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Unexplained changes in circuit output

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Andrew Leigh, Sep 28, 2009.

  1. Andrew Leigh

    Andrew Leigh Member

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    Hi,

    I have searched for a the right 393 but have got nowhere??? I can't seem to find the LM393A symbol file?

    Cheers
    Andrew

    Attached is the .asc
     

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  2. Andrew Leigh

    Andrew Leigh Member

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    OK,

    I got it, don't know how but I think it has something to do with when or how I refresh the model. Will carry on playing for a while and substituting differing R9 values.

    Thanks
    Andrew
     
  3. Andrew Leigh

    Andrew Leigh Member

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    Hi Eric,

    this puppy is frying my brain. I have;

    - Altered the resistances at the pots and brought them to the levels for -8°C (the evaporator temp where the thermistor is located. I increased the two 5V setting in the Thermistor model to 6.5V the OFF temp for 0°C. Is this correct?

    - Inserted the Triac as per the MOC data sheet but could not find the identical one. Note how current continues to flow in the OFF period? When increasing the value of R15 from the data sheets 39R to 100k I can get the current to stop flowing on the OFF period, is this right?

    - Have altered the heater resistance to 275R (the heater is 175W). The heater should draw 0.8A, the circuit is drawing about 1.15A, is the resy going into the circuitry?

    I hope I am getting somewhere, I need some help please!

    Cheers
    Andrew
     

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  4. dave

    Dave New Member

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  5. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Where did you get the Triac model from... I am having problems with it.??
     
  6. Andrew Leigh

    Andrew Leigh Member

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  7. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Andrew,
    Look at this simplified circuit showing only the power side.

    With ref to the 'high' currents you noted, these are peak currents.!

    To see the RMS current value window, press and hold down the kbd Ctrl key and left click on the Heater current label at the top of the plot.
     

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  8. Andrew Leigh

    Andrew Leigh Member

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    Hi Eric,

    thanks for that, I can't begin to tell you how much I value the time you have spent with me on this project.

    Should have realised it was peak as the inverter is rated at 315V. Furthermore that the plots are the traces from a "scope". Been having a bunch of fun checking the interaction of the different components.

    1.) The file I sent you had one error. The value of C4 I entered as 0.01 ...... it would appear as if the model does not understand 0.01 and requires at least a "u" behind for the value to register in the model. With the 0.01uF value correctly entered the file I sent you appears to operate correctly. Sorry about that ommision that is part of my Spice learning curve. Would you please confirm this fact on your side?

    2.) Inadvetently, through playingwith values, I came up with a trace that I am curious about. On the model I sent you last night make R15=680R and the C4=16uF. When running the simulation, a current of about .22A r.m.s. flows during the OFF period. Surely this cannot be as the Opto device is OFF. If this is correct would this not be good news as then in the OFF stage there remains some residual heating not allowing the element to get cold, improving the response time on the next heating cycle.

    Cheers
    Andrew

    Off Topic, when you guys fly out, this weekend? The Rand is going to kill you it is so strong at the minute.
     

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  9. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    The 16uF in the suppression circuit is a no,no...:)

    The residual current is due to the capacitive reactance of the cap at 50Hz.

    So Xcap = 1/[2*pi*50*16*10^-6] = 200R

    So I= V/ ([R15] + Xcap) = 220/[680+200] = 0.244A.!!!

    I would use a 100nF max.
     
    Last edited: Oct 26, 2009
  10. Andrew Leigh

    Andrew Leigh Member

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    Hi,

    excellent, would this be a good idea to maintain this residual current?

    Andrew

    At 100n little happens
     
    Last edited: Oct 26, 2009
  11. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    I would not recommend it.

    Consider this:
    thru the 680R and 16uF you have 0.244A flowing.

    So the heat dissipation in the 680R is W= I^2 * R = 0.06 * 680 = ~40Watts of wasted power!!!

    EDIT:
    Another important point is that 16uF will have to be a mains rated non polarised capacitor.
     
    Last edited: Oct 26, 2009
  12. Andrew Leigh

    Andrew Leigh Member

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    Ok,

    so we are in the finishing straight ........

    Must I bring about the changes you recommended this morning or keep with the original circuit now that we have the 0.01 cap issue sorted out. In hindsight will be reverting to your original suggestion to power the unit off 220V from the inverter so I will need to bring about changes to the board in any event.

    I am extremely excited about bringing this project to a conclusion. If it is as successful as I believe it will be then I would like to post it in the projects section.

    Cheers
    Andrew
     
  13. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Is it possible to fit a 2nd thermistor in contact with the heater tank.?
    We could use the other half of the LM393 comp to provide some feedback to the triac circuit in order to control the tank tempr.?

    As you can imagine I have some chores to do before my trip, so I will be cutting back on the Forum for a while.:)
     
  14. Andrew Leigh

    Andrew Leigh Member

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    Hi Eric,

    Will this be significantly more design work for you? If the design is not finalised before you leave for S.A. I am unable to carry on. The idea sounds great as the accuracy will improve.

    You have plenty to do so really I am in your hands as I don't have the skill. If you are OK with it i'm in.

    Am I correct in stating that the circuit as it currently set works by pulsing the heater in very short bursts of about 100ms on and 60ms off. This is because we have set the time between any voltage change at the thermistor at intervals of 0.1 sec. In reality this time frame is probably too short for any response from the thermistor so the pulses would be a little less frequent?

    Cheers
    Andrew
     
  15. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    As I have said, the only change in this latest version is that the relay was changed to a ZCD triac in order to eliminate the relay clicks which you complained of and also allow 'ac' mains control without too much EMI.
    Please refer to my previous posts regarding the changes.
     
    Last edited: Oct 26, 2009
  16. Andrew Leigh

    Andrew Leigh Member

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    Hi Eric,

    have an awesome holiday. Soak up the sun, swim in the sea, drink copious quantities of of favourite beverage and most of all enjoy the country I hold dear.

    If we don't speak again before you leave then I wish you safe travels and may you have a wonderful X-Mass and great New Year.

    So proportional control is the New Years project, you're on. Will complete this one in the interum period and will report back.

    Many thanks once again.

    Regards
    Andrew

    PS: It is not that I don't read your posts it is that I don't always fully comprehend. Electronics is so far out of my aptitude comfort zone it's simply not funny. I am enjoying it though.
     
  17. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Andrew.
    Thanks for the comments, SA is my second home.

    You have all the Sim tools to check thru most of your ideas and projects, enjoy.:)
     

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