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  • How to wire in a Hobbs meter correctly.

    Hi,

    I have a Rans S6 ESD 503 with a Hobbs meter that's wired into the master switch to record engine on time. Unfortunately it actually records the master switch on time (as it would do!). This is a pain as often I will have the master switch on to test instruments/radios etc and not only to run the engine.

    Is there any other way to wire it in so that it only runs with the engine running? There is a rectifier fitted (obviously) and a charge meter too if that helps anyone.

    I have thought about putting an isolation switch in-line but that rather defeats the object.

    Cheers

  • #2
    How to wire in a Hobbs meter correctly.

    Jock,

    I have the same problem on my Skyranger Swift - I wired my Hobbs meter as per the circuit diagram into the fused live feed for the instruments and the Hobbs meter starts when the master switch is turned on. I am planning to take a spur off the live feed from the rectifier with a 2amp fuse fixed into a panel holder next to the Hobbs meter. The earth will still go to the earth bus bar.

    Regards

    John Armin

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    • #3
      How to wire in a Hobbs meter correctly.

      Why not insert a feed line via a Zener diode ?? When it charges its obviously running and the diode will switch when you establish a threshhold voltage such as 13,8 volts. The battery will obviously have a hold voltage depending on type, may need some tweaking but will work.

      Not an issue to me but if training every minute counts.

      Ciao Paul

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      • #4
        How to wire in a Hobbs meter correctly.

        Zeners dont work that way though do they. Its essentailly a diode in reverse bias so the voltage drop across it is the zener voltage. The voltage across the meter with the 12v diode in line would be 1.8v max You need a small regulator circuit with the zener between ground and the base of a switching transistor. You could use a 13v zener but it does seem a bit hit or miss. What about a coil (you could wind your own) around the ht lead? Or could you use the alternator sense wire? Either would be a much more certain way of determining that the engine was running.

        Laurie

        Paul Lintott wrote:
        Why not insert a feed line via a Zener diode ?? When it charges its obviously running and the diode will switch when you establish a threshhold voltage such as 13,8 volts. The battery will obviously have a hold voltage depending on type, may need some tweaking but will work.

        Not an issue to me but if training every minute counts.

        Ciao Paul

        Comment


        • #5
          How to wire in a Hobbs meter correctly.

          Can't you run it off the MAG AC output? Mine does... Ive never looked at the wiring or even thought about it..

          If it's just 12V operation, you just need to make a little half wave rectifier circuit using a diode, cap and possibly a zener... then power the whole hobbs unit off the mag output.

          Or take the mag output, rectify it with one diode and a cap, divide with resistors and get this to operate a relay or FET to switch the 12V on to the meter.

          I can draw you a circuit if you like...

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          • #6
            How to wire in a Hobbs meter correctly.

            Andy, good idea likewise actually never even entered my head but maybe if a demand for it would knock up a small board with a coil around a HT lead and a power supply in and a supply to the hobbs direct. Much the same as the small tiny tach operates....operates on engine run only. Busy but let me think about it...

            Cioa Paul

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            • #7
              How to wire in a Hobbs meter correctly.

              how much comes out of the 503 mag o/p? It's about 70 volts isn't it (no- that's a 912)? I forget.
              don't see why you need to have a coil around the HT lead, just recitfy the mag output and power the meter. think you just need to drop a load of volts that's all, since the AC output is relatively high from what I remember.

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              • #8
                How to wire in a Hobbs meter correctly.

                Andy, I remember reading 90 volts but may be corrected ??

                Paul

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                • #9
                  How to wire in a Hobbs meter correctly.

                  Is that 90V RMS, peak, or peak to peak?

                  And at 1,000rpm I bet it goes down to about 20VAC.

                  Also how much current does a hobbs meter need at 12V?

                  Comment


                  • #10
                    How to wire in a Hobbs meter correctly.

                    A simple way to drive the Hobbs would be to use a comparator to compare the output of oil pressure sender to a preset threshold. When the oil pressure goes above about 1 bar (i.e. engine running) then it will drive the Hobbs meter. This method only uses the 12V supply that is already available in the fuse box and the signal to the oil pressure gauge which is already behind the panel.

                    I have attached circuit details, a sample Vero board layout and silk screen (to stick to the top of the Vero board) in PDF format.

                    Kevin

                    Attached files hobbs-switch-layout-b.pdf (8.7 KB) hobbs-switch-schematic-c.pdf (25.9 KB) hobbs-switch-silk-screen-b.pdf (4 KB)

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                    • #11
                      How to wire in a Hobbs meter correctly.

                      Well done Kev, Oil pressure switch is much simpler..

                      I looked at the MAG AC output varation on a 503 and it looks to be between 16VAC (rms I assume) and 77VACrms.
                      http://www.challengers101.com/RotaxChargeSys.html
                      That makes it a bit of a PITA for using a very small zener diode solution!

                      Comment


                      • #12
                        How to wire in a Hobbs meter correctly.

                        LOL

                        The only reply I understand here is John's...

                        "I have the same problem on my Skyranger Swift - I wired my Hobbs meter as per the circuit diagram into the fused live feed for the instruments and the Hobbs meter starts when the master switch is turned on. I am planning to take a spur off the live feed from the rectifier with a 2amp fuse fixed into a panel holder next to the Hobbs meter. The earth will still go to the earth bus bar."

                        This seems to be the easiest method for me. Does my air-cooled 503 have an oil pressure swith (bearing in mind it is pre-mix?)

                        Think I might go with John's

                        Thanks all....

                        Comment


                        • #13
                          How to wire in a Hobbs meter correctly.

                          Unless the generator output is diode isolated from the main supply bus bar then the Hobbs will still run when the master is turned on. I am surprised that standard Hobbs meters don't just have an internal comparator that can be driven off the oil pressure signal or the nominal +12V battery voltage rising to the charging voltage i.e. > +13V. The latter option would be very simple, just requiring a 2 wire connection to the Hobbs. I'm sure that most modern aircraft have charging voltages > 13V even with maximum electrical load applied.

                          The circuit in my schematic attached above could easily be modified to operate using the supply only. The circuit could then be mounted to the back of the Hobbs and the existing wiring retained, nice and simple.

                          Comment


                          • #14
                            How to wire in a Hobbs meter correctly.

                            Yes, need to keep the generator output isolated from the main a/c earth.

                            Reckon though that detecting the difference between not charging and charging could end up a bit flaky, depending on how your electrical system is loaded.

                            I can see an alternative idea would be to take the mag output, and pot it down to a small voltage, then use this to switch 12V to the hobbs via an opto isolator (or small relay)... would be very simple.

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                            • #15
                              How to wire in a Hobbs meter correctly.

                              Wht not change the magneto switch for a DPDT version.
                              Connect the hour meter to one side of the switch. The hour meter will only run when that mag switch is on.
                              It could even be connected in series with the master switch so the hours will only count up when the mag switch and master switch are on.

                              Dont forget that all important Mod paperwork :yeah:

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