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  1. #1
    Join Date
    Jun 2008
    Location
    NC
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    79

    Pulsing question ..relays

    I have not done much with pulsing at all but have quick job I need to get done soon.

    It's a totalizer that I will configure to pulse out to turn some equipment on after so many pulses have occurred.

    The meter outputs 24vdc, I was told I need to have the pulses close for a minimum of 25ms for there readings. I know to configure the other stuff like gallons per pulse and such.

    My main questions are

    - do I need to use a certain type of relay for pulsing? They requested a dry contact pulse so imagine I am just going to be pulsing that 24vdc to a relay and use the dry contact side for their two wire..

    - the other question is if I do use a relay and let's say the reaction time is 20ms before it closes. Do I have to figure this into anything on the meter or the number I give the customer for ms?

  2. #2
    Join Date
    Jan 2003
    Location
    USA
    Posts
    1,656
    You might want to use a SSR instead.

    Typical relays last for 1 million cycles. So you may want to guestmate how many pulses per day and figure out how long it’s going to last. It doesn’t sound like a great application for a mechanical relay.
    Propagating the formula. http://www.noagendashow.com/

  3. #3
    Join Date
    Jun 2008
    Location
    NC
    Posts
    79
    Thanks, I was definitely going to go with a solids state relay unless there were other recommendations here.

    I guess finding one with lowest opening and closing time would be what I want

  4. #4
    Join Date
    Jun 2008
    Location
    NC
    Posts
    79
    So the dry contact pulsing did not work for whatever reasons. The equipment i am pulsing to accepts 5-15vdc inputs for also, Couldn't i just drop a resistor cross the 24vdc pulse outs on my meter to drop the voltage to acceptable limits(5-15vdc) for there meter? Anyone know off hand what size resistor to drop it 15vdc or so?

    Thanks

  5. #5
    Join Date
    Jun 2013
    Location
    Austin, TX
    Posts
    641
    If galvanic isolation is not needed, use a 7812 regulator. Radio Shack or any decent electronics store will have that for cheap. You'll need some capacitors to keep the regulator stable (on the order of 1uF) and maybe a resistor (about 10k) to discharge the caps.

  6. #6
    Join Date
    Feb 2005
    Location
    Edmonton, AB Canada
    Posts
    596
    Quote Originally Posted by Shockwave View Post
    So the dry contact pulsing did not work for whatever reasons. The equipment i am pulsing to accepts 5-15vdc inputs for also, Couldn't i just drop a resistor cross the 24vdc pulse outs on my meter to drop the voltage to acceptable limits(5-15vdc) for there meter? Anyone know off hand what size resistor to drop it 15vdc or so?

    Thanks
    Good day Shockwave,

    Yes, you can use two resistors to drop the voltage from 24VDC to your 5-15VDC range. The circuit is called a Voltage divider and the formula is:

    Vout = [R2/(R1+R2) * Vin] where:
    Vin = Input Voltage (24VDC)
    Vout = (your 5-15 V)
    R1 and R2 are wired in series and Vout is taken from across R2.

    However, you need to determine if:

    a. If you need electrical isolation (i.e. Galvanic isolation as mentioned by NiHaoMike) between the equipment
    b. The output impedance and Max supply current of your 24VDC Output
    c. The input impedance of your (5-15VDC) Input

    (a) will determine if you need an electrical isolation circuit/device in addition to the Voltage Divider circuit and (b) and (c) will determine what resistance magnitudes you will need to properly condition your output signal for your input.

    Lastly, depending upon your output signal's characteristics you may or may not require some filtering of the voltage divider output.

    Cheers,

    Sam

  7. #7
    Join Date
    Jun 2008
    Location
    NC
    Posts
    79
    Quote Originally Posted by s2sam View Post
    Good day Shockwave,

    Yes, you can use two resistors to drop the voltage from 24VDC to your 5-15VDC range. The circuit is called a Voltage divider and the formula is:

    Vout = [R2/(R1+R2) * Vin] where:
    Vin = Input Voltage (24VDC)
    Vout = (your 5-15 V)
    R1 and R2 are wired in series and Vout is taken from across R2.

    However, you need to determine if:

    a. If you need electrical isolation (i.e. Galvanic isolation as mentioned by NiHaoMike) between the equipment
    b. The output impedance and Max supply current of your 24VDC Output
    c. The input impedance of your (5-15VDC) Input

    (a) will determine if you need an electrical isolation circuit/device in addition to the Voltage Divider circuit and (b) and (c) will determine what resistance magnitudes you will need to properly condition your output signal for your input.

    Lastly, depending upon your output signal's characteristics you may or may not require some filtering of the voltage divider output.

    Cheers,

    Sam
    Wow there is a lot more to this than i thought!

    So i finally had a chance to play with everything for awhile today for a decent amount of time.

    1. I noticed when no wires are connected at all to the pulse out wires I will read 24vdc across them

    2. Once i connect the wires to a relay it will read around 1.7vdc

    3. I added the 500ohm resistor across the common/24vdc input terminals on the SSR and the voltage drops to .2(I called the meter company and they said this is perfect)

    4. I started the pumps and the first time it pulsed it closed the relay fully and a dry contacted closure was created giving a pulse to the sampler. However as it kept pumping and kept pulsing it would not close the contacts again on the relay. The light on the relay was going on though indicating that it should be closing.. This happened a few times like this, where on that first pulse it would work perfect, but anything after that it wouldnt. Once it did work on this "first pulse only" It would not work for about another half hour or so, almost like a capacitor had to recharge or something on the meter.

    I called the meter company and they said to keep putting 500ohm resistors in series until it works, i was up to about 6 of them in series before i finally gave up..

  8. #8
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    Are you sure the relay is not closing the contacts again? Is it possible that the output is staying closed, and that's the reason you're not getting any pulses?

    I say this because most SSR's are designed to switch AC voltage, they use a triac as the switch. A triac will lock on if it switches DC, it will never turn off as long as the voltage is applied. Triac's use the zero-crossing of the AC waveform to turn off, something that doesn't happen with DC. Make sure your SSR is rated to switch DC!

  9. #9
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    Oops, maybe I misread your post(s). Are you are trying the resistive divider? I thought you were doing an SSR, if not my earlier post doesn't apply.

  10. #10
    Join Date
    Jun 2008
    Location
    NC
    Posts
    79
    Quote Originally Posted by DDC_Dan View Post
    Are you sure the relay is not closing the contacts again? Is it possible that the output is staying closed, and that's the reason you're not getting any pulses?

    I say this because most SSR's are designed to switch AC voltage, they use a triac as the switch. A triac will lock on if it switches DC, it will never turn off as long as the voltage is applied. Triac's use the zero-crossing of the AC waveform to turn off, something that doesn't happen with DC. Make sure your SSR is rated to switch DC!
    Yes! That was happening a lot. The relay would close that first time, and then now that i think of it i checked it a few times afterwards during all this mess and i remember it staying closed and thinking WHY?!?!?!?

    This is one of the relays that i am using, which works on that first pulse..just nothing afterwards seemingly.

    http://www.kele.com/relays-and-contactors/rssdn.aspx

  11. #11
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    Yup, your SSR uses dual SCR's, same as a triac, it won't work with a DC load.

    Sent from my ADR6300 using Tapatalk 2

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