Hi-Pot Testers & Electrical Safety

Electrical safety testing is a universal prerequisite for electrical and electronic equipment!

Testing to specific regional demands can be a daunting task, but today's advanced Hi-Pot testers can help simplify the task.

A Hi-Pot Tester is a safety tester that combines a AC or DC (or both) high voltage generator and a leakage current measuring device associated with a discharge system.

The output capacity of the Hi-Pot tester is usually indicated in VA which refers to the product of the maximum rated AC test voltage and the maximum rated current.

The output capacity actually depends on how much current is required to maintain the test voltage. For example, 500VA can provide 100mA at 5kV AC. 

The high voltage test, also called breakdown test, dielectric strength test, or flash test, is made to stress the device under test with a high voltage AC or DC (Typically around 5kV-6kV) and check that there is no breakdown nor perforation taking place.

In other words, the basic Hi-Pot test applies a high voltage from the conductors to the chassis of the device under test.

This test is often referred to as a Dielectric Test or Voltage Withstand Test.

Its purpose is to confirm that there is adequate insulation and isolation of the non-conducting surfaces from the operating voltage to avoid a shock hazard.

Generally, withstanding voltage, insulation and grounding resistance are the test functions of the electric safety tester.

Among them, the withstanding voltage test is a compulsory test item for electrical equipment before leaving the factory.

In contrast, a Hi-Pot tester or Dielectric Withstand Tester is used for conducting electrical safety tests on electrical components and products to make certain they are in compliance with electrical safety requirements.

The Hi-Pot tester can determine the suitability of the dielectric or insulation barrier between hazardous and non-hazardous parts.

This post was prepared by studying some online guides published by the following companies:

• GW Instek
• Chroma Instruments
• VITREK

OK, see you next week ⫸

High Voltage Transistor Guide

In need of bipolar transistors that can withstand high voltage pulses ? Then go through this post to see some high voltage BJTs with breakdown voltages from up to 400V. 

Let's start with a list of high voltage transistors .

Not to mention, they are still available and often used in many devices today. You can also see some of them in old landline and cordless telephone circuits.

• MPSA92 (PNP)
• MPSA42 (NPN)
• MPSA44 (NPN)
• ZTX458 (NPN)
• FMMT558 (PNP)
• FZT658 (NPN)

So, what exactly is a high voltage transistor?

Simply put, a  high voltage transistor can handle higher voltage levels ranging from a few hundred volts to several kilovolts than standard transistors because they are designed to withstand higher voltage stresses and power levels.

Take a look at the FCX458 transistor data shown below (click image to enlarge).

According to the datasheet, it is a 400V NPN High Voltage Transistor in SOT89.

As you can see, it has an absolute maximum collector-base and collector-emitter voltage of 400V.

Remember at this point that for the very common BC547 transistor we are all familiar with, these are below 50V.

For transistors, the maximum allowable current, voltage, power dissipation and other parameters are specified as maximum ratings.

A transistor composes an input/ output circuit containing an emitter, base or collector. Since either terminal is used as a common terminal in the circuit, the collector-base voltage VCB, collector-emitter voltage VCE, and emitter-base voltage VEB ratings are specified for transistors.

There are two types of breakdown voltages that determine the voltage ratings - those inherent to a transistor such as V(BR)CBO and V(BR)CEO and those dependent on the base circuit conditions such as V(BR)CER and V(BR)CEX.

This TOSHIBA Application Note (PDF) describes the maximum ratings of bipolar transistors ↗ 

To test transistor breakdown voltage we use a small Chinese Multifunction Digital Transistor Analyzer/Tester (see below).

The Duoyi DY294 tester has a breakdown voltage V(BR) function up to 1000V and works very well without damaging a transistor under test so far.

It is also very useful for spotting fake transistors (that often comes with much lower breakdown voltage than specified in the ratings).

Well, until next time, keep on learning! 

Step Recovery Diode Explained

A step recovery diode (SRD) is a semiconductor junction diode with the ability to generate extremely short pulses. 

A step recovery diode can be referred to as a part of the microwave diode (during the high-frequency range, it tends to generate pulses). 

The silicon step recovery diode has a variety of applications in microwave (MHz to GHz frequency range) electronics as pulse generator or parametric amplifier.

These diodes are dependent on the type of diodes that have the characteristics of turning-off fast based on their operation.

Quite recently we caught a bunch of SemiGen’s SSR series of Step Recovery Diodes.

They are epitaxial silicon varactors which provide high output power and efficiencies in harmonic generator applications.

The SemiGen SRDs offer low snap time through voltages ranging from 8 VDC to 120 VDC, and capacitances ranging from 0.2 pF to 3 pF at 6 VDC.

Below is the Capacitance vs. Bias Curve (@1 MHz) of the MA144769-287 & MAVR-044769-1279 Surface Mount Low Power Step Recovery Diodes (MACOM Technology Solutions Inc.).

This is a basic setup to test SRDs (maybe we'll go into this a little more in a later post):

Now to the Drift Step Recovery Diodes (DSRDs) discovered by Russian scientists in 1981. 

The working principle of the DSRD is similar to the SRD but with a substantial difference.

That is, the forward pumping current should be pulsed (not continuous) because drift diodes function with slow carriers. 

Keep note at this point that the main phenomenon used in SRDs is the storage of electric charge during forward conduction, which is present in all semiconductor junction diodes and is due to finite lifetime of minority carriers in semiconductors.

Here you can see an in-depth explanation of SRDs and DSRDs ↣ https://911electronic.com/what-is-step-recovery-diode/

To sum up, SRDs are specialized semiconductor devices that come with their own set of advantages (and drawbacks) tailored to specific applications.

And, one of the principal benefits of SRDs is their ability to produce forward current more swiftly than traditional diodes. 

It seems that SRDs are discontinued by many suppliers. But fortunately a few manufacturers still produce them for customers servicing legacy radio and microwave systems. So, try it out and have fun!

Epicap Tuning Diodes in RF Electronics

The Epicap Tuning Diode (also called the voltage-variable capacitance diode, varicap diode, tuning diode, and varactor diode) is widely used in television receivers, radio receivers, and other communications equipment because it can be used for electronic tuning.

The Epicap Diode has a variable capacitance as the capacitance value can be controlled by adjusting its bias voltage. 

It is in fact a type of PN diode usually used as a  'tuning capacitor' in radio circuits.

The Epicap Diode is connected in reverse bias because when a reverse bias voltage is applied, the depletion zone in the P-N junction varies depending on the magnitude of the voltage (see an application example below - click image to enlarge). 

These diodes are available in a wide range of capacitance values from nearly 22pF to 33pF.

And, low capacitance diodes for VHF and UHF circuits are still reasonably easy to find.

The SVC201SPA from Sanyo is a quite popular quondam Epicap/Varactor Diode.

This Diffused Junction Type Silicon Diode is intended for use in FM receiver electronic tuning applications.

Next one is the MV209 Silicon Epicap Diode which is designed for general frequency control and tuning applications to provide solid−state reliability in replacement of mechanical tuning methods.

See its Diode Capacitance Curve below.

In a nutshell, an Epicap Tuning Diode operates using a reverse bias voltage applied to the P-N junction of the diode. 

As this reverse bias voltage increases, the depletion layer at the P-N diode junction widens.

Thus the resultant is a device which can be used as an 'electronic' variable capacitor or tuning capacitor.

Finally, to the practical project of a Miniature FM Transmitter ⇲

It consists of a simple oscillator with silicon planar RF PNP transistor, and a Varicap Diode is used instead of the conventional tuning capacitor.

You will see the full project details here → https://danyk.cz/sten3_en.html . Have fun!

Bone Conduction Transducer

This post about bone conduction transducers will help you to design and fabricate bone conduction speakers and headphones yourself.

Alright! The general idea was to get a sound signal input, amplify it, put it out through a bone conduction transducer.

Would it be really possible to listen sounds not through your ears but through your skull? Let us see...

A bone conduction transducer can generate sound through direct vibration of the bones in the head.

Also, it is capable of converting surfaces into speakers via vibration when placed on materials such as desks, walls, doors, and many other solid materials.

A quick example is the DAYTONE AUDIO's BCT-3 44 x 32mm Bone Conducting Transducer!

Such a bone conduction transducer can be pressed up against the jaw or ear bone to turn the skull into a speaker cavity.

The outcome then is a great quality audio that is coming from within your head that nobody else can hear. You can also press it against your elbow bone and stick a finger in your ear to hear the audio transmitted through your arm.

Note that this odd loudspeaker does not have a moving cone like traditional loudspeakers, instead, a small metal rod is wrapped with the voice coil.

So, when current is pulsed through the coil, the magnetic field causes a piece of metal to expand and contract (if pressed against a flat surface or cavity it turns it into a loudspeaker).

As Wikipedia says, Bone conduction is the conduction of sound to the inner ear primarily through the bones of the skull, allowing the hearer to perceive audio content even if the ear canal is blocked.

Bone conduction transmission occurs constantly as sound waves vibrate bone, specifically the bones in the skull, although it is hard for the average individual to distinguish sound being conveyed through the bone as opposed to the sound being conveyed through the air via the ear canal.

Intentional transmission of sound through bone can be used with individuals with normal hearing - as with bone-conduction headphones - or as a treatment option for certain types of hearing impairment.

Bones are generally more effective at transmitting lower-frequency sounds compared to higher-frequency sounds...

Well, you can simply connect a bone conduction transducer as you would any other loudspeaker, because it works great with most audio amplifier circuits.

However, note at this point that the sound pressure level (SPL) and frequency response of a bone conduction transducer will vary with what you use as the transducer surface.

Frankly, this intro post contains only a scratch of the details We are about to demo a practical bone conduction audio project a later writeup.

Spoiler »

Now we are doing a little project to make a bone conduction transducer to use as a 'privacy' earphone.

First we started with an existing LM386 audio amplifier circuit design.

The LM386 is a venerable power amplifier IC designed for use in low voltage consumer applications.

But after achieving to create the bone conduction effect successfully, we have decided to design our own monoaural earphone audio amplifier circuit (still in progress).

Keep the questions coming...

Audio Line Out to Mic Input Adapter

 In this post we'll see how to build a simple audio line out to microphone input adapter.

As you already noticed, portable audio systems, computers and smartphones all seem to be happy to accept a microphone input (MIC IN) for adding audio or voice-overs, but very few have a line input (LINE IN) port to overlay music or any other relatively strong audio signal source instead.

So, if you want to plug something there that is not a microphone, you need to use an adapter to attenuate the signal properly to match the audio signal levels.

This is because the line level signals are typically in volt (V) level and the microphone signals are in millivolt (mV) level. Learn more about Line Level ↗

Luckily, you can buy Audio Attenuator Pads for cheap for this kind of application, but it’s not hard to build one yourself.

The circuit looks like (click image to enlarge):

This little circuit is enough to interface consumer audio signals to a microphone input that is designed for two-wire electret microphone capsules.

Basically, attenuation of the adapter is determined by equation:

attenuation = 20 * log10 ( (10K + 1K) / 1K ) = 20dB

So, our passive line-to-mic adapter circuit provides around 20 dB of signal attenuation, typically enough to make things work.

This is quite accurate especially when the impedance of the microphone input where this adapter is connected has much higher impedance than the resistance of the resistor wired between signal line and ground. A typical microphone input in an audio device has an input impedance of 1.5K or higher.

Values of the components shown in the circuit works best but you can alter them if you know what you're doing. Anyway, it's better to use metal film resistors (MFR) in this circuit as they are less noisy than cheap carbon film resistors.

The 10uF/25V capacitor blocks the DC bias used in the electret microphone input to get to the line signal side, and it must be a bipolar audio capacitor (see below).

The free-form soldered circuit itself can be built to a pretty small metal box.

Note that unbalanced microphone input of an audio device is very sensitive to all noise in the system because it handles low level microphone signals, thus interference from nearby devices can make a significant impact.

Ultimately, what is Balanced and Unbalanced Audio?

In general, one of the main differences is that balanced audio has less risk for unwanted noise, while unbalanced audio can pick up humming (buzzing) sounds in certain environments.

An audio cable carrying an unbalanced signal uses two wires, that is, a signal wire  and a ground wire.

The signal wire carries the audio signal to where it needs to go while the ground wire acts as a reference point for the signal (RCA cable is an example).

The balanced audio cable has a structure similar to an unbalanced audio cable but with one addition. 

A balanced audio cable has a ground wire, but it also carries two copies of the same incoming audio signal, sometimes referred to as a hot (+)  and cold (-) signal (XLR cable is an example).

And, once the hot and cold signal get to the other end of the cable, the polarity of the cold signal is flipped, so both signals are in phase, and perfectly in sync.

If the balanced audio cable picks up noise along the way, the noise added to both of those cables is not reversed in polarity.

Therefore when the cold signal flips in polarity to match the polarity of the hot signal, the noise carried along the cold signal cancels out with noise in the hot signal.

This process is called Common Mode Rejection (CMR), with the noise being the common signal between the two.

Simply put, CMR is a process whereby a signal common to a pair of lines opposite in polarity from one another gets cancelled at its destination

On the other hand, the ground wire in an unbalanced audio cable itself behaves like an antenna as well, picking up undesirable noise along the way.

That's it. Don’t miss the upcoming posts. Stay tuned!

Tuning Fork Crystal 32.768 kHz

Why do tuning fork crystals always have a frequency of 32.768 kHz? This post answers that common question!

The frequency of a tuning fork crystal commonly found in quartz watches is always 32768 Hz or 32.768 kHz.

OK but where does the standard frequency for a tuning fork crystal of 32.768 kHz spring up?

Simply, a quartz with a natural frequency of only 1Hz would be so large, and obviously, that would be rather impractical in terms of production and application.

But watch crystals with a frequency of 32.768kHz are relatively easy to produce. Thus real time clock (RTC) applications have primarily been leaning on quartz crystals with a frequency of 32.768 kHz.

Since the 32768 Hz specific frequency can be divided down to 1 Hz (equivalent to one second in frequency) making it the base for all day, date, and timekeeping functions for any electronic device.

Since it has a power of 2^15, dividing down to a 1 Hz stable and accurate signal ideal for timekeeping is an easy task.

Today, oscillator circuits are commonly merged into integrated circuits (ICs) so that only a few more external components are needed to setup a tuning fork crystal oscillator.

Below figure shows the typical circuit of a Pierce Oscillator commonly used in digital processor designs.

In practice, the original frequency of 32768 Hz can be split using T Flip Flop or Ripple Counters. Since each T Flip Flop can halve the frequency, 15 of the series-connected T Flip Flop circuit finally delivers the exact 1 Hz output frequency.

Note at this point that a T Flip Flop (Toggle Flip Flop) toggles its output depending upon on the input. The T Flip Flop will toggle its output every time the clock signal transitions from High to Low or Low to High, hence it divides the frequency of the input clock signal by 2. 

By following the below shown idea (click image to enlarge), you can setup a quick and easy Crystal Controlled 1 Hz Time Base (Source http://www.hackersbench.com/Projects/1Hz/).

As you can see, this circuit uses a standard 32.768 kHz wrist watch crystal and a couple of commonly available ICs as the core components.

Admittedly, we have not tested this particular circuit yet. So, let us know how it turns out. See you next week!

Posistors - An Introduction

Posistors (PTC thermistors) are elements based on ceramics whose resistance rises with an increase in temperature!

Positors find use in applications such as temperature sensing, overcurrent protection, and inrush current limiting, etc.

According to a Murata documentation, Barium titanate (BaTiO), discovered in the early 1940s in Japan, the United States, and the Soviet Union, is generally 10^10 Ω・cm or more at room temperature.

When trace amounts of rare earth elements (Y, Bi, Sb, etc.) are added, the specific resistance becomes 10 ~ 10^6 Ω・cm, and the temperature characteristics of the product corresponding to the Curie point are shown in 1952 by Haayman et al. of Philips (Netherlands).

However, they did not publish literature, only applications for patents, so it became publicly known around 1954.

In 1961, Murata Manufacturing Company, Ltd. began mass production for the first time in the world and acquired the registered trademark POSISTOR.

From around 1963, industrialization progressed in European, American, and Japan companies, and it was applied as a temperature compensation, water level detection, motor overheating prevention, automatic temperature control heater, and degaussing circuit for color televisions.

Like NTC thermistors, PTC thermistors can also be used as temperature sensors by utilizing the change in resistance due to changes in temperature. 

Note at this point that the posistor has resistance-temperature characteristics that cause its resistance to exponentially increase when the part’s temperature exceeds its Curie Point, the critical temperature where the resistance value increases dramatically.

And, typically above the Curie Point, the resistance raise at rate of 15% to 60% per °C.

Coming to applications that utilize the self-heating property unique to a PTC thermistor, it can be used as a resettable fuse (to suppress abnormal current flowing through a circuit due to component failures) that returns to the original resistance value when the abnormal or overload condition is removed.

Now let's look at a PDF Datasheet ↗

Nevertheless a PTC thermistor can also be used to form a constant temperature heater plate (see below).

As an aside, a posistor, as you can see in a cathode ray tube (CRT) color television (CTV) circuit, is a clever combination of a positive temperature coefficient (PTC) resistor and another resistor-element to heat it up and keep it hot.

All color CRTs include an inbuilt degaussing coil, and the posistor activates that coil each time the system is powered up from cold.

The heater element in such a 3-pin PTC thermistor is a disk-shaped resistor across the power line and the thermistor is a disk shaped device in series with the degaussing coil (both are in clamped together to be in close contact thermally).

To sum up, posistor is positive temperature coefficient thermistor (PTC Thermistor).

Normally when the temperature increases, the resistance of a conventional negative temperature coefficient thermistor (NTC Thermistor) decreases. But, the resistance of a PTC thermistor rises sharply when its temperature exceeds a specific level. A posistor can also provide over-current protection if it is series-connected in a circuit.

Last but not the least, we are grateful to the Murata team for the invaluable free documentation on their posistors, without which coming up with this short post would never have been possible.

That's all for now. Any insights you may have will be appreciated!

CR2032 Coin Cell Battery Basics

The CR2032 coin cell battery is a flat cylindrical battery commonly used in a bunch of small electronics applications requiring little current draw over long periods of time.

You might have seen them in a wristwatch, personal security alarm, remote control or any piece of wearable electronics.

The CR2032 is easily the most popular coin cell in use. You may have noticed on the motherboard of your computer and digital video recorder. It contains a Lithium Manganese Dioxide chemistry (Li/MnO2) and provides a voltage of 3V.

The numerical part of the battery's name CR2032 comes from the width and height measurements of the battery.

That is the first two digits (20) reflect how wide the battery is in millimeters whereas the last two digits (32) use the height of the battery in10ths of millimeters.

The CR denotes the battery chemistry classification (manganese dioxide and lithium).

Simply put, CR2032 is a coin cell battery or button battery with a diameter of 20mm and a thickness of 3.2mm, hence the name.

Below you can see a datasheet snip of the Energizer CR 2032 Lithium Coin Battery.

Basically, there are three key segments inside a CR2032 battery:

• Anode, made of Li, releases lithium ions when the battery discharges.
• Cathode, that has MnO2, takes in lithium ions when discharging.
• Electrolyte (alkaline or leak-resistant organic), a conductive paste that facilitates the movement of lithium ions between Anode and Cathode.

When you connect a load across the battery terminals, it forms an electrical circuit between the Anode and Cathode, allowing lithium ions to flow. This flow of ions creates a flow of electrons, which generates an electric current that powers the load.

There is no one single reason you would opt for employing a coin-cell battery in a project, however there are a couple of safety-related things to consider (taken from a product datasheet):

• KEEP OUT OF REACH OF CHILDREN: Swallowing coin the battery may lead to serious injury or death in as little as 2 hours due to chemical burns and potential perforation of the esophagus.

• BATTERY COMPARTMENT DESIGN: To prevent children from removing batteries, battery compartments should be designed with either a tool such as screwdriver or coin is required to open battery compartment, or the battery compartment door/cover requires the application of a minimum of two independent and simultaneous movements of the securing mechanism to open by hand. Screws should remain captive with the battery door or cover.

Now see the MAXELL Coin Type CR Battery Safety Datasheet ↗

Finally, a quick note on its ISR (internal series resistance).

In principle, a battery is an ideal voltage source connected to a resistor in series. 

As you draw current (I) from a battery, the voltage drop (V) across the circuit will change according to the internal resistance (Rint) of the battery.

For most batteries, this internal resistance is pretty low but still results in a small voltage drop relative to current drawn.

The CR2032 batteries also have some internal resistance that affects how we use them.

Ideally, a battery’s internal resistance should be 0Ω to ensure maximum current flow without energy 

However, in reality, internal resistance always exists, and it affects the battery’s current-delivering capacity. 

The higher the internal resistance, the greater the energy loss, and this not only results in energy wastage but also contributes to battery degradation.

Deplorably, the internal resistance of a CR2032 battery is quite a bit higher (15Ω-20Ω) than that of a regular AA alkaline cell (0.1Ω-0.9Ω). As the battery depletes ,this resistance increases further.

Note at this point that the internal resistance of a battery does not consist of the cells alone but also includes the interconnection, fuses, wiring and protection circuits if any.

Also, it is  important to keep note that internal resistance is just one of many factors affecting the  performance and lifespan of the battery.

Certain other factors (such as capacity, temperature, and discharge cycle) also play significant roles here.

To sum up, the CR2032 coin type lithium manganese dioxide battery is a small, lightweight battery with an operating voltage of 3V and the ability to operate over a wide temperature range.

It has a wide range of applications for powering low-current devices such as various wearable devices, healthcare equipment, keyless entry systems, communication tags, beacons, data loggers and IoT sensors.

It also provides a stable operating voltage under long-term low load discharge for Memory and RTC (real time clock) backup circuits.

That's all for now. If you've got any more questions about coin cell batteries, start the conversation below.