Thermostat control of heat and cool

Discussion in 'General Electronics Chat' started by multimediavt, Jan 15, 2015.

  1. multimediavt

    Thread Starter New Member

    Jan 15, 2015
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    Ok, I am not completely inept but working on a small project and hit a snag due to how something I am hacking was originally configured. I bought a device that can do heating and cooling. It originally had a three-way toggle switch (Hot-off-Cold) that was wired in such a way to switch the polarity of a peltier cooling unit to cause it to heat or cool respectively, and be off. I want to wire in a thermostat with two spst relays to now control what the toggle did before and found out that I probably need one or two other relays in order to make it work.

    Here is the toggle switch wiring, it has six posts:

    P0 P1 P2
    P3 P4 P5

    COLD
    P0 = +12VDC
    P1 = +12VDC
    P2 = no contact
    P3 = -12VDC
    P4 = -12VDC
    p5 = no contact

    HOT
    P0 = no contact
    P1 = -12VDC
    P2 = -12VDC
    P3 = no contact
    P4 = +12VDC
    P5 = +12VDC

    Thermostat has 12VDC inputs and two spst relays for HOT and COLD actuation, and an off state where both relays are open and no circuit is complete for either.

    I have ordered a DPDT relay (8 posts; 2 NO, 2 NC, 2 COM, 2 COIL) to replace the toggle switch, but I am not sure if that alone will do the trick nor how to wire it properly with the thermostat. Can anyone help me figure this out? I can provide more info if needed. It seemed fairly simple when I started, but got a little more complicated for my comfort level.
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    Your description of your switch wiring isn't making any sense to me.

    I'm picturing that you have a ganged switch and that when the switch is to one side P0 is connected to P1, P3 is connected to P4, and P2 and P5 are disconnected. In the other side P1 is connected to P2, P4 and connected to P5, and P0 and P3 are disconnected. In the middle position, no pins are connected to any other.

    The problem is that your +12V and -12V power lines connect to a specific pair of pins and they wouldn't change when you flip the switch, yet you are indicating that they do.

    If the switch is as I described and I wanted to reverse the voltage connections to the load by flipping the switch, I would connect one side of the load to P0 and P5 while connecting the other side to P2 and P3. Are you sure that isn't how this is wired?
     
  3. wayneh

    Expert

    Sep 9, 2010
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    What the OP needs is an H-bridge to drive the peltier. Just buy one on e-bay with an adequate amperage rating.
     
  4. multimediavt

    Thread Starter New Member

    Jan 15, 2015
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    I indicated the way the toggle worked. I use the past tense as I want to replace it with a thermostatic control rather than a simple toggle. P0 and P3 of the toggle are fixed (+12VDC and -12VDC respectively) and P2 and P5 are also fixed (-12VDC and +12VDC respectively). P1 and P4 get the reversed polarity when the toggle is switched between HOT and COLD and thus make the peltier device heat or cool respectively. I want to replace the toggle with a DPDT switch connected to a thermostat with two spst switches for actuating HOT and COLD and the OFF state.

    Sorry if that wasn't clear.
     
  5. multimediavt

    Thread Starter New Member

    Jan 15, 2015
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    Will the H-bridge give me an off position as well as the two actuated, reversed polarity positions? The description I found for one isn't clear on that. I need off as well as HOT and COLD.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    Yes, H-bridges are widely used for controlling motors, for instance in robots, and they certainly want an "off" position for those applications.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    Not according to you to the way that you indicated the toggle worked. If Po is fixed at +12V DC, then how is it +12V for HOT and no contact for COLD?

    The wiring of the toggle switch is the SAME for either HOT or COLD operation (unless you disconnect and reconnect wires, which defeats the purpose of the switch).

    But I think I see how it is wired now.

    P0: +12V
    P1: Load+
    P2: -12V
    P3: -12V
    P4: Load-
    P5: +12V

    Load+ and Load- are the terminals of the Peltier and if Load+ = 12V and Load- = -12V then the Peltier cools.

    The problem you will have with a DPDT relay is that it only has two positions while you need three. You could use two DPST relays very easily.

    With an H-Bridge you can get your OFF condition pretty easily, though the easy way would involve taking both sides of the Peltier to the same potential.

    The SPST switches in the thermostat can be used, along with a resistor, to create a node that is pulled up to +12V when the SPST switch is open and pulled down to -12V when it is closed.

    So you will have the following condition on the two nodes, A (controlled by the HEAT side) and B (controlled by the COOL side):

    HEAT: A=-12V; B=+12V (the temperature is below the lower setpoint)
    OFF: A=+12V; B=+12V (the temperature is between the set points)
    COOL: A=+12V; B=-12V (the temperature is above the upper setpoint)

    When in the OFF position, both A and B are at +12V which will turn on both of the NFETs in the H-Bridge tying both sides of the Peltier to -12V, which should shut it off.

    When the HEAT side of the thermostat activates, the A node will go from +12V to -12V so you want this node connected to the PFET on the Load- side of the Peltier (and to the NFET on the other side).

    When the COOL side of the thermostat activates, the B node will go from +12V to -12V so you want this node connected to the PFET on the Load+ side of the Peltier (and to the NFET on the other side).
     
  8. multimediavt

    Thread Starter New Member

    Jan 15, 2015
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    Sorry, I didn't make the thing that had the toggle switch and there was no wiring diagram so I am flying by the seat of my pants. I have also never worked with a peltier before so this whole thing is new to me. All I know is that the inner pins (P1 and P4) reverse polarity depending on which way the toggle switch was moved, so I was looking for a solution to control that polarity swap and prevent reverse leakage while the thermostat drove the swap.

    I will see if I can get my hands on an H-bridge. The ones I saw in the 12VDC/5A category should do it, if this is the correct solution/problem.

    Thanks all.
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    But you are running the Peltier at 24VDC, not 12VDC.
     
  10. multimediavt

    Thread Starter New Member

    Jan 15, 2015
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    Geez, this has been a waste of time with you guys!

    This is the peltier unit ( I was able to get to it and get the info off it): http://www.hebeiltd.com.cn/peltier.datasheet/TES1-12704.pdf It runs on a MAX of 16v and 3.3A.

    I am ditching the H-bridge (unfortunately I can't cancel it nor return it at this point) in favor of two DPDT relays that will have the coils wired to the SPST relays on the thermostat and no contacts on the NC terminals of each; making it actuate the peltier in NO and be off when there's no current going to either DPDT.

    I am simply replacing a damn toggle switch for a peltier and not actuating a motor. The h-bridge looked like overkill and is. I gave it some thought this morning after some sleep and the DPDT relays will work perfectly for the application.

    You guys made things WAY more complex than needed. Thanks for the suggestions, but all I did was waste money.
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    If it has a MAX of 16V, then if you put +12V on one side and -12V on the other, you are running it at 24V and risk destroying it or significantly reducing its life.

    If you recall, I said, "The problem you will have with a DPDT relay is that it only has two positions while you need three. You could use two DPST relays very easily."

    I then proceeded to answer the question you asked about how to get an H-Bridge to give you the OFF condition. Don't blame US for answering YOUR questions.
     
  12. wayneh

    Expert

    Sep 9, 2010
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    Let us know how it turns out.

    Are you planning to run the TEC at full on? They're much more efficient for cooling at lower current, but of course the capacity is reduced.
     
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