Thyratron test questions

Discussion in 'Homework Help' started by xaos, Apr 12, 2012.

  1. xaos

    Thread Starter New Member

    Apr 9, 2012
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    Hi all. I could really use some help trying to decypher this 2d21 thyratron photoelectric switch schematic. I'm hoping that someone here has dealt with these before.

    I took some pics from the test and tried to recreate the schematic as best I could with the info i had (See attachments). Sorry the pics are so bad, but I was trying to not get caught taking them lol. Im not sure which lines hook up where on the 2D21, and im not sure of the ratings on R4, R5, C3, & the relay


    These are the questions asked about the schematic that I'm having trouble with:

    1. Which component allows for the influence of background illumination?
    A. R1
    B. R2
    C. R4
    D. R5
    E. C2

    2. The function of the relay in the circuit is to:
    A. act as the switch for the external circuit
    B. limit the 2D21 tube plate current
    C. Short out C3 when necessary
    D. protect the 2D21 tube from possible damage due to accidentally high voltage
    E. protect the photocell PT from very bright light sources


    3. The resistor R3 is used to:
    A. regulate the anode voltage of the thyratron
    B. regulate the cathode voltage
    C. adjust the bias on both grids
    D. adjust the bias on the first grid
    E. adjust the bias on the second grid


    I could really use any help I can get trying to work through this. I really appreciate your time.
     
  2. xaos

    Thread Starter New Member

    Apr 9, 2012
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  3. BillB3857

    Senior Member

    Feb 28, 2009
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    Have you formulated any answers based upon your classroom experience. Post them first and you will receive any help you need. We typically don't just answer home work questions without first seeing your attempt to answer.

    As a hint, the 2D21 is ta gas filled tube that behaves similar to an SCR.
     
  4. xaos

    Thread Starter New Member

    Apr 9, 2012
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    Basically what i'm trying to do is to break this schematic down so that I can understand what the purpose if the individual elements are in the circuit.

    I'm a beginner when it comes to schematics, but I understand most of the basics.

    This schematic is labeled "A simple photoelectric switch". But to me, there is nothing simple about this. I have done copious amounts of research trying to understand the elements of this particular circuit.


    Ok, so I am going to try and break down this in the most logical way that I can wrap my brain around.

    First we have the photocell. From research, I have determined that this is a variable resistor with the resistence corresponding to the amount of incoming light. This particular photocell (labled PT), according to my question instructions, must receive 50VDC to function. My main power source is a center-tapped transformer outputting 50VAC.

    Disregarding the rectification details, I have come up with this:
    [​IMG]


    If i'm not mistaken, the resistor R1 exists to raise the threshold of the photocell, so that it takes more light in order to lower the resistence and energize the circuit.


    Am I in the right ballpark here?
     
  5. BillB3857

    Senior Member

    Feb 28, 2009
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    Glad to see you back. Was starting to worry about you.:rolleyes: Based upon your answer, you are at least in the right ball game! Good.

    First, lets stay with your first posted schematic. In it, you see the transformer, with the tap being an overall reference that could be for all practical purposes, considered to be ground and would be used as a reference for all other voltage measurements.

    Is there a chance you could post the picture you took of the schematic? Obviously there shouldn't be a wire tied across the relay coil and there appear to be several other errors in the drawing but we will try to ignore them and stay with the general idea of what the circuit should do if corrected. One other obvious error is the wire that appears to be tying the cathode to the grid (the box around the symbol "R5") There seems to be an inconsistency in the use of dots to denote connections in mid-wire joints. Those really help to see the schematic in a more accurate manner.

    The upper section of the winding (50V) is supplying two different rectifiers. This is important for overall operation. Can you identify the polarity seen at the right side of R1 and the left side of R2? How do these voltages relate to the operation of the 2D21?

    Since you posted the data sheet for the 2D21, do you understand the relationship between grid voltage and plate current? If the 2D21 operates in a manner similar to an SCR, what kind of action would you expect to see in the relay?

    I know these are simply a lot more questions, but are intended to help you discover the solution to your original questions.
     
    Last edited: Apr 14, 2012
  6. xaos

    Thread Starter New Member

    Apr 9, 2012
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    Well I guess I should first establish a little background on the reasons for the questions.

    I am trying to upgrade to a better position at work and in order to do so I had to take an electronics test (a seemingly very old test that is probably form the 60s). I passed the test, but I have to pass "bettter" if that makes any sense. There are 3 stages of "pass", yellow, orange, and green. I passed yellow and I have to pass orange or higher. I have 1 more chance to take the test. I did very well on the general questions part of the test, but I struggled on this schematic part.

    The pics I posted (attachments in the first post) are the only pictures I have, since I had to take them "on the fly" so to speak, since I was trying not to get caught.

    I appologize for the poor quality of my recreated schematic, but I did the best I could with the poor quality pics I had.

    The symbol I used in my recreated schematic for the thyratron is just something I found and pasted on. The actual symbol from the real schematic is likely setup different. I am assuming that the 6.3V attaches to the heater element. The Plate (Anode) line is on top, and the cathode line is what connects back to the photocell, am I correct?

    From what I've read, no current will flow out the cathode unless the grid line is powered. How this is supposed to happen in the schematic I am at a loss. I agree that I likely made several mistakes when recreating the schematic.
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. BillB3857

    Senior Member

    Feb 28, 2009
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    Well, now that we know this isn't a homework assignment, more direct answers can be provided but helping you discover is the real goal here. Lets take a look at the questions and apply some logic.

    {Quote}
    These are the questions asked about the schematic that I'm having trouble with:

    1. Which component allows for the influence of background illumination?
    A. R1
    B. R2
    C. R4
    D. R5
    E. C2
    Question 1. What can you say about background illumination. Will it always be the same, if not, how can you adjust for it?
    2. The function of the relay in the circuit is to:
    A. act as the switch for the external circuit
    B. limit the 2D21 tube plate current
    C. Short out C3 when necessary
    D. protect the 2D21 tube from possible damage due to accidentally high voltage
    E. protect the photocell PT from very bright light sources
    Question 2. What is the purpose of any relay?

    3. The resistor R3 is used to:
    A. regulate the anode voltage of the thyratron
    B. regulate the cathode voltage
    C. adjust the bias on both grids
    D. adjust the bias on the first grid
    E. adjust the bias on the second grid
    Question 3. What is the polarity of the voltage going to that tube element from R3. What is the importance of that polarity?

    Do these questions about the questions help? R3 seems to be in question since the original photo isn't really that clear on that component ID but I would say the potentiometer is really R4 after a careful look at the picture, and R3 is the resistor tied to the wiper.

     
  9. xaos

    Thread Starter New Member

    Apr 9, 2012
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    This test question struck me as a bit vague. I can only assume what they meant by "background illumination" is low level ambient light that exists based on random factors such as reflections or seeping sunlight or a light turned on in an adjacent area. So based on this, I would guess that the circuit design would somehow want to make the photocell less sensitive. Since the photocell itself is a type of variable resistor, the only way i can see to do this is to add extra resistence to the circuit to compensate.

    Normally I would say the purpose of a relay is to control an external circuit. But, there do exist overload relays. Due to how fuzzy the only picture I have is of the relay, and the fact that it seems to have a resistence value (looks like 7600 Ohms), I am second guessing myself. It appears that the relay is the first thing energized on the 2nd half-cycle, but it will also be energized in the other direction in the 1st half-cycle due to capacitor C3.

    Yeah... this symbology seems to be a bit outdated. I was wondering myself which resistor was actually the potentiometer. For this question I don't even have a good guess, since I am still having trouble trying to figure out what the purpose of the tyratron is in this schematic. If the purpose of 2d21 is rectification, why? With a center-tap TF, all you need is 2 diodes as here:
    http://en.wikipedia.org/wiki/File:Fullwave.rectifier.en.svg
     
    Last edited: Apr 15, 2012
  10. BillB3857

    Senior Member

    Feb 28, 2009
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    In my earlier posts, I mentioned that the 2D21 functions in a similar manner as an SCR. Are you familiar with SCRs?

    You say you are applying for a different position at work. What do you do now and what position are you wanting to move into, if I can be so curious?
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
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    Re-reading your last post..... The photocell does NOT connect to the cathode.. Look one element above the cathode.
     
  12. xaos

    Thread Starter New Member

    Apr 9, 2012
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    Silicon-controlled rectifier is a latching switch which conducts continuously once gate voltage has been applied and continues to conduct even when gate voltage is removed.

    I wasn't familiar with them, but did a bit of research once you mentioned them.

    And as an answer to your earlier question, I am moving from technician (which is basic mechanical, electrical) to show control (which is more electronic specialized).

    This test has absolutely no bearing on what the job entails. We don't deal with anything like what is on the test. I am already doing the job and can handle it very well. I just don't hold the actual position yet.

    We deal with 3-phase motors and drives, PLCs... Mostly computer controlled industrial electronics. We know what goes in, what is supposed to come out, and if it doesn't come out, we replace the component. The only real troubleshooting we do is on motors.
     
    Last edited: Apr 16, 2012
  13. BillB3857

    Senior Member

    Feb 28, 2009
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    In my past life before retirement, one of my job responsibilities was to screen potential maintenance electricians for a major aircraft manufacturer. Our plant had 4 general areas of focus for our electricians. Construction, Refrigeration, Electronics and General maintenance. Our interview for the electronics type applicant started with Ohms Law, series and parallel resistance and capacitance, transistors, digital logic, relay logic, component schematic identification, and a whole bunch of other things. One question was to have the applicant draw a schematic of a simple one transistor amplifier with AC coupling on the input and output that would provide 180 degree inversion from input to output. Component values weren't required, but the schematic was expected to, at minimum show a means of providing a bias on the base and to explain why that was important. Each applicant was quizzed to the point of not knowing something just to see how he/she would respond to being stuck. We didn't use a written test because if an applicant didn't understand a question, it would be re-worded until they understood what was being asked or if the applicant had a brain freeze, we could help put them back on track. Our electronic type electricians not only found a bad circuit board and replace it. They repaired the bad board, too. We had a lot of folks apply that had worked other places that just swapped boards. We usually didn't hire them because they had no reasoning power. They would have just swapped boards until the system worked. Our people knew why they were swapping a board before it was ever pulled.

    A typical successful applicant for electronics had either military training in electronics or had completed at least a 2 year trade school focusing on electronics. Our standards were high and I suspect your employer also has high standards. Most business don't teach the basics any more. Any training will usually be for specific equipment and will depend upon a solid understanding of basics in order to understand what is being taught about the specific system.

    Good job digging out the information on the SCR. Going back to your circuit, do you have any idea what that additional piece of information adds and what such a circuit could be used for?
     
  14. xaos

    Thread Starter New Member

    Apr 9, 2012
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    Well I was doing a bit of thinking on typical uses of a photocell. They are usually used to turn a light on when it is dark, such as the auto-headlight systems in many new cars, or as a flame detector for heating systems, natural gas, etc.

    Now the only thing I can think of off the top of my head would be some kind of warning system. Say for instance, if the photocell was somehow routed to control the gate current, the SCR/thyristor/thyratron could send power to an alarm that stays on continuously until reset (by removing the anode current).
     
  15. BillB3857

    Senior Member

    Feb 28, 2009
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    Very good. With these thoughts in mind, can you figure out how the circuit may work? Remember to look at the Grid/plate charts in the 2D21 data sheet.

    In industrial electronics, many times you will not have full "theory of operation" to help you. You will be very fortunate to just have a schematic or maybe only an interconnect diagram showing how the unit you are working on connects to other components. Much of the work requires simply going back to basics and figuring it out from there.
     
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