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Pipe Organ Power Supply: Wiring and Fusing

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  • Pipe Organ Power Supply: Wiring and Fusing

    In a recent comment on this Forum, John Kinkennon wrote, with respect to SAM drivers, "... there are potential safety issues if they are not installed in a safe (fused) manner with compatible SAMs, at the designed voltages, and with the right wire gauge."

    Colin Pykett writes, "It is prudent to include fuses in the power supply lines feeding each circuit or group of circuits in an organ so that any fault currents will be of short duration. The current rating of each fuse must be determined by the load it feeds, and this must be the responsibility of the system designer. "

    The installation guide for the combination action/SAM driver boards we are currently installing advises, " The use of fuses to protect all electrical circuits from accidental shorting and compliance with local NEC (National Electric Code) guide - lines is highly recommended. "

    I find the Canadian Electrical Code to be overwhelming in its detail and have no idea how to ferret out the relevant rules. However, I did find the following with respect to Section 16, "The scope (of this section) applies specifically to remote control, signal circuits and voltage or energy limited circuits. The premise of these requirements is that voltages less than 30 volts do not constitute a shock hazard and power levels less than 100 volt/amps do not constitute a fire hazard.

    So, as a "System Designer," where does one begin?

    1. Use the proper wire gauge for the current to be carried. Charts such as:
    https://www.powerstream.com/Wire_Size.htm
    give useful guidance. Even for short runs, inside the console, I like to stay well under the ratings in the column " Maximum amps for chassis wiring ."

    2. Connect the power supply directly to a fuse block so that the current can immediately be subdivided and fused into a series of circuits of 5 to at most most 10 amperes each. I especially like the "Deyooxi 12 Way 12V Blade Fuse Block,12 Circuit ATC/ATO Fuse Box Holder with LED Indicator Waterpoof Cover for 12V/24V Automotive Truck Boat Marine" (Amazon $22). Rated at 100 amperes per panel, it features rugged construction and 12 screw terminals each for positive and negative lines.

    3. Divide up the power needs for the circuits in the organ and select the proper fuse size. For example, here is the description of the power needs for the combination action/SAM driver board for one division of the organ.

    " A standard regulated DC power supply between 12- 24 volts is required. Current draw with all outputs off is approximately 0.1Amps. Each output is capable of switching 0.240Amps continuously or 0.500Amps for 0.6s.
    Each board drives up to 24 stops in a single division ."

    In the organ we are currently converting to solid state, the largest division of the organ has 20 stops/couplers . Although the SAM coils are rated for 0.5 Amps, because of the short duty cycle, and the fact that automotive fuses can briefly withstand an overload, I am going to start with a fuse that assumes 0.250 Amps per SAM. Then the fuse size works out to
    20 stops x 0.25 Amps per stop = 5 Amps for each board.

    4. The stop/primary magnets driver boards we are using have a 5 Amp glass fuse on each board, but I will still power them through a 5 Amp fuse on their own circuit through the fuse block. Yes, this is double fusing, but it protects the wiring inside the console.

    5. Finally, for testing, I like to use a test wire in series with a 10 ohm, 10 watt wire-wound resistor.

    Remember, 60 volts cannot shock you because skin resistance is sufficient to prevent any current from penetrating your body. If you have felt a shock while working on the low voltage DC circuitry inside an organ, it is probably because of the reverse voltage briefly generated when the magnetic field of a coil collapses once the current flow is turned off. It tickles, and may exceed 200 volts, but is harmless (to your body, not electronic circuits).
    But 60 amperes from a power supply is not harmless. Just connect a one foot length of 26 gauge wire across the terminals, and you will see why. The wire will instantly glow red hot and melt. Use pliers to hold the wire and use safety goggles when trying this experiment. You'll develop a healthy respect for high currents.

  • #2
    BTW, I neglected to make clear that I am inviting reactions from those more experienced in these matters than I am. In particular, I sometimes question myself about the wisdom of using automotive fuses in this application. Also, is it wise to derate the SAMs, as I have done, when calculating the appropriate fuse capacity? Please share your experience so that we can all learn. And does anyone know of online information that addresses the whole issue of console and organ wiring?

    Comment


    • #3
      The USA national electric code currently has these requirements which also happen to be the most prevalent standard in the US:
      • The rectifier must not be larger than 30 volts and it must be grounded. The standard for most instruments is 12v but anywhere between 12 and 24 volts can be found.
      • The common return wire must be at least 14 AWG.
      • The chest magnet and console wiring should not be less than 26 AWG. The most common size that I've seen is 24 AWG.
      • Wiring should be grounded in groups of 6 amps or less.
      I hope this helps!

      Comment


      • Coenraads
        Coenraads commented
        Editing a comment
        Thanks John.
        These are sensible requirements and not at all onerous to implement.

    • #4
      Here is an article:
      https://organbuilding.org/post/3

      Comment


      • #5
        Originally posted by johnroper100 View Post
        That looks like the only article there, there is no post 1 or 2 and no home page or anything. Do you know anything about the site and if more will be added?
        Home Organ: VPO Home-Brewed from a former Klann pipe organ console

        Comment


        • #6
          Originally posted by lizny View Post

          That looks like the only article there, there is no post 1 or 2 and no home page or anything. Do you know anything about the site and if more will be added?
          Hi, sorry, the homepage link should work now, more will be added. https://organbuilding.org

          Comment

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