Troubleshooting Resources
AD2CS Quansheng UV-K5 (8) Handheld Radio Review.PDF
The Kelvin Four-Wire Resistance Measurement Method
By Chris Prioli AD2CS - [email protected] - www.ad2cs.com
By Chris Prioli AD2CS - [email protected] - www.ad2cs.com
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Resistance measurement is one of those things that we often tend to take for granted, and don’t think much about. However, it is exactly that mindset that can cause errors to creep into our work without our ever noticing that the errors are present at all. This can happen quite easily, and we might not ever be aware that it is happening, because we have come to expect and accept the erroneous readings as valid and factual. What is this guy talking about, you ask? Read on… Consider the conventional measurement of a 0.2Ω resistance with an ohmmeter whose leads each have a resistance of 0.1Ω. This means that the test leads have as much resistance as the actual device under test, meaning that the meter will indicate a reading of 0.4Ω, or a 100% error. This is unacceptable and is to be avoided if possible. |
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Now consider a 0.773µH toroidal coil wound on a T50-6 core using 0.6mm diameter enameled copper wire. If measured with a conventional ohmmeter, this inductor would yield a resistance measurement of zero ohms, and in fact would cause a tonal continuity tester to emit the indicating tone. However, when measured using my Ascel Æ20218 Milliohm Meter, which uses the Kelvin resistance measuring method, we get a quite different picture. Now we see that this inductor actually has a measurable resistance of 0.0226 ohms.
OK - 0.0226 ohms does not seem like a whole lot of resistance, but it is clearly more than we originally measured with the less-capable meter system. So, what is it about the Æ20218 that makes it “more capable” than the standard ohmmeter as found in my Greenlee DM-510A DVOM? In short, the answer is the four-wire Kelvin measurement method, which is what we are going to explore in this article.
To begin, we have to accept some truths right up front. The first of these is that any test lead will have some inherent resistance within the lead, as a function of the physics of the test lead. The test lead is made of wire, and wire - any wire - has a given amount of resistance per foot (or inch) of that wire. The data in (Table 1 : Copper Wire Resistance By AWG Size, Solid Wire) provides some insight into these inherent resistances. As the caption reveals, these values are for solid copper wire. Similar tables are available for wires that are stranded with various stranding schemes. Because of the wide variety of different stranding schemes on the market, it was decided to ignore the stranded wires and illustrate the solid wire for this discussion.
As the above Table 1 data shows, 22AWG solid copper wire will have an approximate resistance of 16.5 ohms per thousand feet of wire. This works out to 0.0165 ohms per foot of that wire. That is how this table works. Bear with me here, because now I am going to do some magic with the table.
Take a look at the 23AWG wire. It is listed as having a 509 circular mils area, which works out to 22.56 mils diameter. Compare that to the 0.6mm diameter wire used to wind the toroidal coil referenced earlier in this article. A diameter of 0.6mm is equivalent to a diameter of 23.62 mils - very close to our 22.56 mils of the 23 AWG wire. Now let’s look at the resistance of the 23AWG wire, which is listed as 20.8 ohms per one thousand feet of length, equivalent to 0.0208 ohms per foot. Now look at the measured resistance of the toroidal coil referenced earlier. That resistance was 0.0226 ohms. This is remarkably close to the resistance of one foot of 23 AWG solid copper wire, which is remarkably close to the diameter of the 0.6mm diameter magnet wire used for the coil in the first place.
Why did I just go through all of this? I did this to clearly illustrate that these “zero ohms” inductors do in fact have a measurable - and predictable - resistance based on the physics of the wire used to manufacture the inductor. Concept number one is now out of the way, and we can move on to concept number two.
I said earlier that any test lead will have “some inherent resistance within the lead, as a function of the physics of the test lead”, a statement that we have just validated with the above explanation and illustration. The next truth that we have to accept is that for all intents and purposes, the resistance of the leads of a quality voltmeter can largely be ignored when considering the effect on the accuracy of a voltmeter reading. Let’s look at why this is so.
As the above Table 1 data shows, 22AWG solid copper wire will have an approximate resistance of 16.5 ohms per thousand feet of wire. This works out to 0.0165 ohms per foot of that wire. That is how this table works. Bear with me here, because now I am going to do some magic with the table.
Take a look at the 23AWG wire. It is listed as having a 509 circular mils area, which works out to 22.56 mils diameter. Compare that to the 0.6mm diameter wire used to wind the toroidal coil referenced earlier in this article. A diameter of 0.6mm is equivalent to a diameter of 23.62 mils - very close to our 22.56 mils of the 23 AWG wire. Now let’s look at the resistance of the 23AWG wire, which is listed as 20.8 ohms per one thousand feet of length, equivalent to 0.0208 ohms per foot. Now look at the measured resistance of the toroidal coil referenced earlier. That resistance was 0.0226 ohms. This is remarkably close to the resistance of one foot of 23 AWG solid copper wire, which is remarkably close to the diameter of the 0.6mm diameter magnet wire used for the coil in the first place.
Why did I just go through all of this? I did this to clearly illustrate that these “zero ohms” inductors do in fact have a measurable - and predictable - resistance based on the physics of the wire used to manufacture the inductor. Concept number one is now out of the way, and we can move on to concept number two.
I said earlier that any test lead will have “some inherent resistance within the lead, as a function of the physics of the test lead”, a statement that we have just validated with the above explanation and illustration. The next truth that we have to accept is that for all intents and purposes, the resistance of the leads of a quality voltmeter can largely be ignored when considering the effect on the accuracy of a voltmeter reading. Let’s look at why this is so.
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A voltmeter is connected in parallel (Figure 2) with the circuit or device under test. A quality (read : accurate) voltmeter will have a very high input impedance as indicated by the meter’s ohms per volt rating. In the bad old days of analog voltmeters, an ohms per volt rating of 20,000 ohms per volt was considered to be a trait of a good meter. That was, and still is, the DC Volts input impedance rating of the venerable Simpson 260 analog VOM. The AC Volts scale, however, dropped to an input impedance of only 5,000 ohms per volt with the Simpson 260. |
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A more accurate analog VOM was typically found in the form of a vacuum-tube voltmeter (VTVM), which had input impedances on the order of ten or eleven megohms per volt, a considerable increase over the previous impedances discussed.
Modern digital voltmeters also have ten- or eleven-megohm per volt input impedances, due to the fact that they typically employ a field-effect transistor-based amplifier circuit. The FETVOM was the portable, battery-operated successor to the VTVM, again using a FET circuit in place of the vacuum tube amplifier circuit.
The bottom line to all of this is that the connection of a quality voltmeter to an operating circuit should have an extremely limited effect on the working circuit, because we are adding such an immense resistance in parallel to the working circuit. The circuit loading will be nil and the voltmeter reading will be as accurate as possible.
Modern digital voltmeters also have ten- or eleven-megohm per volt input impedances, due to the fact that they typically employ a field-effect transistor-based amplifier circuit. The FETVOM was the portable, battery-operated successor to the VTVM, again using a FET circuit in place of the vacuum tube amplifier circuit.
The bottom line to all of this is that the connection of a quality voltmeter to an operating circuit should have an extremely limited effect on the working circuit, because we are adding such an immense resistance in parallel to the working circuit. The circuit loading will be nil and the voltmeter reading will be as accurate as possible.
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Now for the third truth that must be accepted. In a series circuit, any current flowing in that circuit will pass as the same current value through all series-connected components of the circuit. This is a basic concept, but it is important that it is understood in order to understand the operation of the Kelvin resistance measurement method. It is equally important to understand that an ammeter is always connected in series (Figure 3) with the circuit whose current is being measured, and therefore becomes one of the series components in that circuit and the full series current will therefore flow through the ammeter.
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Let’s now move on to the physical aspect of Kelvin resistance measurement. We start out with the fact that this method requires making two separate measurements and then using the results of those measurements to calculate the resistance.
The Kelvin method uses a set of four test leads (Figures 1 & 4), each of which has some inherent resistance. At one end of each test lead is a connector that makes the circuit into the ohmmeter, e.g., a banana plug or some similar connector. The meter, of course, has a mating receptacle for each of the four test leads. Two of the test leads are voltage leads, and two are current leads. At the working end of the test leads, each pair of leads is brought together to a specialized type of test clip similar to an alligator clip, but the similarity is in general appearance only.
A standard alligator clip is a single connection for a single lead, with both “jaws” of the clip connected to each other whether the clip is open or closed. The Kelvin test clips are quite different in that the two opposing “jaws” are insulated from each other. The clip body is of an insulating material. The two separate test leads are connected separately, one lead to each jaw of the Kelvin test clip. When the test clip is open, the two test leads are isolated from each other.
The typical Kelvin test lead set consists of two leads having red banana plugs on one end of each lead and two leads having black banana plugs on one end of each lead. The two leads with the red banana plugs come together to a Kelvin test clip having red insulation on the handles, and as stated earlier, are isolated from each other when the test clip is held open.
A standard alligator clip is a single connection for a single lead, with both “jaws” of the clip connected to each other whether the clip is open or closed. The Kelvin test clips are quite different in that the two opposing “jaws” are insulated from each other. The clip body is of an insulating material. The two separate test leads are connected separately, one lead to each jaw of the Kelvin test clip. When the test clip is open, the two test leads are isolated from each other.
The typical Kelvin test lead set consists of two leads having red banana plugs on one end of each lead and two leads having black banana plugs on one end of each lead. The two leads with the red banana plugs come together to a Kelvin test clip having red insulation on the handles, and as stated earlier, are isolated from each other when the test clip is held open.
Figure 5 shows the Kelvin test clip with its halves separated, showing that one of the test leads is connected to each half of the test clip, and that when the test clip is open, the leads are isolated from each other. The two leads with the black banana plugs come together to a Kelvin test clip having black insulation on the handles in a manner that is the same as that for the red test leads, and as stated earlier, are isolated from each other when that test clip is held open.
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At the test instrument, the banana jacks are labeled Current OUT and Voltage IN. There is one red and one black banana jack each for current and voltage. So long as the colors are matched, it makes no difference which lead is connected to which banana jack as regards current versus voltage. Figure 6 shows the front panel of the Ascel Electronic Æ20218 Milliohm Meter, a test instrument that uses the Kelvin resistance measurement method. |
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When the Kelvin resistance measurement method is in use, the test instrument sends a DC current out to the resistor under test, using one pair of the connected test leads. The test instrument then precisely measures the current that flows through the unknown resistance. At the same time, a precise measurement of the voltage drop across the unknown resistance is made by the test instrument, using the second set of connected test leads. These two measured values are used to calculate the resistance of the device under test using the Ohm’s Law equation for resistance.
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As an example, suppose that the test instrument measures a current of 783mA and a voltage drop of 3.85V. In that case >>> |
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A quick look at the diagram in Figure 7 might help to make this whole concept a little bit more clearly understood. In the Figure 7 example, a current of 335mA is measured through the unknown resistance, across which a voltage drop of 6.7 volts is measured. Apply these values into the Ohm’s Law equation as shown in the diagram and we get a calculated resistance of 20 ohms for the device under test. Note that the two red leads are connected to the positive sides of the two meters, and the two black test leads are connected to the negative terminals of the two meters.
The inherent resistance of the test leads is nullified differently in the two measurements. In the current measurement, it is a moot point, because the current flowing through the resistance under test is the same as the series current throughout the entire circuit, including the test leads.
In the voltage measurement, as has already been explained, the extremely high input impedance of the voltmeter circuit causes minimal loading to the circuit under test, making the resistance of the test leads negligible.
Because of the fact that the lead resistance is taken out of the measurement, the remaining resistance is purely that of the device under test. Because that resistance is calculated arithmetically using two other non-resistive measurements as the basis for the calculation, we have the strength of Ohm’s Law to support the accuracy of the ultimate resistance determination. These two measurements, as we have seen, are largely unaffected by the inherent resistance of the test leads, reducing the error almost to the point of non-existence.
We cannot get much better than that. The Kelvin four-wire or four-terminal resistance measurement method gives us the most accurate resistance measurement we can obtain without spending thousands of dollars on high-level laboratory test equipment.
In the voltage measurement, as has already been explained, the extremely high input impedance of the voltmeter circuit causes minimal loading to the circuit under test, making the resistance of the test leads negligible.
Because of the fact that the lead resistance is taken out of the measurement, the remaining resistance is purely that of the device under test. Because that resistance is calculated arithmetically using two other non-resistive measurements as the basis for the calculation, we have the strength of Ohm’s Law to support the accuracy of the ultimate resistance determination. These two measurements, as we have seen, are largely unaffected by the inherent resistance of the test leads, reducing the error almost to the point of non-existence.
We cannot get much better than that. The Kelvin four-wire or four-terminal resistance measurement method gives us the most accurate resistance measurement we can obtain without spending thousands of dollars on high-level laboratory test equipment.
Lightweight Paddle Set For CW Operators
By Chris Prioli, AD2CS - [email protected] - www.ad2cs.com
By Chris Prioli, AD2CS - [email protected] - www.ad2cs.com
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This article is a slight departure from my normal writing, in that that the topic at hand here is a description and round-about endorsement of a product. On tap as the subject of this article is the cwmorse.us “Red Lightweight Double Paddle With Steel Base” (Figure 1). On their website, cwmorse.us describes this paddle by stating that it is an… “…Ultimate Lightning Fast Double Paddle Morse Code Key. Super Smooth Action With Dual Precision Self Lubricating Nylon Bearings. Solid Brass Contacts With Stainless Steel Fasteners & Nickel Plated Steel Spring. All Soldered Braided Copper Wire Connections.” I was introduced to this paddle by fellow GCARC member John Zaruba Jr, K2ZA. It did not take me long to read through the small amount of literature available about the paddle set. I then made the decision to purchase one and to see what all the positive press was about. |
Figure 1 : cwmorse.us Paddle Set
Figure 2 : Paddle With Cover Removed
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Now… I am not very capable as regards CW. I am still only learning the requisite skills - both memory and physical - to be able to become competent. However, one of the factors in developing the physical muscle memory to permit Morse competency is having a good, solid, and consistent key system on which to learn. If the key environment is constantly changing, your muscle memory will develop much more slowly as you will be forced to continually re-acclimate yourself to the changing key conditions.
While each CW operator develops his or her own preference as to key type, the paddle system has become quite popular in today’s amateur radio CW circles. Part of the reason for this is the ability to quickly and easily inject opposite morse characters into the middle of a continuing stream of base characters. For example, if the operator is sending the string “091”, it would be coded as :
– – – – – – – – – • • – – – –
or
dah dah dah dah dah dah dah dah dah dit dit dah dah dah dah
or
dah dah dah dah dah dah dah dah dah dit dit dah dah dah dah
using standard International Morse Code. When sent via a paddle, this is accomplished by holding the right lever for the proper count to send the dahs, lifting as needed for the spaces. Then, when it is time to insert the two dits, simply tap the left lever twice with a lift between them.
Another example might be seen in sending the single character “Q” which consists of the code string dah dah dit dah, and is sent by holding the right lever for a four-count, while also tapping the left lever once at the count of three. This will inject a dit between the second and third dahs.
Another example might be seen in sending the single character “Q” which consists of the code string dah dah dit dah, and is sent by holding the right lever for a four-count, while also tapping the left lever once at the count of three. This will inject a dit between the second and third dahs.
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When the new paddle arrived, I immediately set about getting a “feel” for the new device. It is well-weighted, so it stays put on the desktop, using four rubber feet to help keep it from sliding around. The connection to the radio is via a 3.5mm (1/8”) TRS jack (Figure 3), so if your radio uses a 1/4” key jack, an adapting cable or a plug adapter will be required. The paddle is comfortable under the fingertips, and is fully adjustable as to travel and tension. Adjustment is made via a trio of stainless-steel socket-head machine screws (Figure 4). One of these screws is mounted through the left-hand lever and is the rear-most of the adjustment screws (when viewed from the lever end of the unit). That particular adjustment screw controls the spring tension exerted upon the levers. Tightening the screw increases the spring tension, making the lever action stiffer. The remaining two screws are mounted, respectively, through the left-hand and the right-hand adjustment stop blocks, and their tips are directly in contact with the side of the lever toward which each screw is pointing. These screws adjust the lever travel by changing the at-rest positions of the levers. |
Figure 3 : Connection End View
Figure 4 : Adjustment Screw Arrangement
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The looser the screw, the greater the travel. As a nod to “attention to detail”, the Allen wrench required to make adjustments to this paddle set is included with the unit, and it has a convenient storage location machined into the plastic upper base of the paddle, which is visible in the Figure 3 illustration.
Due to the nature of the paddle design, the three wires of the TRS jack are connected as follows :
- Tip (T) to right lever (dah) via the RED wire;
- Ring (R) to left lever (dit) via the YEL wire; and
- Sleeve (S) to common via the GRN wire.
This is the standard configuration for a right-handed paddle setup. However, if your radio uses the opposite configuration, you will need to place your radio’s paddle setting in reverse mode (if available) or else use an adapting cable that has the wires crossed to accommodate your radio’s key/paddle circuit.
The base of this paddle is a steel plate 1/2” thick and three inches square. All edges and corners are killed so that there are no sharp aspects to this base. The remainder of the unit, apart from the TRS jack, the wire, and the hardware, is all of the 3-D print manufacturing process. Even the cover that snaps on over the works is a 3-D printed item. When we think of 3-D printing as a manufacturing method, we often think of the product as being of lesser quality somehow. That is clearly not the case as regards this paddle. All of the parts are cleanly made and they all fit - and work - together quite well.
The paddle is available in several different colors, including army green, black, blue, green, grey, indigo, orange, purple, red, and yellow. The price for the basic paddle is $42.95, while the price of the paddle with the steel base is $64.95. The base is available as an accessory for $24.95. The web address for the sales site is https://cwmorse.us.
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For those who are able to interpret it, there is a message in the label (Figure 5) that is affixed to the top cover of each paddle sold by this company… only it is actually printed in Morse. It says “USA” – the location in which all of this company’s products are proudly manufactured.
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W2MMD Clubhouse Test & Repair Bench : Coaxial Cable Tester
By Chris Prioli, AD2CS - [email protected] - www.ad2cs.com
I am very pleased to announce that I have just added a new piece of equipment to the W2MMD Clubhouse arsenal on the test and repair bench. The new tool is a multi-connector-type coaxial cable tester that will test cables of several types and of any length for continuity on the outer shield braid and on the inner center conductor, as well as testing the cables for shorts between the braid and the center conductor.
The tool indicates the results of the two-stage testing process via LED's on the front panel. The test procedure instructions are labeled on the unit, which is extremely easy to use.
This device will test cables with SMA, BNC, UHF, and N connectors, in any combination of those connector types. It is compact, portable, and completely self-contained. It is powered by a standard nine-volt alkaline snap-top battery, which should have an extensive service life due to the very limited current drawn during the testing of the cables.
I am fully open to suggestions for additional test equipment that may be suitable for the Club and for the T&R bench. Please feel free to pass any such comments or suggestions on to me, as well as any comments on the new cable tester.
By Chris Prioli, AD2CS - [email protected] - www.ad2cs.com
I am very pleased to announce that I have just added a new piece of equipment to the W2MMD Clubhouse arsenal on the test and repair bench. The new tool is a multi-connector-type coaxial cable tester that will test cables of several types and of any length for continuity on the outer shield braid and on the inner center conductor, as well as testing the cables for shorts between the braid and the center conductor.
The tool indicates the results of the two-stage testing process via LED's on the front panel. The test procedure instructions are labeled on the unit, which is extremely easy to use.
This device will test cables with SMA, BNC, UHF, and N connectors, in any combination of those connector types. It is compact, portable, and completely self-contained. It is powered by a standard nine-volt alkaline snap-top battery, which should have an extensive service life due to the very limited current drawn during the testing of the cables.
I am fully open to suggestions for additional test equipment that may be suitable for the Club and for the T&R bench. Please feel free to pass any such comments or suggestions on to me, as well as any comments on the new cable tester.