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Unexplained changes in circuit output

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

  1. ANTlabs

    ANTlabs New Member

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    I would check all solder points and if that doesent work, get a new thermosister.
     
  2. Andrew Leigh

    Andrew Leigh Member

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    Hi Eric,

    have been working some 18 hour days but got some time now to bug you all with more questions.

    The simulations you depicted above the circuit, a couple of questions please cause I don't think I am reading it correctly.

    The plot shows the resistance of the thermistor rising to 14.4k at this point the relay switches of, when the resistance drops to 14k then the relay come back on. So discount the lag in the heating element to get back up to speed the circuit is responding to a swing of about -0°C to +0.6°C. Would you concur with this?

    Cheers
    Andrew
     
  3. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    I'll check that out, get back to you.:)

    EDIT:
    Checking that 14K/14.4K.

    If you recheck the d/s for the thermistor you will see the tolerance range of the resistance at 10K is quite wide.
    This is one of the reasons you need set pots on the project.
    If you find that the On/Off switching band of the tempr is too sensitive, you may have to reduce the 1meg hysteresis resistor value to say 470K or 330K.
     
    Last edited: Oct 17, 2009
  4. dave

    Dave New Member

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  5. Andrew Leigh

    Andrew Leigh Member

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    Hi again,

    done some work. Produced a spreadsheet which allows me to simulate the on off voltages and thereby play with the hysteresis. It also is a much faster ways to balance the circuit resistance as all the formulae are interdependednt. I am learning a lot from this process. The spreadsheet has allowed me to see the interaction of the resistors on the workings of the circuit. I can physically see the results as I make the changes.

    Apart from the LM741 which is not suitable it is fairly difficult to balance the circuit resistance to be in the sweetspot of the temperature range required. I have had to change the one resistor (R1) from 15k to 22k as the evaporator (where the thermsitor has been located) gets down to 24k when the internal fridge temperature is 1.5°C.

    I also swopped out the feedback resistor to a 1meg which now has halved the hysteresis to about 2% (ON 6.32V OFF 6.19V) or so my calcs say.

    A couple of days ago I was reading about the new generation of compressor fridges which have a variable speed compressor. This got me thinking, would a PWM circuit not work and give better results, I need to check with a fridge technician?

    Finally with the thermostat cycling much faster now is is difficult to get the transistor to drive a SCR or suitable device rather than the relay which audibly clicks on and off. Could you point me in the right direction?

    Cheers
    Andrew
     
  6. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    If the relay contacts are switching 'ac' you could use a SSR, these normally accept a 'dc' control signal of 3V thru 28V, so it would be compatible with the existing relay driver circuit, get the correct switch sense.

    With 'dc' load switching you could use an SCR to switch ON the load and a SCR shorting transistor to switch it OFF, thats messy and would mean a circuit change.

    You could switch the 'dc' load using a suitable power FET/heatsink, this is the solution I would use.

    OK.:)
     
  7. Andrew Leigh

    Andrew Leigh Member

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    Hi Eric,

    I may be boring you as this has probably becoming a fridge rather than circuit thread but though I would let you know what I have found.

    There is a large hysteresis component in the fridge (not controller). The fridge is only cooling when the ammonia is boiling, this takes some time to achieve as the heater first needs to heat up. Anyway I think a picture tell a thousand words so prepeared a sketch to show you my reasoning.

    In my case this hysteresis is in the order of 3 - 4°C. This hysteresis is not fixed and seems to change as it gets warmer, this makes sense as the fridge will tend to lose temperature faster in the higher ambients than what it will gain cooling allowing the fridge to settle at a higher temperature ..... I think.

    I can see a solution to this, keep the ammonia about 20% below it's boiling point permanently with one heater and add a second heater for the top up. Will get onto some refigeration forums to see what they have to say.

    Thought I would share

    Cheers
    Andrew
     

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  8. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Andrew,
    Its not boring, in fact I find it interesting, especially the data feedback that you are posting.

    An obvious question would be, does the fridge have it own inbuilt tempr controller.??
    If yes, what is it set too.?

    Also I would expect it would have some type of safety controller.??

    OT: only 12 days to go for sunny SA.!!:D
     
  9. Andrew Leigh

    Andrew Leigh Member

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    Hi Eric,

    Its not boring, in fact I find it interesting, especially the data feedback that you are posting.
    Excellent, is is good to have a sounding board.

    An obvious question would be, does the fridge have it own inbuilt tempr controller.??
    Yes it does, it is the capilliary tube with type that had a hysteresis of 14.5°C. So in real terms, and based on my results (past 24 hours was 2.5 to 4.4°C and the 24 hour period prior to that was 1.7 to 4.3°C if me memory serves me correctly), this means that we have managed to reduce the hysteresis to less than 3°C. This in my book is significant progress.

    Also I would expect it would have some type of safety controller.??
    Herein lies the rub. There is no safety mechanism that I can see, what I believe they have done is to limit the element wattage. In so doing they have also increased the hysteresis.

    So have been thinking on this, one can increase the wattage of the element and this will reduce the hysteresis. I would be a little concerned on this option as they have obviously sized the element for safety. I do believe that the controller you designed will be able to control it well and the relay if there is any problem on the board will drop out.

    The second thought I had was that in crude terms a thermostat is nothing more than a coarse PWM circuit. Therefore if we were to employ PWM in our thinking this should solve all our problems. Let me explain my reasoning.

    When running on gas the flame is always on, when the thermostat kicks in the flame increases to full, so there is always heat on the ammonia and therefore the response time is much quicker, also gas is instant heat and more efficent than an element attached to the side of the tube as the contact is not as good as an engulfing gas flame. Remember I mentioned that gas is the one medium that produced the best results from a hysteresis point of view, now I fully understand why.

    Therefore in my limited understanding of PWM I believe that PWM would emulate running on gas but will be more efficient. This is exciting, when we are done I think I will do a write up and stick it on the web for others to benefit from as I can assure you I am not the only one who has experienced these frustrations. In fact most seasoned caravanners will immediately replace their absorption fridge with a compressor type on receipt of a new van.

    I did a two part feature in the local caravan magazine on fridges. I was also able to prove to CI the local manufacturer that they disregarded every fitting instruction found in the Dometic manuals which alone gave a further 3-5°C cooling. They also had ignored the safety aspect when it came to having a gas escape route. I should send you the article is is in word format and about 5Meg's you will see I have travelled a long road in the quest for perfection .... and am nearly at the end of it I hope.


    Your thoughts?

    Cheers
    Andrew

    Only 12 days to go for sunny SA.!! That excellent, will try arrange the best weather for you, time to change from a pint of bitter to a pint of lager.
     
  10. Andrew Leigh

    Andrew Leigh Member

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    Hi,

    just had a thought. I have 1A Variac at home. This weekend will connect the heater to it and dial in a lower voltage, hopefully 175V will be low enough. If the hysteresis improves dramatically this will surely be proof that PWM will be an option?

    Been looking at PWM circuits but can find no way of integrating a thermistor.

    Cheers
    Andrew
     
  11. Andrew Leigh

    Andrew Leigh Member

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    Hi,

    could something like this work, it is from Light dimmer circuits.

    The variable resistor VR1 controls the dimming, just need to find a way to get the pot substituted for the thermistor. The problem with the thermistor is that the operating range is between 20 to 26k for the the desired temperature range.

    Is this kind of circuit a good starting point?

    Cheers
    Andrew
     
  12. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Andrew,
    The circuit in the link is for 'ac' control.
    Its difficult to read, I dont understand why circuits are posted on the web using ASCII characters,
    when there are so many free circuit drawing programmes available.?

    The fact that the thermistor range is 20k to 26k isnt a problem.:)
     
    Last edited: Oct 22, 2009
  13. Andrew Leigh

    Andrew Leigh Member

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    Hi Eric,

    I did the schematic in Eagle but forgot to attach it, it is at home now.

    Can we discuss the general power issue before getting back into the circuitry. I was thinking carefully about power facilities where we camp. 95% of the time I camp on 220V. For the other 5% I have gas and a 110Ah battery in the car and a 50Ah battery in the caravan.

    I need to have a circuit to control the fridge while we are driving, this is critical. A 4 hour trip in December, without cooling, and the inside of the fridge gets up to 15°C. I had been looking at using a 12V element and control circuit.

    I also have a APC pure sine wave 24/220V 680W rms UPS. The problem is that it is 24V input, which messes with my battery plans. If it was 12V powered then life would have been easier. For this reason I built "chemelec's" inverter, the board is made and 99% populated. I put the project on hold. This is a 12V modified sine wave inverter but should be OK as the element is a resistive load.

    My new car will be fitted with a dual battery charging system and a 105Ah battery specifically to power the caravan whilst enroute.

    So how is this for a thought.

    a) Complete "chemelec's" 12 to 220V inverter and make it handle ~500W.
    b) Make the control circuitry all 220V and instead of using the 12V element only use the 220V element and drive this through the inverter.

    Doing the above allows me to run with the freezer as well and will allow for a stop over without the engine running of ~ 90 to 120 minutes. Total element wattage is 335W and assumed 30% losses.

    The question is, will the "dirty supply" from the inverter mess with the control circuit?

    Your thoughts

    Cheers
    Andrew
     
  14. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Andrew,
    I guess from this the 'control' circuit will be the therm and comps, OK.

    Are you planning to use the 12Vdc direct from the battery to power the control circuit or from a simple 230Vac to 12Vdc 250mA converter.?
     
  15. Andrew Leigh

    Andrew Leigh Member

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    Hi Eric,

    I can do either to power the circuit, from the battery makes the circuit a little easier?

    I guess from this the 'control' circuit will be the therm and comps, OK.
    No too sure I follow what you mean?

    Cheers
    Andrew
     
  16. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    I would go for a 220Vac to 12Vdc 250mA, driven from the step up invertor.
    The reason for that suggestion is that when the 'system' is running from the van site mains outlet, no secondary battery is required.

    You mentioned the word 'control' unit.
    I was just confirming that we are talking about the same unit.;)
     
  17. Andrew Leigh

    Andrew Leigh Member

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    Hi Eric,

    been out the office for a while. Thanks for all the help once again. OK let me clear my thoughts now, we have been through a few iterations and thought processes and before I get totally confused a few questions please.

    a) In order to reduce the element hysteresis do you agree that we employ some form of PWM? If I understand it correctly, no matter how good the comparitor is we will never get more accurate unless we manage to remove the inherent hysteresis in the element. Or am I missing the boat here?

    b) If yes to a) then would that ASCI circuit I posted be acceptable as a starting point?

    c) If yes to b) then you mentioned that the value range of the thermistor would not be a problem. Just as I was getting my head around comparitors a new technology LOL.

    Just to get some clairity and direction on the next step. Gotta get as much info before you fly out.

    Cheers
    Andrew
     
  18. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    The PWM approach, without some form of zero crossing detection sync, would mean that the triac will be firing at any point on the sine wave cycle.
    This could be a problem for the 12Vdc to 220Vac invertor.???

    You can still get proportional control of the heating element by using a ZCD triac module.
    This means the triac would always trigger at zero crossing.
    By applying the triac control voltage over complete 'full' cycle in an On/Off way using a comparator controlled mark/space ratio the amount of heat could be controlled.
    Look at the MOC3081 and the higher current Sharp S102S02 ZCD triacs.
     
    Last edited: Oct 22, 2009
  19. Andrew Leigh

    Andrew Leigh Member

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    Hi Eric,

    So we still get to use the comparitor but instead of the LM393A driving the transistor and in turn the Relay we now effectively insert the MOC3081 and the S102S02 ZCD triacs in place of the tranny and relay? The reason for zero crossing is to avoid nasty transient voltages?

    It is so frustrating not understanding electronics, I run a business with three factories, on different sites, emplying 240 people and I swear that is 100X easier than trying to understand electronics:eek:. My aptitude is mechanical.

    If my assumption above is correct then will have a go at the new schematic.

    I have downloaded PSpice in the hope that it will be of some assistance, is it easy to use? (for me that is)

    Thanks again
    Andrew
    Andrew
     
  20. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Andrew,
    Look at this intermediate circuit step, from Relay to Triac [ZCD, same operation as the present circuit], when using 'ac' for the heater no more irritating clicks.:)

    The MOC can switch max 1Amp at 220Vac 50Hz, if you need more umph the Sharp S102S02 can switch 6Amps.

    If you want the ltspice.asc files for the project, just ask.

    BTW: if you get into LTS, join the Yahoo spice group for support.
     

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  21. Andrew Leigh

    Andrew Leigh Member

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    Hi

    I am not ignoring your recommendations but you will understand I have to find existing circuits to modify due to my skill level, could find no complete circuits using the MOC3081. In my hunt I have located one that hopefull could do what I want, it is also a Proportional Control Triac Triggering Circuit that appears to be Zero Crossing. http://www.electro-tech-online.com/custompdfs/2009/10/TDA1023_T_CNV_2.pdf This is more for me as hopefully I can calculate the values based on the data sheet and don't have to design ..... that is of course assuming the circuit is appropriate.

    They quote 5 to 30°C control range, surely this can this be altered, could not find anything.

    They speak of special capacitors?

    They appear to have two thermistors?

    I have included a schematic from their datasheet.

    Cheers
    Andrew
     

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