programmable output level between 30 and 55dBmV.

Javier Ruano
Denver, CO

Need Suggestions For A New PIC Programmer 2020 Issue-2

I haven’t done much with PICs recently and I used to use a PICKit1, which is now obsolete.

Now, I want to re-program something I made about 10 years ago, my PICKit1 is no longer usable, and my old development software won’t play nice with Windows10.

I look at the array of products available and I am overwhelmed. So, what do I need to have a modern equivalent of a PICKit1 and what software do I need to program PICs using C?

Also, in addition to a Windows set-up, is there a Linux-compatible option as well? I tried contacting MicroChip but got no help there.

Keith Ujvary
Oliver, British Columbia


I would suggest using a PICKIT 4, part number PG164140 for hobby programming. I would suggest moving to MPLABx for the development environment and the MPLAB XC compiler for a C compiler. Most can get away with the free version. The tools also work with a MAC or Linux. I do suggest using the native version and not running under emulation so the directory structure can be the default. Software is all available on the Microchip web site and the programmer/debugger can be bought direct or from many different distribution options.

You will probably need to make some code changes to align with the changes made over the years to the software but it should not be too much effort. If you are doing more severe development you may want to consider the ICD4 but the cost is higher and may not be needed or justified depending on your use.

Larry G Nelson Sr
Webster, MA

I use picprog under linux which you can find in the repository to program PIC chips using a home built usppicprog programmer. You can either install it through the GUI software manager or via the command line by running "sudo apt install picprog" in terminal.

Derek Tombrello
Shelby, AL

Protection Of Device Against Automotive Transients, Reverse Voltage 2020 Issue-2

I was wondering what would be suggested to properly provide protection of a device with a 5V low power regulator and LED driver against automotive transients, and reverse voltage that is connected to an automotive starting battery.

I presume a TVS diode could be used, but should it be bi-directional or uni-directional? A Schottky diode could be used for low voltage drop, first in line for reverse voltage protection, but it would have to withstand the clamping current of the TVS diode.

Would it be be necessary to include a resettable fuse if the transient lasted too long and exceeds the power rating of the TVS diode?

Wayne Carpenter
Omak, WA


Using a uni-directional TVS diode and series Schottky diode is an excellent start. This will take care of garden variety transients caused by relay switched inductive loads. Cars are full of those. Some capacitors on the input will help too. In addition, there are a couple of unique automotive transients that require attention.

The first is called double battery jump start and happens when the tow truck guy jump starts your car and has a second battery in series with his truck battery hooked to his jumper cables. This results in about 26 volts and a typical automotive device must withstand this for one minute.

The second transient is called load dump and occurs when the battery becomes disconnected while being charged. The alternator regulator cannot react instantly and will generate a transient that can reach 60 volts peak and then slowly return to normal charge voltage over about 400mS.

I solved these problems by using regulators that would stand the 26 volt jump start. You will need a bit more heat sink but it only lasts one minute. There are some automotive regulators that will stand the 60 volt load dump as well. An example is the LM2931. However, it is low current (100mA).

Another way to deal with load dump is use a resettable fuse (PPTC) in series with the input diode and use a large enough TVS to blow the fuse. In case you have not dealt with automotive 12 volt systems before, the normal run voltage is actually 9-16 volts with nominal about 14 volts.

Jim McGrew
Saline, MI

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