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water meter pulse wiring

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  • 18-12-2013 8:41pm
    #1
    grousedogtom
    Registered Users Posts: 463 ✭✭


    Hi, I'm looking for advice on something I'm not 100percent sure of. I have to replace an existing water meter which has a magnet type contact mounted on the side of the meter with a 2 core cable wired back to a 24 v dc power supply unit. How it works at the moment is as follows the power supply unit sends out 0 volts to a2 on a relay coil and 24 v to one core of the meter cable, when 10 liters pass through the meter the magnet contact returns the 24 v as a pulse for a second or less back on the second core to energies the coil ,as soon as the coil drops back out again this activates a chlorine dosing pump. The new meter Ive been giving has a switch unit on it with 6 cores, I ll be using just 2 of these wires the black and red . My problem is as the turbine spins on the new meter after 10 liters has passed through I get a similar pulse but my multimeter needs to be set to mega ohms to detect the pulse ,will this be enough to energies the relay contacts. Ive attached the manual for meter wiring. Thanks


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    #2
    whizbang
    Registered Users Posts: 1,935 ✭✭✭whizbang


    You need to swap the power supply wires. So 0v to black wire, +24v to relay coil, 2relay coil to red wire.
    But you need to check if 30mA is enough to switch the relay. Bit doubtful IMO.


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    #3
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    whizbang wrote: »
    You need to swap the power supply wires. So 0v to black wire, +24v to relay coil, 2relay coil to red wire.
    But you need to check if 30mA is enough to switch the relay. Bit doubtful IMO.

    Assuming 30mA is not enough to switch my relay coil what are my options?


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    #4
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,594 Mod ✭✭✭✭2011


    I would not use a standard "mechanical" relay for this.
    A solid state device would be preferable.
    These have no moving parts (so they don't wear out) and they use far less current.
    Check out "Phoenix Contacts" and you will find something suitable.


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    #5
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    If I'm looking at it right, the pulse is of too short a duration to properly operate a relay directly probably. It looks like a pulse width of 10 ms for the 10 litre pulse cycle.

    As for current to switch relay, a small transistor can be used to switch it from the pulse, but again, duration is too short. A good one for the microcontroller chips again.


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    #6
    ted1
    Registered Users Posts: 23,543 ✭✭✭✭ted1


    Which model did you get?
    Switch your meter to continuity and you'll here the beep with every pulse


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    #7
    ted1
    Registered Users Posts: 23,543 ✭✭✭✭ted1


    Bruthal wrote: »
    If I'm looking at it right, the pulse is of too short a duration to properly operate a relay directly probably. It looks like a pulse width of 10 ms for the 10 minute pulse cycle.

    As for current to switch relay, a small transistor can be used to switch it from the pulse, but again, duration is too short. A good one for the microcontroller chips again.
    It's either 10, 80 or 100ms. 80ms or longer is standard. Minutes don't come into it.

    A small 24v 5 amp mechanical relay has no problem operating at 80ms or longer. I've used them to split pulses before or to change npn outputs to pnp and have never had a problem


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    #8
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,594 Mod ✭✭✭✭2011


    Bruthal wrote: »
    As for current to switch relay, a small transistor can be used to switch it from the pulse, but again, duration is too short. A good one for the microcontroller chips again.

    Solid state relays from Phoenix Contacts can be used for this type of application.
    Here is an example, it has a response time of 20 µs turn off time 300 µs and a transmission frequency of 300 Hz.

    See data sheet.


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    #9
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    ted1 wrote: »
    It's either 10, 80 or 100ms. 80ms or longer is standard. Minutes don't come into it.

    A small 24v 5 amp mechanical relay has no problem operating at 80ms or longer. I've used them to split pulses before or to change npn outputs to pnp and have never had a problem

    When using the pulse output from the red wire, its output is 10ms for every 10 litres measured but I could be reading it wrong of course, and looking again, it looks like its actually 80ms in this case.

    It was litres I meant rather than minutes which is fixed now.A mechanical relay won't be great for a 100th of a second pulse width.

    I might test one later for curiosity by setting up a 10ms PIC output, and see do the contacts actually close by connecting them to a PIC input to sense closure. . Even if they do close though, it would be a fairly crude setup.

    The relay likely won't have a problem closing if the pulse width is 80ms.


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    #10
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    2011 wrote: »
    Solid state relays from Phoenix Contacts can be used for this type of application.
    Here is an example, it has a response time of 20 µs turn off time 300 µs and a transmission frequency of 300 Hz.

    See data sheet.

    Since it looks like the pulse is 80ms, and not the 10ms I thought it was, the 24v relay should close momentarily, and will probably work perfectly as it is, in the new setup.

    OP may have been thrown by the multimeter having difficulty measuring a very short pulse, rather than having actually tried the new setup.

    If the 24 volt relay is not closing properly, a 12v or 6v one could be used, which would close more positively, but the setup as is will probably work.


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    #11
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,594 Mod ✭✭✭✭2011


    Bruthal wrote: »
    Since it looks like the pulse is 80ms, and not the 10ms I thought it was, the 24v relay should close momentarily, and will probably work perfectly as it is, in the new setup.

    OP may have been thrown by the multimeter having difficulty measuring a very short pulse, rather than having actually tried the new setup.

    If the 24 volt relay is not closing properly, a 12v or 6v one could be used, which would close more positively, but the setup as is will probably work.

    Yes, it may well work without a relay.

    However relays offer many distinct advantages, such as:
    1) It permits switching of a different voltage.
    2) It makes switching a larger current possible.
    3) In many circumstances it is not desirable to have field instrument connected directly connected to a control system.
    4) Relays can have multiple poles with NC & NO contacts providing different functions, such as dosing pump & indication lamp.
    5) Timer relays can be used to increase the duration of the pulse if required.


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    #12
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    2011 wrote: »
    Yes, it may well work without a relay.

    However relays offer many distinct advantages, such as:
    1) It permits switching of a different voltage.
    2) It makes switching a larger current possible.
    3) In many circumstances it is not desirable to have field instrument connected directly connected to a control system.
    4) Relays can have multiple poles with NC & NO contacts providing different functions, such as dosing pump & indication lamp.
    5) Timer relays can be used to increase the duration of the pulse if required.

    Sometimes its like you believe you're posting to a junior infant with explaining why a relay should be used:)

    But I never said don't use a relay. What I said was the existing setup, which has a relay, might work fine, since the pulse width is 80ms, not the 10ms I thought it was for some reason.

    If 30ma is too low a current to switch the relay, a transistor could be used to switch it, with transistor being switched by the pulse.


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    #13
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,594 Mod ✭✭✭✭2011


    Bruthal wrote: »
    Sometimes its like you believe you're posting to a junior infant with explaining why a relay should be used:)

    Perhaps I am, there are lots of people on the internet.
    Not everyone reading this will have an understanding of the advantages of relays.

    However I would install a relay if I were doing this.
    What I said was the existing setup, which has a relay, might work fine, since the pulse width is 80ms, not the 10ms I thought it was for some reason.

    I would think that the relay such the Phonenix one in the link I posted above work at both 10ms and 80ms. Do you agree?


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    #14
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    2011 wrote: »
    Perhaps I am, there are lots of people on the internet.
    Not everyone reading this will have an understanding of the advantages of relays.

    Advantages over what though? No relay?

    I think they are basic enough for anyone with interest. Not all the advantages in your list are exclusive to relays. Transistors can switch different voltage than the signal side, they just have a common 0v in NPN ones. And can also switch large currents. Solid state DC relays are simply opto couplers switching mosfet transistors.

    Where a relay differs is they isolate the signal side from the switched side, which is what the opto coupler does in the SSR.

    But its academic here, as a relay is in the setup already I believe.
    However I would install a relay if I were doing this.
    There already is a relay. The OP question was if the 30ma capacity of the pulse would energise the relay.

    I would think that the relay such the Phonenix one in the link I posted above work at both 10ms and 80ms. Do you agree?

    Yea, but the 10ms is not a factor now, so why spend the money on it? How much are them ones you linked anyway?

    If the pulse 30ma is too low at 30ma, a transistor, resistor, and diode would do perfectly, for about a euro. This would switch the existing relay, in case you still believe im saying the transistor will substitute for the relay.

    As I said, if i was doing it and it was 10ms, id use a pic to sense the input, and output a 500ms pulse to operate the relay, but its all if`s and buts, since im neither doing it, nor is it a 10ms pulse by the look of it.


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