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How to wire in a Hobbs meter correctly.

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  • #16
    How to wire in a Hobbs meter correctly.

    I like this one Conrad :-) There we were with zeners, comparitors, wound coils, etc etc when a simple dpdt would do the trickk. I was even thinking in the car on my way to work this morning about putting a microphone on the engine block, or a bimetal switch on the exhaust.

    yew after larf.

    Conrad Beale ConAir Sports wrote:
    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 [IMG]/forums/emoticons/yeah.gif[/IMG]

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    • #17
      How to wire in a Hobbs meter correctly.

      Bloody engineers :-)

      I did a circuit (back of fag packet design) to run the hobbs meter directly off the MAG.. it will work ok, but the problem is that the variation in MAG output voltage between idle rpm and full rpm means that the excess energy when at full power needs to be disspated. So in this case the electronics needs to have a ~6W power rating to dissipate it all... bit silly really, rather inefficient (although so is the shunt regulator anyway!) but it would work...

      Of course, could just trigger it at a higher RPM than idle, but that kind of defeats the whole point! :-)

      sure with a bit of fettling it could be made more efficient.

      does anyone really give a toss? :rofl:

      Attached files

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      • #18
        How to wire in a Hobbs meter correctly.

        Perhaps we could have a competition for who can come up with the most ridiculous design. It would have to work and have no redundancy. Anyone remember the old Heath Robinson creations?

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        • #19
          How to wire in a Hobbs meter correctly.

          Right here you go.. I know I'm a bit of a sad bastard, but I have refined it a bit! So the Hobbs meter now runs direct off the generator output via this circuit. That way it's only on when engine is running.

          Anyone wanna try it cos I can't be bothered :lol: (mine already works fine!).. I just stuck it in the spice simulator and reckon it should work over a range of generator outputs from idle to full chat.

          That is assuming the hobbs meter takes no more than say 20mA.

          Oh, I drew the fuse on the wrong side, but can't be bothered to draw/scan it again... better off putting the fuse in line with the mag output and the circuit (i.e. before 1N4003 diode).

          Of course, I haven't tested it... only simulated it... try this at your own risk as this is merely a 10minute hack.. but I don't see any reason why it shouldn't work...

          Reckon it would cost about 3 in bits from Maplins and about 100 in paperwork, excellent... see the Bill of Materials on the bottom of the page.

          PS don't connect the hobbs -ve to the aircraft chassis.. it's a different earth!

          (whoops forgot transistor part number: BD911, from Maplin: N01AJ)

          PPS.. or you could just use the MAG switch :lol:

          Attached files

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          • #20
            How to wire in a Hobbs meter correctly.

            That will work Andy, and I was sitting designing a device to work from a coil on a HT lead, that ensured that the plug was actually firing!

            All this just for a accurate Hobbs reading..

            I am reading with interest the uprated front suspension for a Quik.

            With this level of British ingenuity for such a simple problem we can easily understand now how Bletchley Park cracked Enigma..(yes I am an expat Brit).

            Paul

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            • #21
              How to wire in a Hobbs meter correctly.

              Hi Paul,

              Well it'll work fair enough, the challenge is making it operate correctly over a wide range of magneto output voltages, from loaded at low rpm to lightly loaded at full rpm (without toasting the components!). There is such a large variation that there is a balance between low end operation and protecting itself from a lot of volts at the high end.

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              • #22
                How to wire in a Hobbs meter correctly.

                To use the mag output to drive the Hobbs I would go for a simple rectifying and smoothing circuit followed by a wide input range DC-DC with a single 12V output. A low power device would suffice as there would be little in the way of power dissipation due to the efficiency of the switcher.

                I could design you a SIL 4 circuit with a full safety case if you really wanted ;-)

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                • #23
                  How to wire in a Hobbs meter correctly.

                  Yeah but that's more than 3!! :-) If it gets more complicated than a bit of stripboard and couple of beans bought from Maplin, then people won't bother.

                  come on we need proper microlighter tight arsed-ness :rofl:

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                  • #24
                    How to wire in a Hobbs meter correctly.

                    Ah, I see my problem now, having come from owning a CofA aircraft anything else is cheap in comparison.

                    If you want cheap then just blag some samples from some of the power supply compaines, the only problem is that they will then pester you for the rest of your natural life!

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                    • #25
                      How to wire in a Hobbs meter correctly.

                      Kevin wrote: ..will then pester you for the rest of your natural life!
                      :lol: tell me about it :-)

                      Problem is I'm only coming from approx information on the mag output voltage (I've never measured it - I don't have a 503).. in reality I'm not sure if it'll be that high or not, depending on how much it's loaded, so the linear version probably wont get that hot in reality.

                      About the most useful info I found was here http://www.800-airwolf.com/pdffiles/...9 - 4 page.pdf.

                      The linear design above should work from a 16VAC rms input to a 77VAC rms input. At the top end it dissipates about 2W - but in reality I'm not sure it'll ever get there.

                      I took a look at some of the maxim and lineartech parts. Most of the small simple DC-DC parts have max input volts of ~75V. MAX5035 for example has 76V. but when you work out mag output at 77Vrms, that is peak at 108V, after rectifier. Efficiency would be much better but comes at the expensive of not being able to get them from the shop and also would probably need a proper PCB, so not as easy for DIY.

                      I think for a 4-stroke though, your pressure switch circuit is definately the best approach... I think it's also the way they do it on other GA aircraft?

                      Anyway I shall shut up now, I have clearly passed the geek test. :freaked:

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                      • #26
                        How to wire in a Hobbs meter correctly.

                        I remember us paying a lot of money for a new Hobbs in our PA38, that worked off the oil pressure. It lasted a couple of weeks and then no more :-( Unfortunately it would have meant a flight to our maintenance organisation and another bill so we didn't bother.

                        Sadly, I was thinking about this problem last night. What you could do is to use a simple 8 bit micro (Microchip PIC would suffice) with a built in ADC it could measure the supply. On power up it would make an immediate measurement of the 12V supply and this would be the reference level with the engine not running. It would then look for a change in the supply from the reference to the reference + deltaV where deltaV is an increase caused by the generator coming on line. The micro would then drive the Hobbs until there was a negative step change in the supply indicating that the generator had gone off line and the engine stopped. I reckon < 1 worth of components and a tiny bit of vero board that would sit on the back of the Hobbs itself with only 2 wire connection ;-)

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                        • #27
                          How to wire in a Hobbs meter correctly.

                          I have a feeling this could go on..... ;-)

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                          • #28
                            How to wire in a Hobbs meter correctly.

                            Wahay!

                            Conrad's idea is the one for me!

                            Simple and easily installed. Brilliant!

                            Cheers

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                            • #29
                              How to wire in a Hobbs meter correctly.

                              I know this thread is a tad old now but I have just seen a very interesting schematic in the Skyranger manual.

                              The schematic shows the standard Ducati regulator used on a lot of Rotax 912 installations. They show the 'L' terminal on the regulator connected to the +ve of the Hobbs. I then found an internal circuit diagram of the Ducati module and discovered that the 'L' terminal is an open drain MOSFET drive that becomes energised when the generator is not running. They have a buzzer connected from +12V (via the master switch) which acts as a current source for the Hobbs. This would therefore seem to be the simplest and most effective method of driving the Hobbs.

                              In the Skranger schematic it also shows the 'R' terminal connected to the 'C' terminal which I believe is standard.

                              Still not sure whether this would constitute a modification.

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                              • #30
                                How to wire in a Hobbs meter correctly.

                                er... .the L terminal on the 912/ducati regulator is the Lamp connection, the lamp lights when the generator is not producing output but the battery is connected to the C terminal. It is for the generator fail light which is connected between the L terminal and the battery positive, ie this line only drives low and it does it when the generator is not working (like when the engine is not running). It won't do what you want.

                                If you want to use this line to switch an hour meter you will need to use a transistor to invert the signal on this line, ie to drive low when the line goes high. Don't forget to put a couple of resistors in to pot down the signal or you will find that your generator fail warning light stays on and/or your transistor burns out.

                                I still favour the use of a spare terminal on a mag switch which becomes grounded when the mag is live. (I didn't fit an hour meter in my new aircraft!)

                                Andrew.

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