2014 Tech study guide: DC power calculation

Below is the “DC power calculation” section of the 2014 edition of the No-Nonsense Technician Class License Study Guide. This section is mostly unchanged from the last edition. As always, comments welcome…Dan

Power is the rate at which electrical energy is generated or consumed. The formula used to calculate electrical power in a DC circuit is power (P) equals voltage (E) multiplied by current (I). (T5C08)

P= E × I

138 watts is the power being used in a circuit when the applied voltage is 13.8 volts DC and the current is 10 amperes. (T5C09)

P = E × I = 13.8 V × 10 A = 138 W

When the applied voltage in a circuit is 12 volts DC and the current is 2.5 amperes, the power being used is 30 watts. (T5C10)

P = E × I = 12 V × 2.5 A = 30 W

Just as with Ohm’s Law, you can use algebra to come up with other forms of this equation to calculate the voltage if you know the power and the current, or to calculate the current if you know the power and the voltage. The formula to calculate the current, if you know the power and the voltage is

I = P / E

For example, 10 amperes are flowing in a circuit when the applied voltage is 12 volts DC and the load is 120 watts. (T5C11)

I = P / E = 120 W / 12 V = 10 A

2014 Tech study guide: electrical principles, Ohm’s Law

As some of you may know, the Tech question pool is being updated this year. That means, of course, that I’ll have to update my study guide. 

Below are the first two sections. These are basically unchanged from the last edition, except that I removed the questions about voltmeter and ammeter from the first section and questions about calculating power from the second. This allows readers to focus on the electrical concepts. We’ll cover the voltmeter and ammeter questions, and the power calculation questions later.

pier-chartsI am considering adding the charts shown at right to aid people in remembering what formulas to use when calculating the various parmaters. What do you think? Should I add them, or would that just muddy the waters?

The study guide will show the correct answers in bold. I don’t know what the deal is, but for some odd reason, the bold text doesn’t really show as bold here on the website.

Electrical principles, units, and terms: current and voltage; conductors and insulators; alternating and direct current; resistance; power

You don’t have to be an electronics engineer to get a Technician Class license, but it does help to know the basics of electricity and some of the units we use in electronics. The most important concepts are current, voltage, resistance, power, and frequency.

Voltage is the force that causes electrons to flow in a circuit. Voltage is sometimes called electromotive force, or EMF. Voltage is the electrical term for the electromotive force (EMF) that causes electron flow. (T5A05) The volt is the basic unit of electromotive force. (T5A11)

The letter V is shorthand for volts. About 12 volts is the amount of voltage that a mobile transceiver usually requires. (T5A06)

Current is the name for the flow of electrons in an electric circuit. (T5A03) Electrical current is measured in amperes. (T5A01) Direct current is the name for a current that flows only in one direction. (T5A04) Batteries supply direct current, or simply DC.

Alternating current is the name for a current that reverses direction on a regular basis. (T5A09) Frequency is the term that describes the number of times per second that an alternating current reverses direction. (T5A12) Alternating current, or AC, is what is available from your home’s wall sockets. Power supplies convert the AC into DC, which is required for most modern amateur radio equipment.

Resistance is the term used to describe opposition to current flow in a circuit. The basic unit of resistance is the ohm. The Greek letter omega (?) is shorthand for ohms.

Conductors are materials that conduct electrical current well, or, in other words, have a low resistance. The copper wires that we use to connect a power supply to a radio are good conductors because copper is a good electrical conductor. (T5A07)

Insulators are materials that that have a high resistance. They do not conduct electrical current very well. Plastics and glass, for example, are good electrical insulators. (T5A08)

The term that describes the rate at which electrical energy is used (or generated) is power. (T5A10) Electrical power is measured in watts. (T5A02)

Ohm’s Law: formulas and usage 

Hams obey Ohm’s Law!

Ohm’s Law is the relationship between voltage, current, and the resistance in a DC circuit. When you know any two of these values, you can calculate the third.

The most basic equation for Ohm’s Law is

E = I × R

In other words, when you know the current going into a circuit and the resistance of the circuit, the formula used to calculate voltage across the circuit is voltage (E) equals current (I) multiplied by resistance (R). (T5D02)

When you know the voltage across a circuit and the resistance of a circuit, the formula used to calculate resistance in a circuit is resistance (R) equals voltage (E) divided by current (I). (T5D03) We can also write this formula as

R = E / I

When you know the voltage across a circuit and the resistance of a circuit, the formula used to calculate current in the circuit is current (I) equals voltage (E) divided by resistance (R). (T5D01) This formula is written

I = E / R

The resistance of a circuit in which a current of 3 amperes flows through a resistor connected to 90 volts is 30 ohms. (T5D04)

R = E / I = 90 V / 3 A = 30 ?

The resistance in a circuit for which the applied voltage is 12 volts and the current flow is 1.5 amperes is 8 ohms.(T5D05)

R = E / I = 12 V / 1.5 A = 8 ?

The resistance of a circuit that draws 4 amperes from a 12-volt source is 3 ohms. (T5D06)

R = E / I = 12 V / 4 A = 3 ?

The current flow in a circuit with an applied voltage of 120 volts and a resistance of 80 ohms is 1.5 amperes. (T5D07)

I = E / R = 120 V / 80 ? = 1.5 A

The current flowing through a 100-ohm resistor connected across 200 volts is 2 amperes. (T5D08)

I = E / R = 200 V / 100 ? = 2 A

The current flowing through a 24-ohm resistor connected across 240 volts is 10 amperes. (T5D09)

I = E / R = 240 V / 24 ? = 10 A

The voltage across a 2-ohm resistor if a current of 0.5 amperes flows through it is 1 volt. (T5D10)

E = I × R = 0.5 A × 2 ? = 1 V

The voltage across a 10-ohm resistor if a current of 1 ampere flows through it is 10 volts. (T5D11)

E = I × R = 1 A × 10 ? = 10 V

The voltage across a 10-ohm resistor if a current of 2 amperes flows through it is 20 volts. (T5D12)

E = I × R = 2 A × 10 ? = 20 V

SDR: Success this time

Back in May, I purchased a DVB-T mini-dongle to play around with software-defined radio (SDR). Unfortunately, I purchased the wrong type of dongle, and I couldn’t get it to work with any of the SDR software packages out there. The reason for this is that it the dongle did not use the Realtek RTL2832U.

A couple of months ago, I purchased another dongle, this time making sure to purchase the correct one. As you can see from the Amazon ad at right, this dongle cost less than 15 bucks. When I first got it, I was able to get it to run with the Windows laptop I was using down in the shack, but wasn’t able to get it to work with the Mac I use in my office. So, I put it aside, meaning to get back to it sooner or later.

Well, later came on Sunday, as about a foot of snow fell on outside. I downloaded an OSX port of the SDR program gqrx, and in short order, I was actually up and running, and listening to FM broadcast stations here in Ann Arbor. Very cool! I have since used it to listen to not only FM broadcasts, but also the local repeaters and the NOAA weather station on 162.55 MHz.

At first, I was pretty disappointed with the performance. That was the fault of the antenna, though. The antenna that came with the dongle is pretty much useless. What I did was to cut off the whip, solder on a couple of alligator clips and clip the coax to an FM broadcast dipole that came with a stereo I bought ages ago. If you don’t want to do this, you can purchase adapters cable, like the one shown at right, which have an SO-239 on one end and the MCX right angle connector, which plugs into the dongle on the other end.

That greatly improved the performance on FM broadcast as well as on 2m. I had planned on clipping to the 450-ohm ladder line J-pole that I have to see how it improves reception on 2m, but haven’t gotten around to that yet.

While gqrx is a nice program, it does have some limitations. For example, you cannot set up “memories” and switch between them. So, switching frequencies can be somewhat of a hassle. It would also be nice to have a scan feature, so that I could set it up just like I do my handheld and scan all the local repeater frequencies. The good thing is that this program is still very much under development.

The support is really good, too. There is a  gqrx Google Group where I got a very quick answer to  a newbie question.

What else can you do with this dongle? Quite a bit as it turns out. Yesterday, while composing another blog post, I learned about an RTL-SDR spectrum analyzer using a BeagleBone Black, which I already happen to have, and its 7? Touchscreen cape. Again, very cool.

You need a blivet

Brad, AA1IP, wrote to the Glowbugs group:

Disappointed and grumpy because ol’ Sanitary Claus didn’t being you any radio-related presents? Put on your 22, get on your toes and tapdance out an order for a Blivet or Blivette to restore your holiday cheer!

For $9.87, you receive a small USPS Priority flat-rate mail carton– a Blivette– stuffed with miscellaneous electronic parts: some are old, some are new, some are used and removed from equipment, and some are blew, er, blue.

For $23.23, you receive a medium USPS Priority flat-rate mail carton– a Blivet– stuffed with the same contents as above, only more so.

Note that the price of a Blivet or Blivette includes  postage to U.S. addresses only. Overseas postage charges are steep.

Questions welcomed, PayPal honored. Please make sure that your PayPal invoice includes your preferred USPS delivery address.

Now go build a radio!

I just sent off for a Blivette. I’ll let you know what I get.

From my Twitter feed: Santa, kits, PCB design

HamHijinks's avatarHam Hijinks @HamHijinks
Trying a new mode tonight: Santa Scatter! #hamr #hamradio


RigolTechUSA's avatar

Chris Armstrong @RigolTechUSA
The Rise, Fall, and Rise of Electronics Kits | @IEEESpectrum #IEEE goo.gl/wzTuv1

ke9v's avatar
Jeff Davis @ke9v
Design it. Order it. Build it. Very cool. fplus.me/p/4wc8

From my Twitter feed: bad soldering, tequila,

hackaday's avatarhackaday @hackaday
New post: Impersonating FBI Agents And People Who Can Solder bit.ly/JQRKil

2e0sql's avatarPeter Goodhall @2e0sql
Interesting talk on using NodeJS and websockets within Amateur Radio specially when using hardware.- youtu.be/r0svcHERWrM #hamr

NS0D's avatarPete NSØD @NS0D
The #Arrl Centennial QSO party starts 1-Jan-2014 – should be a real marathon with a few different awards available arrl.org/centennial-qso…

This isn’t related to ham radio, but I do like a nip of tequila from time time….Dan

Gizmodo's avatarGizmodo @Gizmodo
How tequila geniuses made the best-tasting spirit I’ve ever had: gizmo.do/qo176k0 pic.twitter.com/XAoQTSrcCz

From my Twitter feed: bypass caps, VHF propagation, SMD rework

dangerousproto's avatarDangerous Prototypes @dangerousproto
app note: properties and application of bypass capacitors goo.gl/YFpqLx

georgesmartuk's avatarGeorge Smart @georgesmartuk
Not sure if everyone has seen @ng0e‘s fantastic VHF Propagation Map from #APRS data. How genius! aprs.mountainlake.k12.mn.us #hamr #hamradio #vhf

DIYEngineering's avatarDIY Engineering @DIYEngineering
app note: Rework method for surface mount MLCCs – Things you might need to know about doing SMT rework on Multi La… ow.ly/2C95Qu

From my Twitter feed: learn electronics, visualize cellphones, NTS newsletter


imabug's avatarimabug @imabug
LearnAbout – Electronics learnabout-electronics.org/index.php


TheZerocool's avatarDenis S @TheZerocool
If your in the school of thought that mobile phone mast RF will cook your mind… check this out and begin to worry: goo.gl/XcrkAN


VA3QV's avatarBob Sharp @VA3QV
A link to a NTS Themed Newsletter : ve3gna.wordpress.com/2013/12/07/lat…

Transforming impedances: Question G5C07

In the last two weeks, I’ve received e-mails from two readers of The No-Nonsense General-Class License Study Guide. Both questioned my explanation of how transformers transform impedance. I wrote:

Transformers are also used to transform impedances. The impedance ratio is also related to the turns ratio, but the transformation is equal to the square of the turns ratio. The turns ratio of a transformer used to match an audio amplifier having a 600-ohm output impedance to a speaker having a 4-ohm impedance is 12.2 to 1. (G5C07)

Doug wrote, “The only way I can reproduce the calculation is by taking the square root of the turns ratio.” His comment made me see where my explanation could be a bit misleading. I wrote back:

Think about it this way. An impedance transformation can go either way. When transforming from a higher impedance to a lower impedance, you divide by the square root of the turns ratio. When transforming a lower impedance to a higher impedance, you multiply by the square of the turns ratio. In either case, the impedance ratio is “related” to the square of the turns ratio.

I love getting feedback from my readers. Feedback like this helps me improve my study guides. If you have used one of my study guides, and have a comment or question about any of the material, please feel free to contact me.

EE Times on PCB design

Are you interested in designing your own printed circuit boards (PCBs)? Then EE Times has a couple of things that you’ll be interested in:

  • Webinar: Fundamentals of PCB Design. This free, introductory overview of printed-circuit design treats the main difficulties you will likely meet when planning, designing, and manufacturing printed circuit boards for digital applications. From this lecture you will take away many nuggets of wisdom concerning manufacturing technology, signal integrity, EMI, power quality, thermal analysis, and project management.
  • A Guide to Low-Cost PCB Tools. This short blog post, written by Adam Carlson, Senior Mechanical Design Engineer, Eagle Technologies, lists eight different PCB design tools that are available for free or are low cost.

If you’re already doing your own PCB design, what tools are you using?