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12VDC to 12VAC

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  • 12VDC to 12VAC

    I have a RS232 relay board that I want to install with my carputer so I can control 8 relays with the touchscreen for things like my garage remote, spot lights, etc. According to the manual, it says that it requires 12VAC to run. However, I have used it connected straight to the car battery and with 12VDC from a hacked ATX power supply.

    Should/could I run it straight from the ‘dirty’ 12VDC from the cars battery or should I be using a converter of some sorts to supply the 12VAC it requires? Would such a converter be expensive or could I make it using normal electronic components and a bit of breadboard?

  • #2
    AC ???? - I think there is a mistake - DC must be.

    You can build simplest lm7812 voltage regulator ( follow ic's datasheet) and enjoy it - there must be DC I think!
    Good luck .
    400Watt CarPC power supply for powerfull systems- thaks for Beta testers- it was perfect.

    Remember - AMD Athlon XP (socket A) powers from 5v rail in most times !

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    • #3
      model #

      Originally posted by airborne_cookie View Post
      I have a RS232 relay board that I want to install with my carputer so I can control 8 relays with the touchscreen for things like my garage remote, spot lights, etc. According to the manual, it says that it requires 12VAC to run. However, I have used it connected straight to the car battery and with 12VDC from a hacked ATX power supply.

      Should/could I run it straight from the ‘dirty’ 12VDC from the cars battery or should I be using a converter of some sorts to supply the 12VAC it requires? Would such a converter be expensive or could I make it using normal electronic components and a bit of breadboard?
      If you tell us more about the relay board we could probably figure out what it really needs. Manufacturer, model, etc.???
      ~Jimmy

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      • #4
        No, sadly it is AC.

        Here it is:

        http://www.velleman.de/ot/en/product/view/?id=351282

        I bought it a few years back.

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        • #5
          DC is OK

          Originally posted by airborne_cookie View Post
          No, sadly it is AC.

          Here it is:

          http://www.velleman.de/ot/en/product/view/?id=351282

          I bought it a few years back.
          Sadly is not true. Be happy. You can use 12V car power because the board will pass it through a bridge rectifier and then on to a 5V regulator. This is from the assembly manual which also lists the parts used.
          Attached Files
          ~Jimmy

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          • #6
            If you look at the schematic you'll see that it full wave rectifies and smoothes the 12vac to produce dc. You can feed it dc if you bypass that.

            Edit : too slow.
            Old Systems retired due to new car
            New system at design/prototype stage on BeagleBoard.

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            • #7
              Originally posted by Rob Withey View Post
              If you look at the schematic you'll see that it full wave rectifies and smoothes the 12vac to produce dc. You can feed it dc if you bypass that.
              No need to bypass. DC fed into a bridge rectifier will simply use two of the four diodes all of the time since the polarity never reverses. Just feed 12V in and the on-board regulator will work.
              ~Jimmy

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              • #8
                Awesome. Thanks guys. When I had a closer look and saw the diodes I thought that would be the case but wasn't certain.

                Would the fact that the 12VDC that comes from the battery isn't 'clean' (ie spikes and dips) cause a problem?

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                • #9
                  Originally posted by JimmyFitz View Post
                  No need to bypass. DC fed into a bridge rectifier will simply use two of the four diodes all of the time since the polarity never reverses. Just feed 12V in and the on-board regulator will work.
                  True, but you get 2 diodes worth of voltage drop and the ground will be different.
                  Much cleaner to bypass it.
                  Old Systems retired due to new car
                  New system at design/prototype stage on BeagleBoard.

                  Comment


                  • #10
                    Originally posted by airborne_cookie View Post
                    Would the fact that the 12VDC that comes from the battery isn't 'clean' (ie spikes and dips) cause a problem?
                    12v doesn't always come from the battery. You'll get up to 14v out of the battery when the alternator is charging it.
                    Old Systems retired due to new car
                    New system at design/prototype stage on BeagleBoard.

                    Comment


                    • #11
                      Originally posted by Rob Withey View Post
                      12v doesn't always come from the battery. You'll get up to 14v out of the battery when the alternator is charging it.
                      On the flip side of this, 12v ac, when full wave bridge rectified and smoothed will give you about 1.4 times that voltage (this is because ac voltage is specified using rms).

                      So the 12v ac input turns into nearly 15v (taking 1.4v off the ac figure for the two diode drops) : 1.4 * (12 - (0.7 * 2)) = 14.84.

                      Given that this is smoothed with a 1000uF electrolytic, only used to drive the relays, and there is a dropper resistor in parallel with the indicator LEDs on the relay circuit I would feel quite happy putting unregulated car voltage into this circuit.
                      Old Systems retired due to new car
                      New system at design/prototype stage on BeagleBoard.

                      Comment

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