Search for intermittent faults, and Pupin coils using a reflectometer.

Intermittent faults ('floating' defects) are damages that manifest themselves periodically and are caused by poor-quality core connections or reduced insulation resistance. Customer complaints about short-term connection losses are evidence of defects of this kind. Such defects may appear due to mechanical damage to the cable (for example, in the event of vibration from heavy vehicles, rotary equipment, etc., nearby).

Typically, when a technician encounters this type of damage, he has to wait patiently for it to manifest itself, hoping the effect will last long enough to determine its location. There is no guarantee that the damage will reveal itself while the technician is on duty. The use of reflectometers allows one to automate this process and maximize productivity.

Some reflectometers have a special function for detecting intermittent faults. The device connected to the line accumulates all reflectograms over a certain period and displays them superimposed on each other. Where the reflectogram differs, the intermittent fault is located.

Finding intermittent faults

For example, consider the following situation: a particular pair of cables works fine for the better part of the day, but there is a momentary failure out of the blue.

We get two reflectograms for the same pair (with different gain settings) when checked. In the first one, with a gain of 12 dB, a surge of positive polarity is observed on the reflectogram of a working pair at a distance of 6760 feet, corresponding to the end of the cable. In the second one, when the gain increases by 14 dB, an additional spike appears on the reflectogram, the nature of which indicates the presence of a coupling in the cable at a distance of 3280 feet. By further increasing the vertical gain level, the reflectogram will not reveal the slightest sign of damage along the entire length of the cable being tested.

We will need the 'intermittent fault detection' function mentioned above. By continuously monitoring the pair's condition, the OTDR shows any deviations from the cable's rated impedance, allowing the location of intermittent faults to be pinpointed.

The reflectometer display will show the current reflectograms obtained during testing. Periodic inspections allow one to determine whether signs of malfunction have appeared. Once the non-persistent damage has been captured, the result should look approximately as shown in the figure.

The differences will be evident if one compares it with the previous one. A noticeable drop appears where there was nothing before. The location of the fault can be determined by simply moving the cursor to the front of the pulse reflected from the break and reading the distance from the display.

Random vibrations or other irregular events cause the connections to loosen and electrical contact to be temporarily lost, resulting in a fault similar to a partial break. Note that at the moment this fault occurs, the pulse reflected from the far open end of the line decreases because, due to a poor connection in the cable coupling, the magnitude of the electrical signal reaching the end of the cable is reduced.

What conclusions can be drawn? Almost every type of cable system is susceptible to intermittent faults. Such damage creates severe problems for users and technicians. The intermittent fault detection mode of reflectometers allows one to continuously monitor the cable over a long period, so the technician does not have to waste working hours waiting for the damage to manifest itself.

Pupin coils

Pupin coils can still be found on an analog telephone line. Pupin coils disrupt the homogeneity of the copper pair, turning it into an ideal low-pass filter with more substantial high-frequency attenuation.

Therefore, a prerequisite for using any xDSL technologies on existing phone lines is the removal of Pupin coils, which have been found to have extensive applications in US telephone networks. Servicing xDSL systems can always...

How does binary logic work? Shift registers

Sometimes, a microcontroller does not have enough pins to receive signals from buttons or display them on LED indicators, control relays, etc.

Sometimes, one needs to interconnect two digital devices with a single cable, and it would be great to transmit eight, sixteen, or more signals over two to three wires to avoid needing a thick cable.

Or, let's say we just want to make a lighting effect for a street sign. One does not need a whole computer or a microcontroller for this task. All these cases (and many others) should be designed with shift registers.

As children, many of us had an NES (Nintendo Entertainment System) game console. Its gamepad had 8 buttons: a plus-shaped button for left, right, up, and down, then Select, Start, A, and B. And there were only five wires in the gamepad cable: ground, +5-volt power, and three signal wires. Meaning the state of eight buttons was transmitted over three wires.

In the core of the gamepad is a single CD4021 chip. It is an 8-stage parallel input/serial output shift register. Here is a diagram of its internal logic: the chip has eight inputs for parallel input and outputs from the last three flip-flops.

This should look familiar to our audience: a sequence of synchronous D flip-flops passing the torch of data bits from one to another. Oh, that's our combination lock from the post on flip-flops!

The CD4021 chip has two operating modes: serial and parallel. In parallel mode, eight flip-flops store information from eight inputs, each individually, regardless of clock pulses.

In serial mode, at the edge of the clock pulse, each subsequent flip-flop receives a data bit from the previous one, and the first flip-flop gets an incoming one from the serial input.

Then, where is the input pin to reset all flip-flops? The answer is there's none. However, you can pull the serial input low and send eight consecutive clock pulses. If necessary, we can write zeros to all memory cells. Although, in the case of a gamepad, one can do without it.

Simply switch the chip to parallel input mode, and it will save the state of the buttons. Pressed-down buttons correspond to logical zeros; released buttons correspond to logical ones because parallel inputs of the CD4021 in the NES gamepad are pulled by resistors to the power supply positive.

In this case, the DATA wire connected to the output of the eighth flip-flop will contain the state of the button S8 ('A'). We switch the chip to serial mode, apply clock pulses, and read S7 ('B'), then S6 ('Select'), all the way to S1 ('Right').

Congratulations! We have read the state of eight buttons via three signal wires (plus two power wires). Then we toggle to parallel mode again, rinse and repeat. This mode toggling is performed lightning fast, and the player will feel like the console responds to button presses instantly.

But what if it’s the other way around, and one doesn’t need to read information from buttons but to write it into cells, for example, by lighting LEDs? Then, a shift register with serial input and parallel output will help.

An example of such a shift register is CD40194. Unlike CD4021, it has not 8, but only 4 digits. Yet it's got parallel output and input, as well as serial input, with the ability to shift both to the right and left!

Does the CD40194 have a serial output, though? I hear you asking. Of course, it has! Q3 will be the serial output when shifted to the right, and Q0 will be the serial output when shifted to the left.

The CD40194 also has a general reset input. And there are also two mode selection inputs: S0 and S1.

When S0 = 0 and S1 = 0, nothing happens. The chip does not respond to signals other than a general reset, retaining the saved 4 bits of information present at its outputs Q0..Q3.

When S0 = 1 and S1 = 0, a shift to the right occurs at the leading edge of the clock pulse, from Q0 towards Q3. And the value from the left-most serial input is written to Q0.

When S0 = 0 and S1 = 1, a shift to the left occurs at the leading edge...

LED strip amplifier / LED amplifier / RGB amplifier. Features, problems, choice.

What is the difference between LED strip amplifiers, LED amplifiers, and RGB amplifiers?

Only in words and channels.

• LED strip amplifier is a general definition.
• An LED amplifier usually refers to a single-channel device that operates with a white light strip. Another name is the DC amplifier.
• WW LED Amplifier is a dual channel device for warm and cool white light temperature strips.
• An RGB amplifier is a three-channel device that produces red, green, and blue color strips.
• The RGBW amplifier is a four-channel device for red, green, and blue color strips, with additional white LEDs.
• The RGBWW amplifier is a five-channel device for red, green, and blue color strips, with additional warm and cool white LEDs.

Why do you need LED amplifiers?

Conductors always have resistance. Imagine we want to install an LED strip around a 500-square-foot room (16*32 feet). We need 96 feet of LED strip, and it is impossible (!) to connect it to the power supply at one point (!). Why?

For example, a strip has a power consumption of 3 watts/foot (a 16-foot reel has a power rating of 48 watts and a current of four amps at 12V). A 16-foot strip comprises 96 sections (cut lines) of two inches each. Each strip section will have an internal conductor resistance of 0.005-0.02 Ohms, depending on the manufacturing quality. The total native resistance of the strip is 0.48 to 2.8 Ohms. The supply voltage drop for the last sections of the strip will be 2 - 7.7 V. The voltage across the last sections of the strip will be 10 - 4.3 V. This is very low!

When all three channels are on, we will clearly see the difference in brightness between the beginning and end of a 16-foot strip. For an RGB strip, it will look like a color change. The start of the strip will be white, and the end of the strip will be yellow.

=======================================
Details are in my earlier post:
Effect of reducing the LED strip supply voltage on the light emitted
https://teardownit.com/posts/effect-of-reducing-the-led-strip-supply-voltage-on-the-light-emitted
=======================================

Several solutions to the power problem exist for a long line of LED strips. The first option is to install a thick power cable next to the entire LED strip and connect it to the strip several times every 10-20 feet. For example, a 4*14 AWG cable. The solution is excellent and reliable but expensive.

The second option is the use of an LED strip amplifier. The device is a set of transistor keys for powering a powerful load controlled by a special signal. LED amplifiers allow us to use multiple power supplies, synchronizing powerful loads with a control signal. We don't need to run four thick conductors along the entire length of the RGB strip, but just install a few power supplies and amplifiers every 10-20 feet. We can also combine power supply options depending on the situation.

We will also need an RGB amplifier to connect more loads (LED strips) to the RGB controller output than it supports/allows. For example, the RGB controller is designed for a 100W load, but we want to connect 300W LED strips.

So, what can go wrong with such simple devices? When choosing from catalogs and online stores, you will see only two significant characteristics - operating voltage (5/12/24V), maximum output current, and the number of channels - three for RGB and four for RGBW. However, my experience has shown that not everything is shown in the documentation.

Lying about the maximum current

Amplifiers are DC-powered, and when all channels are switched on, the total current of all channels flows through the common power wire (5/12/24V). Therefore, the maximum total current through the amplifier is critical. Since we are describing an electrical circuit, it is crucial to know how strong the weakest link is.

Let's take a look at such an amplifier.

In the housing and documentation, the maximum current through the amplifier is 24A. But! The device uses disconnectable connectors, which...