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Need someone to check my circuit

Discussion in 'General Electronics Discussion' started by C-41, Aug 12, 2016.

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  1. C-41

    C-41

    5
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    Jun 25, 2016
    Hi,

    I'm working on a small project : building a controled water bath to process my own photographic films.

    I will use a PID to control the T°C (with auto-tune), a CAMCO 1500W 120V water heater, an aquarium pump (even T°C) and a PT-100 probe.

    Can someone take a look at my circuit to be shure that everything will be OK? Polarity, etc.

    Should I use a heat seak for the SSR relay?
    Should I use a specific relay of the ones that comes with PID controllers (eBay) should be OK?
    What wire gage should I use for 120V? And DC coming out of the PID?

    Sorry for all those questions, but I am no pro when it comes to electrical circuit.

    Thanks for your time.

    R.
     

    Attached Files:

  2. Sunnysky

    Sunnysky

    489
    128
    Jul 15, 2016
    Your PID is an ON OFF control? or triac phase control?

    +- AC symbols are incorrect. It should be switching LINE and not NEUTRAL.

    Normally you just need a comparator , thermistor and pot with a Vref to drive a relay, since heating time is slow. If you use PID control then D gain must anticipate rate of change or rise in temp to shutoff before set point, for good stability.,But this may be overkill.

    e.g. waterbeds just had a thermistor and triac control with a pot and cap
     
  3. Minder

    Minder

    3,149
    680
    Apr 24, 2015
    It is always best to use a heat sink of some kind, a square aluminum plate is good.
    14g should be suitable and easily obtainable.
    Is any of it DC? you usually require low voltage dc to switch a SSR.
    M.
     
  4. Sunnysky

    Sunnysky

    489
    128
    Jul 15, 2016
    I would use home thermostat with thermistor removed on shielded twisted pair and insert in water and use contact closure to drive a relay for 800W water bed heater.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I have SSRs switching 5A and they do not need a heat sink. However, at 10A I probably would. It also depends on the size of the SSR. A physically larger device has greater surface area to transfer heat to the ambient environment. The datasheet would give you all the information you need, but in its absence, try switching the load on for a few seconds before disconnecting the power and feeling the SSR. If it isn't more than slightly warm, try longer and longer periods (double the period each time) until you find out that it either gets hot or remains just barely warm after several minutes.

    I have mine mounted on 100mm x 65mm x 3mm aluminum plate with a ribbed rear surface, which is almost certainly overkill in my case
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    All mains wiring should be done with wire of a similar gauge (same or thicker) as that used in 10A rated cable. Use switches and cable rated at higher currents if practicable.

    If your power circuits are rated at 20A, then it would not be overkill to use 20A wiring (and heat sink for a 20A load) just in case someone connects a huge load sometime in the future. If the writing cannot support the full current available on the circuit then I would also write in an appropriate fuse (frankly I would have a fuse anyway)
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The power coming out of your pid? I assume you mean the low voltage control signals? Just use hookup wire as this is low current stuff.

    Btw the thickness of the wire is determined by the current (not the voltage). The thickness of the insulation is determined by the voltage.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Oh, and your mains can kill you (and it will hurt the entire time you're dying).

    If you value your own life and that of others, get someone competent to work with you, or at the very least to check your work before you apply power.
     
  9. Sunnysky

    Sunnysky

    489
    128
    Jul 15, 2016
    Home thermostats don't use PID since the time constant is so long, just threshold detection with 0.5'C or 1'F hysteresis.
     
  10. Sunnysky

    Sunnysky

    489
    128
    Jul 15, 2016
    I suspect 1500 W is overkill and will boil water on the surface of the element, which is why I Suggested an 800W insulated Rubber heater for the bottom like those used in water beds.
     
  11. C-41

    C-41

    5
    0
    Jun 25, 2016
    Wow... thanks for all your imputs!

    To give you a better idea of my project : this is my inspiration.


    So...

    I dont know much about PID Controllers. Someone told me that about any PID controller with "Auto-tune" should do the trick (like this one). The Auto-tune should maintain a very good T°C precision (along with a water pump to get even temperature across the water bath).

    1. +- AC symbols are incorrect. It should be switching LINE and not NEUTRAL.

    So, the red wire (+) goes in the on/off switch and to the relay. The black wire (or white - ) goes in the PID and to the outlet. But what about the ground (or neutral; green wire)?

    2. Heat sink
    Go for a heat sink. Some PIDs are sold with a heat sink and a relay.

    3. Is any of it DC? you usually require low voltage dc to switch a SSR
    Good question! I dont know. Here are the specs of one PID model with relay:
    - power supply:AC110-240V
    - relay output: contact capacity 250V AC 3A (resistive load)
    - Insulation resistance: >50M ohm(500V DC)
    - Insulation resistance: 1500V AC/min

    From info from the web (as good as it can be), I concluded that the current from the PID to the relay was DC. But maybe I'm wrong here.

    4. Water bed heater instead of heater element + 1500w overkill

    The idea is good, but I need 38°C +/- 0,5°C for the C-41 color developer. Is 1500W overkill considering the fact that the heating element will be controlled by the PID - Auto-tune (aka not working at 100%)?

    Would a portable teacup heater (like this) be good enough to maintain 38°C some 4 or 5 gallons of water?

    5. Wiring
    What I had in mind was "house wiring" : 10/2 with naked wire for ground.

    6. Fuse
    In my diagram, I have a bathroom outlet (Ground Fault Circuit Interrupter) : it is along the main power line. Is that enough? If not, where should go the fuse in my diagram? With a good plan, I can build about anything. But the plan needs to be 100%.

    7. Oh, and your mains can kill you (and it will hurt the entire time you're dying).

    That's why I have a Ground Fault Circuit Interrupter along the main power. Is it enough? And yes, I will ask a professional to check my circuit.

    8. hysteresis...
    I have tried to understand this concept, and then, I the «Auto-tune» mode came along... :)

    All I need is a good plan. With a good plan I can build about anything. And yes, I will ask a professional to check everything before I power the whole thing! I value my life!

    I will post a new version of my plan. Thanks to all ! Your help is much appreciated.

    R.

    And you end up learning everyday! ;)
     
  12. Sunnysky

    Sunnysky

    489
    128
    Jul 15, 2016
    IF the tank is thermally insulated, then it is simple on/off with setpoint temp.

    Otherwise, you end up with stability issues needing complex PID gain adjustments.

    What temp? and what tolerance?
     
  13. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    The power required to maintain a certain temperature will depend on the heat loss and not the volume of the bath. The time to reach the temperature required will depend on the mass which is heated.
    PID will not be needed if you have a time constant of a few minutes.

    Thermally insulate the tank.
    I would use a low voltage heater for safety. You can get 12V heaters to make hot drinks in a car.
     
  14. Minder

    Minder

    3,149
    680
    Apr 24, 2015
    A earth ground should be used where the equipment is not classed as double-insulated, in that case then a earth ground should be connected to any metallic case of the equipment, if should NOT contact neutral at any point.
    M.
     
  15. Sunnysky

    Sunnysky

    489
    128
    Jul 15, 2016
    That's why I suggested CSA/UL approved rubber mat heater for water beds.
     
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