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Help with 3v relay

Discussion in 'General Electronics Discussion' started by BarryTX, Feb 14, 2016.

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  1. BarryTX

    BarryTX

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    Feb 14, 2016
    Hi, new to the forum and it has been many years since I messed with electronics. I have a small project where I want to control the opening of a solenoid valve (24V DC, 0.25A) with a 3V DC signal. I have verified that he valve amperage is 0.25A with a meter.

    I thought the answer was to use a relay that triggers with 3V and allows 24V across the poles on the other side. I got a Songle SRD-03VDC-SL-C relay, which has a 3V coil and shows 10A for 250VAC, 125VAC, 30 VDC, and 28 VDC. I can hear and feel that the relay is toggling when 3V is applied across the coil, but it is clearly not completing the circuit on the other side of the relay. I have put 24V DC on that side without the solenoid valve in the circuit and used a voltmeter to check, I get no voltage flow across the poles when the 3V side is triggered / closed. I must be missing something basic here, can anyone assist?

    Thanks in advance,

    Barry
     
  2. Externet

    Externet

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    Aug 24, 2009
    Welcome.
    The 3 Volts source in not capable of delivering the current that relay needs to overcome its spring *
    A weaker/weakened spring may help. If not, a power transistor stage powered by the same 24VDC supply of the solenoid may be necessary to do the relay activation. The same weak 3V source can itself trigger a ~1 Ampere rated power transistor to do it even without a relay.

    *You can confirm that measuring the 3V when trying to activate the relay : It will drop well under 3V, uncapable of sustaining the voltage.
     
    hevans1944 likes this.
  3. Minder

    Minder

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    Apr 24, 2015
    Those relays have a 3v version, if you have this one check the resistance across the contacts when energized you may be using the wrong contact pair?
    M.
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    The relay you specified requires a whoppin' 120 mA to actuate. The coil resistance is only 25 Ω. Make sure your 3 V source can supply the required current. See @Externet's comment on post #2 which is spot-on.

    [​IMG]
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
  6. BarryTX

    BarryTX

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    Feb 14, 2016
    All,

    Many thanks for the responses, they are greatly appreciated. I am still confused, however. Please go easy on me as I know this is basic stuff but it has been 40+ years since I sat in an electronics class and I'm either not finding or not understanding what I read online.

    I am using 2 rechargeable AA batteries as the power source, they are rated at 2500 mAh capacity when fully charged. I do understand that capacity isn't amperage, but from what I've read I believe I should expect the fully charged batteries to supply 3v with 250 mA current load for 1 hour. So I believed that the 2 AA batteries should be able to toggle the relay.

    The other confusion for me is that I can hear and feel that something is indeed moving in the relay when I apply the 3v power to the coil side. I assume this is closing the switch on the high power side, but if not I don't understand physically what is moving and clicking inside. But having said that, I know the circuit is not closed on the other side because I've tested it. So if the reality is that something is moving but 'not enough' to close the switch I'm curious what kind of power supply would be more appropriate - at this point I really want to understand and learn from what is going on.

    Harald, thanks for the suggestion on the transistor, that gives me another thing to learn about! :)

    Again, many thanks for the responses and help!

    Barry
     
  7. signalman72

    signalman72

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    Jan 26, 2014
    If it's clicking it sounds to me like the contacts are closing. Are you sure you are not reading the N/C side when energized? The N/O are the ones that will close when the relay is energized.
     
    Last edited: Feb 15, 2016
  8. Externet

    Externet

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    Aug 24, 2009
    The two AA rechargeable cells are likely 1.2V each. That sums 2.4V with no load.
    Still when fully charged, are uncapable of reaching 3V under load.
    If the relay is encapsulated, you will not see or able to weaken its spring.

    Try D non-rechargeable cells instead to confirm. The best option is to use a transistor instead of a relay.
     
    BobK likes this.
  9. signalman72

    signalman72

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    Jan 26, 2014
    Shouldn't his batteries still (if fully charged ) be able to supply the 75% of rated operating voltage and pick the relay?
     
  10. BarryTX

    BarryTX

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    Feb 14, 2016
    I'm sure I'm on the correct side, the relay actually has a diagram on the bottom where the pins are. putting my AA power on the coil side actuates something, but the high power side is not switched closed. I have tried 2 different copies of this relay and both did the same thing.
     
  11. Minder

    Minder

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    Apr 24, 2015
    Did you do as I suggested and measure the continuity across the contacts when closed?
    M.
     
  12. BarryTX

    BarryTX

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    Feb 14, 2016
    Thanks Externet. The relay is encapsulated, I'll try Ds tonight to see what happens.
     
  13. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Just to get a feeling for the relay you can also try 3 rechargeables in series, or 3 alkaline cells (which have ~1.5V per cell). The relay will tolerate the overvoltage for a short time.
    AAs should easily be able to deliver 250mA, I see no need to go for D cells. D cells won't help you with the low voltage issue, they too have 1.2V per cell.
     
  14. BarryTX

    BarryTX

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    Feb 14, 2016
    Minder, I put the 24v between one side of a voltmeter and one contact on the switch (high power) side, and attached the other side of the voltmeter to the other contact on the switch side. I then turned on the 3v power on the coil side, heard the relay click but saw no voltage across the contacts on the switch side. I did make sure the scale on the voltmeter was proper and tested it directly on the 24v battery pack, it showed the proper deflection when the relay was not in the circuit.
     
  15. BarryTX

    BarryTX

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    Feb 14, 2016
    I'll give that a try tonight also, thanks.
     
  16. Minder

    Minder

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    Apr 24, 2015
    I would have done a resistance check just to make sure, if you have a Diode range, this passes a small current through the device and shows the voltage drop across the contacts.
    If you do not have then just a simple ohms test.
    M.
     
  17. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Sure sounds like the relay is actuating, so the problem must be in the contacts. Do what @Minder suggested: ditch the 24 V power supply and just perform continuity and resistance checks with your multimeter. Use all possible combinations of the relay pins (except the two pins connected to the relay coil) in your continuity checks. Check continuity with and without the relay energized. Report back here if you find two pins that show continuity and the condition under which this occurs: relay coil energized or relay coil not energized. If after all this effort you do not find continuity between two relay pins, whether the relay coil is energized or not, then you should abandon this relay and purchase one that actually works.
     
    Harald Kapp likes this.
  18. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    This is the relay schematic from the datasheet:
    upload_2016-2-15_20-54-58.png
    The current path in red is the one you're looking for. There should be a very high (infinite) resistance between these two pins (use the diode range as suggested by @Minder ) when the relay is not energized.
    There should be a very low (almost 0Ω) resistance when the relay is energized.
     

    Attached Files:

  19. BobK

    BobK

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    Jan 5, 2010
    So your voltmeter is across the relay contacts.

    This should show 0V when they are closed, which is apparently what you are reading.

    Use the meter on Ohms across the contacts, and it should read 0 (or very small <1) when the relay is closed, and infinite when the are open.

    Bob
     
    Harald Kapp likes this.
  20. Externet

    Externet

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    Aug 24, 2009
    Hoping there is not an undisclosed built-in diode paralleled to the coil, and being reverse fed...:rolleyes:
     
    hevans1944 likes this.
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