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Thread: [B]How the SSR, the PID and the thermocouple work together.[/B]

  1. #1
    Boolit Master bbogue1's Avatar
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    [B]How the SSR, the PID and the thermocouple work together.[/B]

    How the SSR, the PID and the thermocouple work together.
    The wall connected AC powers the PID. Utilizing the same connections, (high voltage as a terminal connection pass through) power to the SSR. When the PID senses an error in temp it provides a DC triggering signal to the low voltage side of the SSR. The PID is generally powered by AC line voltage, but will in most configurations, will only output a low voltage DC triggering signal. When the triggering signal is received the SSR closes the “switch” on the low voltage side which in turn closes the “switch” on the high voltage side. When the high voltage “switch” is closed high voltage can flow to the load. The PID is monitoring the change in temperature through the thermocouple. When the thermocouple reading indicates the temperature change is in accordance with the selected temperature value setting (SV) the PID stops sending the low voltage to the SSR which opens the “switches” and stops the action. When the PID senses the temperature is in variance with the SV the process begins again.

    A tip when using SSR units: if you connect a DC control voltage to the input side and try to hook up a meter to measure continuity on the output/control side like you would with a dry contact relay, you get no continuity on the load side even though the triggering LED is lit. This has caused some people to think their SSR was bad. SSR's need an AC voltage on the load side and a feedback loop is created within the circuitry of the SSR to "switch" the load on or off. In the end it functions like a plain old analog "switch" but there is no physical switch or moving parts in a SSR. But for all appearances a SSR will function like a simple analog Single Pole Single Throw switch (like your homes light switch) with the PID triggering signal acting like the "finger" that flips the switch.

    Many thanks to dward51 of Smoking Meat Forum and Mike W1 for the clarification and the tip.

    What Do The Letters Mean? This answer is from Dave M at Google Groups:
    The character "R" in any of these positions means that it has relay contacts
    at the output.
    The character "S" in any of these positions means that it has a solid state
    relay (SSR) at the output.
    The character "T" in any of these positions means that it has a silicon
    controlled rectifier (SCR) at the output.
    The character "N" in any of these positions means that there is no output
    for that position.
    For example, the number for model TA4-RNR indicates that the first output is
    a relay; the second output is None; and the third (Aux) output is a relay.

    Here is a diagram that explains the 110V AC side:
    Click image for larger version. 

Name:	SSR at work diagram.jpeg 
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    Left side no signal from PID Right side with signal from PID
    Here is another that explains how no power passes through the SSR at ANY time:
    Attachment 204279
    Thank you to Dward51 of Smoking Meat Forum
    Last edited by bbogue1; 09-18-2017 at 08:43 PM. Reason: Added SSR diagrams

  2. #2
    Boolit Master

    Mike W1's Avatar
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    Well actually the PID doesn't "provide" power to the SSR. It just sends a signal to the SSR (DC) and the "switch" side of the SSR closes (not the right word, it's not a mechanical relay) and power flows to the unit, pot, etc. The AC is hardwired to the "switch" side of the SSR.
    Mike

    Benefactor Member NRA
    Life Member Iowa Firearms Coalition
    US Army Vet

    There are two ways to conquer and enslave a nation.
    One is by the sword. The other is by debt.”
    John Adams 1826

  3. #3
    Boolit Master bbogue1's Avatar
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    Thank you. I struggled with wording that. Yes the AC that powers the PID makes its next connection at the SSR. Thank you for enhancing the post.
    Last edited by bbogue1; 09-18-2017 at 08:45 PM.

  4. #4
    Boolit Master bbogue1's Avatar
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    Here is the wiring diagram to hook up the least expensive PID fleabay has to offer. (Yours may be different - check the diagram on the side of the PID) From wall power black comes in then splits. One wire of split goes to PID the other goes to the SSR. White comes in from wall power in then splits. One wire of split goes to PID the other goes to the load (melter or hot plate, or whatever you want to control). Note the wiring from the PID to the SSR is DC and low voltage. Remember + on the PID goes to + on the SSR and - on the PID goes to - on the SSR.
    Attachment 204565
    Last edited by bbogue1; 09-24-2017 at 01:14 AM.

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