Zener: – Potential, Region and Diode

Zener potential is the reverse biased potential that results in a dramatic change in the characteristics of a diode in the reverse biased. (Vz.)

SEE THE ABOVE IMAGE.

When the voltage across the diode increases in the reverse bias the velocity of the charge carriers also increases, which are responsible for the reverse saturation current Is. This velocity of theirs is associated with:

Wk= 0.5 * M * V^2;

This is sufficient to release additional carriers through collision which is otherwise in a stable state.

• Zener Breakdown mechanism is significant only for LOWER levels of (zener potential.).
• The maximum reverse bias potential that can be applied before entering the zener breakdown voltage is called the Peak Inverse Voltage or the Peak Reverse Voltage.
  • To increase the peak inverse voltage of a diode, connect another diode with same characteristics in series with the former one.
  • To increase the current capacity, connect them in parallel.
• At fixed temperature reverse saturation current increases with the reverse bias voltage.

Temperature Effects on Diode:

• In forward bias, the graph moves to left by 2.5mV for a one-degree centigrade rise in temperature.
• In reverse bias, the graph moves downwards as reverse saturation current of silicon diode doubles for a one-degree rise in temperature.

Zener Region:

Image.

• It is the region wherein we see a characteristic drop in an almost vertical manner at the reverse biased potential (Vz.).
• The fact that the curve drops down and away from the horizontal axis rather than up and away from the positive Vd region reveals that the current in the zener region has a direction opposite to that of forward biased.
• The slight slope of the characteristic curve is the indication of resistance in the circuit which is associated with Zener diode.

• This clearly represents that the direction of current flow in zener diode is opposite to that of a normal diode.
• Location of zener region can be controlled by the doping level.
  • And an increase in doping will increase the impurity and would decrease the zener potential.

Parameter values of Zener diode (AT 25-degree centigrade temperature):

1. Zener voltage: Vz = 10V.
2. Test Current: Izt = 12.5mA.
3. Max dynamic Impedance: Zzt = 8.5 ohms at Izt.
4. Max Knee Impedance: Zzk = 200 ohms at Izk = 0.25mA.
5. Max Reverse Current: Ir = 10uA. at Vr.
6. Test Voltage: Vr = 7.2V
7. Max Regulator Current: Izm = 32mA.
8. Temperature Coefficient (%/degree.Centigrade): +0.072.

The zener potential of a zener diode is very sensitive to the temperature of operation.

Tc = [(ΔVz  / Vz ) /  (Ti – To) ] * 100 (%/degree.Centigrade).

Zener as Voltage Regulator.

A Regulator is the combination of circuit elements in a way so as to get a fairly constant output voltage.

• Fixed Parameters:
  • Vi and Rl are fixed.
  • Determine the state of zener diode by removing it from the network and calculating the voltage across the resulting open circuit. V = Vl = (Rl * Vi)/ R + Rl; (Voltage Divider)
  • If V >= Vz then, Zener is ON.
  • If V < Vz then, Zener is OFF. Therefore, we substitute an open circuit in place of zener diode.
  • Substitute the appropriate equivalent circuit and solve for the desired unknown parameter.
  • Vl = Vz as both are in parallel connection.
  • In zener diode circuits current through it must be found out using KCL(Kirchhoff’s Current Law).
  • Power Dissipated: Pz = Vz * Iz.
  • If the zener is in ON state than the voltage across the diode is not V volts as No sooner we would turn on the network zener diode would reach Vz volts and would never reach the higher values of voltages like V volts. This is known as locking in the voltage to Vz.

• Fixed Vi, Variable Rl:
  • Due to the OffSet Value of Vz, there are limited values of Rl that can be taken (Hence, the load current is also controlled).
  • Small values of Rl will lead to low values of Vl and once the value of Vl is less than Vz zener diode turns OFF and thus enters into it’s Off state.
  • Minimum Rl can be found when zener diode is in its ON state.
  • Rl(min) = (R*Vz) / (Vi – Vz); this also gives Il(max).
  • Il(max) = (Vz) / (Rl(min));
  • Diode in ON state makes voltage across R fixed.
  • Vr = Vi – Vz;
  • Iz = Ir – Il;
  • Il(min) = Ir – Izm;
  • Rl(max) = Vz / Il(min);
• Variable Parameters:
  • Vi(min) = [ (Rl + R) * Vz ] / Rl;
  • Ir(max) = Izm + Il;
  • Vi(max) = Ir(max) * R + Vz;
  • Maximum value of Vi is limited by maximum zener current Izm in this case. (therefore, Second Equation).
  • For a fixed value of Rl, Vi most is sufficiently large to turn ON zener diode. (therefore, first equation).
  • ~Jay Mehta
    

CLAMPERS

Before starting with clampers, we must know the basic working of the diode:
 (Refer this link http://wp.me/p6vMKz-4)

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Clampers : –

• Clamping circuits, also known as dc restorers or clamped capacitors, shift an input signal by an amount defined by an independent voltage source.
• The entire input reaches the output in a clamping circuit – it just shifts the waveform so that the maximum (or minimum) value of the input is “clamped” to the independent source.
• Additional shifts can also be obtained by introducing a DC supply to the basic clamper circuit.
• In this circuits we need to take care of the time constant as the rate of charging and discharging of the capacitor will change the output waveform.
• So, we need to choose the values of resistor and capacitor according to the equation ” T=RC “. – where T-time constant, R-value of resistor and C-value of the capacitor. We must keep this value sufficiently large to ensure that the voltage across capacitor does not discharge significantly during the interval diode is not conducting.

∗*We will always start our analysis from the point the diode is forward biased because, if we start our analysis from the time interval the diode is reversed biased we can not predict the voltage across the capacitor for that time interval and hence we can not do further analysis.   And capacitor might not always be charge up to input voltage.

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 Negative Clamper : –

1.)Unbiased :

• In this case, we do not have any DC supply connected to the circuit.
• When the input waveform is a positive half cycle the diode in this case ( negative clamper) becomes forward biased and thus acts as a short circuit.
• This leads to the charging of the capacitor to a value Vm volts(which can be calculated using KVL equation).
• We get Vm=Vi(input voltage) and Vo(output voltage)=0.(as diode acts as a short circuit).
• For the half-cycle, the diode is reversed biased and hence acts as an open circuit.(polarities of capacitor does not change)
• Now we again apply KVL to the present state of the circuit and get the value of Vo. Which comes out to be Vo=-(Vm+Vi)=-2Vi.
• Hence, the whole input waveform is shifted down by Vi volts.

2.)Biased :

• Considering the first circuit in the above image.
  1. When Ei > 0; i.e. diode is forward biased;
     • We get Eo=E;
     • And applying KVL to the mesh we get,
     • Em =Ei – E.
  2. When Ei<0; i.e. diode is reversed biased;
     • We Apply KVL to the present state of the circuit and get ,
     • Eo = – Ei – Em = – Ei – Ei + E = -2Ei + E. 
• Similarly, we can deduce the waveform for the second circuit in the above image.

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Positive Clamper : –

1.)Unbiased :

• In this case, we will start our analysis by keeping the input waveform in the negative half cycle. As our diode should be forward biased when we start our analysis.
• Just like negative clamper circuit this circuit also does the same thing but the only difference being it shifts the waveform upwards.

2.)Biased :

• Considering the first circuit in the above image.
  1. When Ei < 0; i.e. the diode is forward biased;
     • We get Eo = E
     • And on applying KVL to the circuit; We get,
     •  Em = Ei + E
  2. When Ei >0; i.e the diode is reversed biased;
     • on applying KVL we get,
     • Eo = Ei + Em = Ei + Ei + E = 2Ei + E;
  3. Hence, the circuit will move the  input waveform upward by E volts.
• Similarly, we can deduce the waveform for the second circuit.

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~Jay Mehta

CLIPPERS

Before starting with clippers, we must know the basic working of the diode: (Shown in the pic Above)
(Or Refer this link http://wp.me/p6vMKz-4  or click on this link DIODE – A Basic SemiConductor Device.)

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Clippers :

• Clipping circuits (also known as limiters, amplitude selectors, or slicers), are used to remove the part of a signal that is above or below some defined reference level.
• A Half-Wave Rectifier is a Clipper circuit that basically cut off everything at the reference level of zero and let only the positive-going (or negative-going) portion of the input waveform through.
• To clip to a reference level other than zero, a dc source is put in series with the diode.
• – Depending on the direction of the diode and the polarity of the battery, the circuit will either clip the input waveform above or below the reference level.

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Series Clippers : –

1.) Unbiased clipper : –

•  In the above images, the image (b) shows negative clipper(removing the negative half cycle) and the second image (a) shows a positive clipper(removing the positive half cycle).
• As we can see clearly in the first image that the circuit is open in the negative half cycle as the diode becomes reverse biased when the negative half cycle of input waveform passes through the circuit.
• Hence, we get a clipped output waveform.

2.) Biased Clipper : –

• In the above, images we see four different biased clipping circuits. In this, circuits we are just moving the reference voltage(or threshold voltage – Em) from zero to a value of our choice (E) using a DC battery.
• In the ‘ a ‘ part of the image, initially, when a positive half cycle of input wave is passed through the diode, the diode is still negative biased due to the voltage sources connected to it further in the circuit.
• Due to this diode acts as an open switch and we do not get any output waveform until the input voltage reaches a voltage value similar to that of the applied voltage source(E).
• As soon as the input voltage exceeds its voltage value with respect to E, the diode is forward biased and the current starts flowing through the circuit giving us the output waveform. We get this waveform till the input voltage again reaches to E voltage after which the circuit does not conduct till the next cycle exceeds the voltage value of E.
• Similarly, we can deduce the waveform for other three circuits using the basic characteristic of a diode.

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Parallel Clippers : –

1.)Unbiased Clipper : –

• In the image alongside, we see two parallel clipper circuits.
• In the first circuit of the image (a), we clip the positive part of the input wave, as the diode is forward biased in the positive half cycle and hence, diode acts as a closed switch resulting a current flow through that branch of the circuit giving no output at the terminals of the resistor. In the negative half-cycle, the branch of the circuit having diode will act as an open circuit resulting in the flow of current through  the resistor thus showing us the output waveform.
• Similarly, we can deduce the waveform for the second circuit in the above image.

2.) Biased Clipper : –

•  We can get the waveforms this circuits by following the above procedures.

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~Jay Mehta

DIODE – A Basic SemiConductor Device.

Q:What do we use diodes for?

a) Protect circuits by limiting the voltage (clipping and clamping)
b) Turn AC into DC (voltage rectifier)
c) Voltage multipliers (e.g. double input voltage)
d) Non-linear mixing of two voltages (e.g. amplitude modulation)
• Symbol for Diode:

Q: What is Diode made out of?

• Semiconductors: A solid substance that has a conductivity between that of an insulator and that of most metals.Example- Silicon and Germanium.
• We then, dope this semiconductors with trivalent or pentavalent impurity to make ap-type or an n-type semiconductor respectively.
• Then we form a P-N junctions by joining n-type and p-type semiconductor materials.
• And, hence a diode is formed.
• Here’s a image of unbiased diode :

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A) Diode is Forward Biased :

When, Vanode > Vcathode. 

Characteristics of diode in Forward Biased :

• Diode conducts current strongly
• Voltage drop across diode is (almost) independent of diode current
• Effective resistance (impedance) of diode is small.

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B) Diode is Reverse Biased :

When, Vanode < Vcathode.

Characteristics of diode in Reverse Biased :

• Diode conducts current very weakly (typically < mA).
• Diode current is (almost) independent of voltage, until breakdown.
• Effective resistance (impedance) of diode is very large.

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C) Current-voltage relationship for a diode :

I = Is*(e^(eV/ kT) -1)
known as: “diode”, “rectifier”, or “Ebers-Moll” equation,
where,

• Is = reverse saturation current (typically < mA),
• k = Boltzmann’s constant,
• e = electron charge,
• T = temperature.

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Specification That Must Be Checked For A Diode :

• Reverse Saturation Current – is the current flowing through the diode due to the minority charge carriers across the junction when the diode is reversed biased.
• Peak Current and Voltage
• Transition Capacitance – when the diode is  reversed biased, the majority charge carriers move away from the diode resulting in the increase in the width of depletion region.This leads to the increase in the immobile charges, which eventually starts to act like a capacitor.
• Diffusion Capacitance – is the change in the charge ‘ Q’ due to change in applied voltage across the diode in forward biased condition.
• Reverse Recovery Time – is the time taken by the diode to change its state from conducting to non-conducting state, when the voltage across the diode is reversed. It is useful to for we need diode for fast switching action.
• Sensitivity to Temperature

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Types Of Diodes :

• Junction Diode (ordinary type)
  
  
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• Light emitting (LED)

• Photodiodes (absorbs light, gives current)
  
  
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• Schottky (high speed switch, low turn on voltage, Al. on Silicon)

• Tunnel (I vs. V slightly different than jd’s, negative resistance!)
• Veractor (junction cap. varies with voltage)
• Zener (special junction diode, use reversed biased)

~Jay Mehta

A Trek To A Fort At Karnala

~Jay Mehta

It all started with me getting an invitation from a colleague of mine to join her for a trip that she was going on.

I had just completed my semaphore year and my summer vacations had begun just before the monsoons. As, I had 45 days long vacation and absolutely nothing to do to while away my time, I decided to take up an internship program. It was indeed a wise decision. In the one month long period of my internship program, I made some really good friends and even made a best friend. It is strange how we humans get attached to other humans in such less span of time. Well, it is not wrong if one says, “it is not the period of time you have been together, it is the time when you have been together that matters.” It was this new bestie of mine who gave me an invitation for an adventurous trekking trip to Karnala.

It was not my first trek but it was definitely my first monsoon trek. No wonder I was excited about it, and the thing which made my enthusiasm boost was - I was going to meet a make new friends. Before this trek, I had been to Matheran which you wouldn’t call it trekking as it was more like walking amidst the dense woods – which had its own uniqueness. The second trek that I been to was a place named Cyprus in Mulund. This was quite like a trekking may be that was the first real trek that I had been to. No doubt doing trekking at Cyprus was a lot more fun and new friends that I had made was like icing on a cake. Irrespective of the places and the people with whom I had gone did not stop me from making good memories and lifetime moments. Well, I did have hopes that the upcoming trip that I was going to go on would be a lot better, but to the contrary …… I was hoping a way to less it had been a hell of a fun. It was a trip that killed the year-long stress that had been built up. It was the real refreshment when could ever find at almost no expense.

I had got the invitation just a day before the commencement of the adventure. It was weird as I said yes to it without giving a second thought or even bothering to invite friends of my age group. Although, I did make an attempt to invite them which actually was of no use as no one thought it wise to go in such a place during monsoons. But this time I was firm enough and I couldn’t let the invitation be ignored as it was from my bestie. Moreover, I felt as if the nature at Raigarh was calling me to feel the warm embrace of hers in the cool monsoons.

The only difficulty that I was worried about before the trip even began was – me waking up early in the morning. That was a herculean task for me. But I obviously had it sorted – I just requested my buddy to give me a call in the morning so that I wake up, alarms were also mandatory for me in such situations. You see I am not that much of a morning person. After dismissing all the four alarms that I had kept and picking up the call I finally woke up around ten minutes past six in the morning which kind of was a bit late, but then when am I not late? As the only difficulty that I was worried about was over, things were ought to be good, nothing could go wrong now or could they?

Well, I missed two trains but, the good part was I somehow was able to catch the last train that could make it to the required destination – Panvel on time. My bestie was going to catch up with me with her friends on the next stop itself, and she even had but we had to get down at Rabale station as her other friends had been left behind. With this, we had to change two other trains so as to reach Panvel. It took us more than an hour and a half to reach there whereas it actually should have taken forty-five minutes only. We were the late comers. This was nothing new for me, though.

We met rest of our trekking mates at Panvel station itself. No sooner the six of us had reached the station and paid our share of contribution of mere two hundred rupees, and then the introductions and the important announcements had begun. Due to heavy rains and moving trains we couldn’t hear each other that clearly, hence we were forced to stop the introduction session and move on towards our first stop - the breakfast point. It was just five hundred meters outside the Panvel Railway Station, where we almost got all type of snacks and beverages that usually a person would like to have. Most of us did not eat or drink any liquid to that extent as we all knew we had to walk and climb for a long time and thus to avoid any kind of digestive problems eating a bit less was a wise option. As we all had given two hundred rupees beforehand from that point on we had no need to pay a single penny for the whole trip now. Our group then moved forward to the ST Bus Depot to board a bus to Karnala Bird Sanctuary.

Photo and video sessions had already begun. We all had smiles on our face and excitement was vivid in each one of us. Once the tickets were taken and everyone had settled down, the entertainer guy who was even the part of the organizing committee started cracking random jokes on the people he knew. Once the bus had started he started chanting ‘shlokas’ we all involuntarily did the same thing and at the end, we all hailed Lord Ganesha. It is a common practice in India to do such things as it is said that it helps us keep our positive thoughts at the pick and requesting the Lord to give us a safe journey. There was not a single moment of silence. No sooner did the guy completed chanting prayers, then he removed his mobile camcorder and directed us to say our name followed by the phrase – “we are enjoying a lot and are going to have loads of fun!!”. Everyone followed his instruction and added few expressions and dialogues here and there but I tried to stand out amongst the crowd by saying, “No buddy!! It is no fun at all. All of us are damn bored. – followed by a smile and giggles”. The video came out beautifully and his comments on every one of us made it more jovial. He did bring some life with him to the trek. Well seeing him one could say that he was an enthusiastic brave soul, who could do anything and transform a dead party into a booming one. Most of us became a part of all the goofing around – ‘dialogue_bazzi and full_on_masti’, this, made the approximately fifteen-kilometer long journey seem too short. The bus almost got emptied at our stop as most of the people got down with us. I got to know a lot of people with whom I was going to travel with for the rest of the day. Indeed it was a good idea to take an introduction.

The formalities to get an entry in the Sanctuary were completed by the finance guy of the organizing committee. It took him not more than five minutes to do that, until which we took photos in different poses and videos that showed our classic entry in the gates of the nature of Karnala. We were apparently the noisiest group out there, at least till that time. The guards had to ask us to keep our voice levels low or a heavy fine would be imposed on us. Well, it was fun listening to that.  It was time. The time had come when we were actually going the start the new adventure, trekking had commenced. We all marched forward into the dense woods without looking back or worrying about the difficulties that we might face ahead. We all lived in the present moment and just had one objective – enjoy as much as we could and reach the summit to see the world with a new perspective. I was way too excited and moved forward with an exceptional pace at times. I took photos of nature and selfies were a mandatory disclosure of the trip. There were points when I had to wait for my team-mates. Most of the photos that I took were the group photos from the top as most of the times I was on the higher level. But then I slowed my pace and walked along with them as I felt that I was not been included in the group photos. –Smiles sheepishly- we did lots of stuff as we moved towards our first halt. We a did dance in rains, splashed muddy water on one another irritating those who easily got offended with that act. We did all kinds of crazy things. We all went insane in the cool warmth of nature. There was no doubt I was going to make wonderful memories of my vacation’s last Sunday. And it was just the half way to our journey.

One thing that I can’t ignore in such places is the thing that makes me wonder a lot – Are the stones and the routes strategically planned or is it the mother nature that fits everything so perfectly in such imperfect patterns?  On my way to the summit, I felt many types of textures. I felt the smooth but sticky stony rocks, I felt the wet cold surface of different leaves some of the leaves which had fallen on the ground made a slippery cushion, I felt how nice it feels when our feet sinks into the marshy and swampy regions. It felt that nature was pulling me to feel its soul. These were just some of the feelings that I experience on my way top.

We reached the first halt very quickly if we ignore all the photo sessions, a dance that we did in the woods and the jokes which made us laugh so hard that we had to stop holding our stomachs tight. We all settled down under the shade where we all stood in a circle, most of them kept their bags in the middle, after a few instructions we all started sharing our snacks. We all enjoyed the different variety of food that we were getting to eat. You do experience joys in the simplest of things. It was like India under a roof kind of feeling. We almost waited there for an hour, before we begun the last part of our journey to the zenith.

After resting and eating we all moved towards our next destination, keeping two of our colleagues behind. As, we started achieving greater heights some of the crew sat behind for their particular reasons, what so ever it may have been, but 80% of our group had been at the top. Trekking from the first halt to the zenith was not a cake walk and necessary precautions and self-balance had become the priority. People became more cautious and people from one group helped others who felt scared and needed help to get down or climb up. Nature does bring humanity with, doesn’t it?? Be it natural disasters or jovial place where people go to hang out, humanity is always there. We climbed through really narrow paths and some were too steep. But after an hour of climbing, we all were at the apex, our excitement was at its apex, happiness had reached its summit, and the shocking top view of the dense forest that we walked through was just breathtaking. Most of us stood at the edge of the cliff and felt the cool storming high-speed breeze that just refreshed you and made you feel that the hard work of climbing was worth. I closed my eyes for some moments and just took deep breaths of the air so different, of the air so cold, of the air so clean. I just sat on the edge keeping my legs inside. As I sat and enjoyed the calm nature my new friends played loud music and did a group dance worrying about nothing in this ruthless world. They seemed to have transformed to their true self. It really looked as if the child inside of them had broken all the social norms and had finally got a chance to be free. I watched them dancing and jumping around with some of their friends taking the video about the whole thing to make it a long lived moment eternal. As I was observing all that I eventually I fell into deep thoughts. Maybe not that deep!!

I saw my surroundings. It was happy. Everything up there was in complete bliss. Nobody seemed to have cared about the world outside that territory. Nobody would have had a feeling to hear the clichéd city noise that surrounded all of us. Everything seemed so perfect. There were no fights, no swearing, no gossips, nothing at all. Well, it did seem like I had found a heaven on earth. The scenery, the fog, and the eternal cool wind just added truth to the hoax. Although, saying it to be a hoax could also be a hoax but that would be relative. As I went on to think such things I made myself a promise that I would go on to such new places and discover more such heavens because let us be true to ourselves – we all are going to hell anyway, so why not make a trip via heaven. Suddenly, the music was stopped and everything got silent. That’s when I realized that the entertainer who had turned the rocky ground into a dance floor with the help of his Bluetooth speakers. Sometimes I wonder technology aren’t that bad it helps us find happiness at our finger-tips and nobody really cares if it just for few moments.
Once everyone had taken all their belongings and made sure nothing was left behind we all started descending to the base. As I had been climbing I just had one fear and that was to get down because the rains made the ground slippery and to get a grip on it was a herculean task. But to the contrary, it wasn’t that difficult and we actually reached the base lot faster than we had reached the pinnacle. It is well-said that ‘to reach the top you need to invest lot of time but you come down in a matter of seconds!’

We all ate our lunch at the cafeteria near the entrance of the premises at around five or six in the evening. After having a delicious lunch we finally bade goodbye to the amazing nature that we had seen and with heavy hearts we moved out to start our mundane life just to experience yet another adventure and find new heavens and learn the language of the world.

It was indeed a day well spent.

Meet you soon with yet another exciting blog.

~Jay Mehta