Tech Forum

Capacitors...there are MANY types, physical size/confirguration, chemistry, range and ratings. Choosing capacitor types depends on many variables. Size constraints, environment (i.e. voltage, temperature and frequency).  Most capacitor applications are either in AC circuits or used as timing circuits and storage. Application of the types depends a lot on cost also. Tantalums are more effective in some cicuits due to size and tight tolerance, but are also much more expensive over electrolytics. In RF circuits, typical types are mica, ceramic, glass, vacuum, air and even oil dialectrics. (there are 100s of them) Each application again depends on cost considerations, size and power levels.

I suggest if the reader wants more data on capacitors to consult one of many texts covering the theory and application of capacitors. A good start would be -Electronic Communications- by Robert Shrader (McGraw-Hill)

I 'think' Nuts & Volts did a series on capacitors many months ago that was good reference material.

There are many types of capacitors in common uses today, with many different properties. The key difference is the dielectric, or insulating material, between two metal plates. Here are some common types. All values shown are just rough approximations. Maximum capacity is expressed as nF (nanofarad), (µF) microfarad and F (Farad), maximum voltage as V (volt) or kV (kilovolt), and maximum frequency as Hz, (Hertz), kHz (kiklohertz) or MHz (megahertz).

   Air variable capacitor: used in radio receivers and low-power transmitters. Value may be changed, but bulky for capacity. Max. ~1 nF, ~1,000 V, ~100 MHz
   Vacuum variable capacitor: used in high-power transmitters. Value may be changed, but bulky for capacity. Max. ~1 nF, ~10,000 V, ~1,000 MHz
   Mica capacitor: used in receivers and transmitters. Value is stable. Max. ~1 nF, ~1,000 V, ~1,000 MHz
   Ceramic capacitor: used in digital electronics, receivers and transmitters. Value is less stable than mica, but more compact. Max. ~1 µF, ~1,000 V, ~1,000 MHz
   Al or Ta electrolytic capacitor: used in audio-frequency and power supplies. Al is cheap, Ta more efficient. All require a DC offset and are destroyed by true AC or reverse voltage. Max. ~100,000 µF, ~300 V, ~30,000 Hz
   Ultracapacitor: double-layer dielectric capacitors for power supplies and power backup. It offers very high capacity, and might eventually approach that of electrochemical cells, but has low voltage per capacitor, so are usually placed in series. Max. ~100 F, ~1 V. ~100 Hz

Other dielectrics are used, such as paper, plastics, glass and water (yes, pure water has a high dielectric constant, low conductivity and is environmentally friendly):
[url=http://www.rle.mit.edu/cehv/documents/34-Phy.Tech..pdf]http://www.rle.mit.edu/cehv/documents/34-Phy.Tech..pdf[/url]

Ceramic caps are small, low loss, low leakage and inexpensive but the temperature coefficient varies from COG (very stable, 2% is available) to X7R (moderately stable, 10% is available) to Z5U (typically +20%, -80% over temperature). There is another type of ceramic called porcelain that is used in microwave applications; ordinary ceramics are too lossy at those frequencies.

Aluminum Electrolytics are widely used because they are low cost and smaller than a film capacitor. Electrolytic caps are leaky (measured in microamps), have internal resistance that may be significant in some applications, and vary with temperature (+- 20% typically) and frequency (not useful at high frequency). After 6 or more months on the shelf, aluminum electrolytic caps should be re-formed to regain their voltage rating.

Tantalum caps are also an electrolytic but smaller than aluminum, lower leakage, better temperature coefficient and operate at higher temperature. Cost is more than aluminum but does not de-form (loose its voltage rating) with non use.

There are other capacitor types to consider: mica, polypropylene, metalized polyester,  polystyrene, and paper. Each has pluses and minuses to consider.

Multimeters, particularly digital VOM's, and some high impedance analog types, have a Diode test postion on their scale selector. This position presets the full scale range, typically 1000 - 2000 ohms FS, to measure the front to back ratio of a diode and also the forward resistance.  Many DVM's do not provide enough voltage (up to 3 volts) in their 'normal' resistance/ohm scales to cause a forward 'breakdown' of the junction to measure it. (Especially in auto-ranging units). Also, this pre-set range allows one to test diodes rapidly without waiting for the unit to count down.

The diode setting measures the forward drop of a diode at a current of a few milliamperes. To ensure the diode is conducting, the potential across the meter probes is a few volts. On my aged Radio Shack DVM, the open-circuit potential is 2.88 volts, sufficient to "turn on" most diodes, including red, yellow and green LED's, which glow dimly. Note that blue, ultraviolet and white LED's, as well as Darlington transistors and some high-voltage diodes (that are, internally, series-connected diodes) show as open circuits, since they need a higher voltage to conduct.

The resistance setting is intended not to "turn on" a diode, so, on that setting, the voltage across the meter probes is only a few tenths of a volt. That allows accurate measurement of a resistor in parallel with a diode or across other semiconductor device, except perhaps Schottky diodes, which may have a forward voltage drop of 0.2 volts.

See [url=http://en.wikipedia.org/wiki/Diode]http://en.wikipedia.org/wiki/Diode[/url] for more information, but the list below is a rough guide to forward voltage drop at a few mA:

• Ge Schottky diode:   0.2 V
• Ge signal diode:        0.3 V
• Si rectifier:                0.6 V
• SiC Schottky diode:   0.6 V
• SiC junction diode:    0.6 V
• Red GaAsP LED:      1.3 V
• Blue InGaN LED        3.0 V

Using the diode setting, you can usually identify the type of a diode or junction transistor, as well as determining if it's open or shorted.

BTW, thanks for asking that question, which inspired me to check the open-circuit voltage and current draw of my DVM diode and resistance settings. Now my measurements should be a bit more meaningful.

The diode selection on a multimeter is for continuity and diode checking. Digital multimeters do not produce enough voltage to forward bias a diode ordinarily so this position outputs several volts at low current and sounds a buzzer if current is drawn. If the diode is good, the forward voltage will be displayed in one polarity and open circuit will be indicated in the other.

The "diode" range on digital multimeters is used to "measure" diodes (any type) to see if they are good or not. To use the range, connect the BLACK meter lead to the diode wire with the "stripe" on the diode body (cathode lead) and the RED meter lead to the other diode wire (anode lead). The following are "typical good" forward-bias readings for various diodes:

• "Silicon" diodes: "0.6-0.7"
• "Germanium" diodes: "0.3-0.4"
• "Tunnel" and "Schottky" diodes: "0.2-0.3"
• Zener diodes: similar to silicon diodes
• "Selenium" rectifier: "0.9-1.2" (approx)
• "Power" (i.e., 10A rating) and "high-voltage" (i.e., kilovolt rating) diodes: "0.7-0.8"
• LEDs: "1.6-1.8"

Reverse the leads (reverse-biasing) will show "OL" - any "numerical" reading usually indicates the diode is "shorted" in reverse-bias. This range is also useful for checking the "polarity" of BIPOLAR (PNP, NPN) transistors, if you don't know what kind you have:

• The emitter-base drop will read "0.5-0.6"
• The collector-base drop will read "0.7-0.8"
• The collector-emitter junction will read "OL" ("anything less" indicates a blown transistor)
• Reverse-bias the emitter-base and collector-base junctions and you'll read "OL" ("anything less" indicates a blown transistor)

NOTE: The BLACK lead will tell you which lead(s) are the "N" lead(s) for determining "PNP" or "NPN" polarity (use the voltage drop difference to  determine collector from emitter). It's also useful for determining the polarity of "unmarked" diodes: BLACK is cathode ("stripe") and RED is ANODE. DO NOT use this range to measure FET (field-effect transistors) as modern FETs are "static-sensitive" devices and will be probably be destroyed from the action of taking the reading. SCRs, triacs (i.e.,"thyristors") and similar "exotic" junction-type devices cannot be accurately "tested" on this range. Finally, you can measure LOW-VALUE resistors (<2000 ohms) and the range usually has a "buzzer" associated with it for audible continuity readings (i.e., wire tracing, testing fuses).

The diode function on a digital multimeter is part of the Ohms function. This function doesn't autorange. It provides a constant current to forward bias the diode under test and displays the voltage drop. You should measure about .5 to .6 volts for a standard silicon diode. Other junctions will read higher or lower depending on the construction of the device.

Your son should consider the local community college, and look at other careers related to electronics. While engineers are often mentioned, technicians with with associate's degrees are now more in demand (USA TODAY).

Electronics is just one option - also consider other related fields such as  machine tool technology, industrial electronics, instrumentation repair, or machine maintenance. These programs include electronics along with motor control, programmable logic controllers, instrumentation, fluid power, sensors, pumps, valves, process control, and industrial robotics. These people are in demand and command high incomes.

For info on career options and certifications, check out the Electronics Technicians Association, International - [url=http://www.eta-i.org/]http://www.eta-i.org/[/url]

Your son needs basic skills in practical math and physics. With those, he can look at many technical-education programs, from vo-tech to an advanced degree. I recommend a visit to a nearby technical school and to an engineering department at a college or university. After your son sees what students and professors do, he should have a better idea of what preparation for a technical career entails. But don't force him into a technical field. No matter what your son wants to study, he'll need a life-long interest in learning and reading.

Technical careers can vary far and wide, and many of them do almost require a degree of some kind. If he/she is interested in engineering, then a degree will be extremely helpful. An easy way to figure out whether it's necessary is to take a look at local technical job listings. Unfortunately, the degree is often very arbitrary. It's required to get hired, but most of what you learned to get the degree is useless.

I recommend trying to test out of as many classes as possible. Look into CLEP testing through College Board, as well as specific college's Credit by Exam terms. If programming is more up his/her alley, then a degree is likely very necessary to get hired. However, unless he/she can learn very well from professors, I recommend "playing" with as much code outside of college as possible. Some of the top (non-web) languages in demand are: Java, Python, C/C++, C# and Perl.

One of the few tech jobs that does not require a degree is freelance or semi-freelance web design/development. You can learn all that you need to know through free and low cost resources. You can also get a degree in web design/development, and that might help you get hired by a design house.

The absolutely required technologies to know for web are: HTML, CSS, Javascript+JQuery and SQL. SQL doesn't get you anywhere without a server-side scripting language such as PHP or Ruby on Rails. I am admittedly biased toward freelancing web design, as that is what I do.

But web design has one of the biggest markets for selling yourself in, and a lot of money can be made. It's certainly not for everyone, though. It requires that you keep up with current technology, are good with selling to people, and are willing to put some hard (but fun!) work in. Have a look at this link for learning (web and non-networked) programming: antsmagazine.com/web-development/20-amazing-places-to-learn-code-from-scratch Ignore W3Schools and anyone who says they are a good resource. codecademy.com is where I first learned HTML and CSS (then did it again in college...).

It needs to be a field that he would like to do. Get to the book store and find stuff on getting a job. Also, find out what jobs are out there. I will tell you a mistake I made — leaving a job before I had another one. Some jobs want a high grade-point average and it may take four months or more to find one. You can get a book or two from Amazon online. Getting into a co-op job should be considered, before you get your degree. You may want to consider getting a job near home.

Yes, they have likely been damaged, I had to look up what "equalizing" a battery was since I wasn't sure. Equalizing may help but I doubt that full capacity will be restored, or if it is, it will last very long. I'm going to assume that these batteries are the deep discharge type and so you may be able to restore them at least partially.

I've noticed with my backup 12 Volt deep discharge 70 - 80 AHr batteries for amateur radio & emergency use that once I let them get even partially dry I can never get them back up to full capacity. On the other hand, I do not own a charger that can perform the equalization charge either. So, I'd say since it likely won't hurt, give it a try.

My question is: Since you never run them down, you have no way to compare to see if this has improved anything, and also, since you never run them down, why worry about it? You never seem to need whatever the full capacity is anyway.

You may contact me if you like. I'm interested since I've done battery work both professionally at Marsh McBirney, Inc., and  Gardner Labs, as well as in my amateur radio hobby.

I have heard that If you let car batteries completely discharge, they will lose one-third of their capacity.

How about 6V lantern battery?

First, a 9V "transistor" battery doesn't have enough current to run a cell phone. The Lantern battery might work. Look at your cell phone battery, it will be a Li-ion probably 3.7V, so you will need to lower the voltage of the lantern battery to that.

Next, measure or just read on the battery, which pins are + and -. Then, figure out how to connect to the small pins/strips inside the phone. In one of mine, I found that using the little hook-type clips worked fine.

That should do it. If you need to learn how to make a voltage reducing circuit, I believe that both linear types, using a LM317 chip as well as some of the newer switching chips have already been published in N&V. I found the LM2575. http://pdf.datasheetcatalog.com/datasheet/SemtechCorp/mXvqxtq.pdf

First, determine how long you want the phone to run, and choose a battery with enough milliampere hours for the duration. (A 9 volt radio battery may be inadequate).

That said, for efficiency I recommend a switching converter such as the On Semi MC34063. See the datasheet for how to use. For an inexpensive inductor consider the toroidial inductor CR-345 from AllElectronics.com. This should have more than enough capacity to not run dry. If, in the future, you need a voltage inverter, be aware that I have found other datasheets to be in error with their circuit for this part, but the ON Semi circuit works well. Simply use the formulas in the datasheet. If the source voltage is close to 12 volts, than a surplus car adapter for your phone from Goodwill is the low cost option.

I've been an electronics hobbyist since junior high school (I'm 57 now) and I've salvaged many components from PC boards and other electronic hardware for use in various projects. Sometimes the old stuff, in the many boxes of salvaged parts I have, has made it possible to complete a project. Don't forget to save hardware items also! I can't count how many times I've pulled out a bin of fastener hardware or brackets and found just what I need.

Another fantastic (and usually very inexpensive) resource for electronic and hardware parts is the surplus electronics market. In most larger cities there is at least one, and sometimes many, surplus electronics stores. I would say you have a very good chance of that in Houston. For example, anyone who lives in the SF bay area of California ("Silicon Valley") has a goldmine of surplus stores! One thing I usually do when visiting a larger city (if I'm driving, not flying - too expensive to transport) is to check the phone book for an Electronics - Surplus section and make some phone calls to get an idea of what's available.

If you decide to go ahead with salvaging parts from boards, you can begin inexpensively by using a soldering iron and a "solder sipper" (spring loaded vacuum device) and some solder wick to remove components. You will need to be careful not to heat individual component legs very long so as not to damage the components. It can be done. As you progress with this you can do some web searches for some less expensive desoldering equipment that will make it easier to remove components. You may even find some of this equipment at one of the surplus stores! Good luck!

Absolutely! Since I was a kid, I've kept a stash of boards from dismantled equipment from which to salvage usable parts. While it started out of necessity, even now that I can afford to buy new parts for projects, it's still handy to have a selection of salvageable stuff on hand, and the best form of recycling is reuse.

If you are anything like me, sooner or later it will be 2am on a Saturday and you will find yourself needing a specific part you don't have and more often than not the junk box will save the day.

Most components hold up well with age. Semiconductors are almost always good, unless you are unlucky enough to find the part that resulted in the equipment being discarded in the first place. Reasonably modern components don't tend to drift much, although electrolytic capacitors may be suspect, particularly if they were used in a switchmode power supply.

It is certainly worth salvaging parts from old electronics. My favorite tools for this are a desoldering iron (Parts Express model 372-202) and a Black & Decker heat gun (model HG1300). A good first device to take apart would be a VCR. Look for a heavy one made in the 80's or early 90's, it will have many basic parts like capacitors and inductors, along with more interesting bits such as a VFD display and a motor control chip.

In terms of what else to take apart: I would not recommend TVs, you can get a nasty shock from one, and the boards don't have much on them. Older computer boards have more through-hole chips than newer ones. (Heat gun works well here.) Stereos have a lot of good parts, from heavy duty transistors, to graphic equalizer chips.

I have not seen much deterioration of component values. Very old electrolytics sometimes can't hold a good charge. Most of the resistors in an old Vidicon camera I took apart were out of spec. In comparison, stuff from 20-30 years ago normally holds up very well. Capacitors with 'J' markings and resistors with gold bands almost always measure as expected.

Also, to dispel a couple of 'myths' about salvage that I frequently hear. “Don't bother removing LEDs or transistors, they always burn up.” This is not true, you can save anything if you work quickly enough. “Remember that salvaging costs money in electricity to run the iron/heat gun.” Not really, at 12 cents per KW/H, it costs about half a penny to use the 45 watt iron for an hour. If I use the heat gun, the cost per hour rises to a measly 16.5 cents.

The oscilloscope only needs to be flat to 15kHz, so almost any one with at least 100kHz bandwidth will work. I did it with one of the 5MHz/5" scopes that were readily available since the 1950s (Dumont, Eico, Heathkit, KnightKit, RCA, etc.). You should be able to find a used one for less than $20. The connections are the same: FM detector to the X input and AGC to the Y input.

One of the suggestions was one that I was first tempted to suggest, but did not as it is potentially dangerous. The use of a filtering capacitor to dampen the voltage should not be tried. The sender circuit is low impedance and needs a very large value capacitor to be successful. Unfortunately that capacitor will store quite a bit of energy and the sender rheostat, in the presence of fuel and air, could create a spark that is sufficient to explosively ignite the fuel vapor. (Think TWA-800).

You have gotten a lot of good suggestions. The best ones deal with repairing the system as it was designed. You might want to make sure the sender is grounded properly and try an external wire back to the gauge to bypass the original one as a test.

The type of filter you need is called a "median" filter. I designed it in software when I worked for Marsh-McBirney, Inc developing flow meters. The data you have can be filtered with a PIC to keep anything from three to seven readings & then just select the value in the middle of the pack. Use even higher numbers if the readings aren't stable enough. With our units taking five readings, it did the trick and was immensely better than an "averaging" filter which took forever to ramp up or shut down, not to mention you never saw anything close to a real time value.

Most of the older vehicles used a +5.0 VDC voltage for all of the instrument cluster. And, the sending unit in the gas tank is one of the resistors in the Wheatstone Bridge circuit for the gauge. In my 1977 GMC 1/2 ton pickup I had the exact same problem. I fixed it by removing the dash group and then removing the gas gauge. The gauge had two long screws with loose nuts that depressed into some clips that connected to the flimsy circuit traces. I carefully tightened those nuts. I still had the typical GM problem of driving for a week to work, and at 1/2 tank I could only make it to work one more day, because the gas needle went to empty in 30 miles. I'd suggest checking your gas gauge connections in the cluster, loose nuts on the gauge's protruding screws, and the flimsy circuit trace connections.

On my old Dodge, (and this applies to Fords and Chevys too) the instrument panel has a thermal voltage regulator. The instruments operate at a voltage of about 5-7 volts. The guages have built in damping.

The thermal voltage regulator is nothing more than a bi-medal strip with a heating coil on it, that opens and closes a contact, thus maintaining a poor, but semi constant voltage. I've replaced these devices with simple voltage regulators like a LM317, set to a voltage that approximates 6 volts. More reading here. www.chevytrucks.org/tech/gasgauge.htm

Mentioned in the above link, is to check for a GOOD ground at the sender in the tank.

I saw the removal of a gas tank and the sending unit was a variable resistor. This resistor sits in the gas! I would not play with this system period!

As you know, a gas sending unit is just a variable resistor acting as a voltage divider. There are several possible reasons for a jumpy reading. One would be if the float is not well placed in the tank and the gas sloshing around just moves it up and down. If that is the case, a filter may help.

Since the gas level should change very slowly, a simple series connected capacitor/resistor combination connected from the output of the sending unit to ground would be a good filter. A large electrolytic with a high enough Voltage rating to withstand any spikes in the 12V system would be best.

You need a time constant that is longer than the period of the gauge's jumps; perhaps a couple of seconds. But you also need a resistance that is small enough to allow enough current to pass through it when needed to maintain the dial reading. TC(Time Constant) = R * C or C = TC/R. R is the value of the series resistor. I would take it to be a few Ohms, perhaps 10 as a first try.

It is a bit hard to calculate the power dissipated in this resistor as it depends on the frequency and severity of the fluctuations but I would start with at least a 1 Watt size and if that burns up, try 5 Watts. With a one second time constant we have: C = 1/10 = 0.1 Farads That is 100,000 uF which is probably impractical. I would try a nice large electrolytic and see what happens. 20,000 uF at 25 Volts would be a good first choice.

If that helps but is not enough, go larger. The negative (-) terminal on the capacitor would go to ground. It does not matter which side of the capacitor the resistor is on as long as it is in series with it. This will not effect the accuracy of the readings as long as the added resistor is not in series with the line to the gauge.

 Another possible cause would be a bad sending unit, but you say you have already replaced it so that is not likely. A third cause would be bad wiring between the sending unit and the gauge. Before you try anything else, you might run an extra, temporary wire to see if it helps. If it does, repair the harness.

Finally, it could be a bad gauge. After trying the above filter and extra wire fixes, you could temporarily rig a 100 Ohm pot at the dashboard to activate the meter. Disconnect the wire from the sending unit and observe the meter as you drive down a bumpy street. If it still jumps, it is the gauge itself and that must be replaced.

I think the simplest way would be to buy this servo adjuster (about $20), remove the pot and add your own wiring to your throttle control pot. With a low cost RC servo connected to the carburetor you should be good to go. Just do a search on the internet to find sources (even Ebay).

These RC servos are for 5V only, so you need to convert any 12 volts to 5V before using them.

Of course another way is to use a microcomputer like the Picaxe. This puppy has analog inputs (for the pot) and servo output. With a microcomputer you could add your own software to do other things, like keep the rpm constant with load changes. Again, it would be 5 volts only.

Also keep in mind a failure of the system may leave you without throttle control. Since you didn't state what this is for, be safe.

Any further questions or help you may contact me directly at: [email protected]

A stepper motor should work and there are already stepper motors being used in cars under the hood. You will need:

• An encoder to measure the position of the gas pedal
• A device to convert this position into pulses
• Another device to use the pulses to power a stepper motor
• And of course, the stepper motor itself

Digi-Key sells an encoder with 16 different pulse counts per revolution, set with tiny switches, for 26 bucks, part number AMT103-V. US Digital sells a chip that will convert the encoder output into pulses that a stepper driver can use. Part # LFLS7184, $3.50. Interinar sells a stepper driver board for $36.00. Stepper motors are available surplus everywhere.

You will have to have an idea which stepper you need if you decide to go this route. You will also need a 5 volt power supply for the encoder and the 7184 chip. US Digital also sells encoders but none that have 16 different resolutions, as far as I know. I have used all the devices mentioned, bought from the companies mentioned. I have used them for a similar application, so I know they will work. The encoder mentioned is a robust device that will be better over time than a cheap pot to measure the foot pedal position. Since the encoder has so many different resolutions it should be fairly easy to find a ratio between the foot pedal position and the throttle opening.

You could also use an RC servo to open the throttle. These servos need a  pulse train of a particular width to know how much to move. The pulses must be constant, because when the pulse train stops the servo returns to the starting position. So, you need some kind of servo controller that will use a pot connected to the foot pedal to tell it how wide to make the pulse it sends to the servo.

The stepper solution suggested above, pretty much just requires connecting the different parts with wires according to the info in the data sheets. But, you can make your own servo controller from a 555 timer, some fixed resistors, some variable resistors (AKA pots), and some capacitors. You will need to solder all this together in an enclosure. This link: pcbheaven.com/wikipages/How_RC_Servos_Works/ will explain how RC servos work and has a sample circuit that you can build to control the RC servo.

One advantage of the RC servo is that it is an absolute positioning system. A certain resistance measured at the foot pedal results in a certain throttle position. The stepper system described above does NOT work that way. The throttle must be at the zero position when power is removed. If the throttle is at half way when power is removed, then the pedal will start to move the throttle from the half way position when power is restored,  unless some type of mechanism always returns the throttle to the fully closed position, when power is removed.

I have two slinkys that I've used as a dipole for 20-80 meters, with a tuner. If you make a coil with the wires close together, you have an inductor. So the larger you make the loops and the further apart the loops are from each other, the lower the inductance will be. That said, I don't think you'll really want to do that with regular copper wire. It might work with copper coated steel wire, but I've never used it, so I don't know. On the other hand, a slinky is not made of copper, it is probably some kind of steel. You can buy the original slinky on-line or at a Cracker Barrel restaurant, that's where I got my two originally.

If you do try to build your own, then try using the length you want for the longest wavelength you want to transmit on and add some extra. Then make a coil with large loops, 3" diameter or larger, and space them apart about an inch or so. If you do not want to use a tuner then you'll probably have to add some capacitance to it at the center of the dipole and this value will change depending on the transmitting frequency. Or, perhaps cutting wire off to make it shorter will be all you need to do. Buying a tuner makes using strange things as antennas much easier.

I hope that helps.