Zener control relay?

Discussion in 'The Projects Forum' started by David Pate, Nov 4, 2013.

  1. David Pate

    Thread Starter New Member

    Oct 29, 2013
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    Can I use a zener in series with a relay solenoid to control the voltage at which the relay will drop out? Most relays will only drop out at 10% of nominal coil voltage. I need a relay to drop out at 6V. Would a 6V zener accomplish that?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The energized/De-energised hysteresis range of a DC relay is quite large.
    What is the maximum voltage applied?
    Max.
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    What are you trying to do that you think thats important?
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Yes that should work ok provided the relay voltage and zener and supply voltages are all chosen correctly.

    I would put a large cap (1000 uF) in parallel with the zener, that will give good pull-in of the relay as while the cap is charging (at first turnon) there will be more voltage and current in the coil.

    You should characterise the relay by using a variable voltage bench supply and find the pull-in and pull-out voltages, ie the hysteresis. You will need that data to choose the correct zener and PSU voltages. :)
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    This is similar to a recent post by someone who had a AC sense relay fed by a alternator where the relay coil chattered as the pull in point was approached, I mentioned I had solved this in the past by placing a bridge off the AC with a zener in series with the bridge output feeding the coil and with a 150μf cap across the coil.
    Max.
     
  6. David Pate

    Thread Starter New Member

    Oct 29, 2013
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    My problem is that there are a couple of circuits in parallel with the source voltage. However when the source is switched off the circuit I am trying to isolate can generate enough voltage feedback that other circuits are negatively affected for several minutes, and this is not acceptable. I could put in a large diode to block reverse current, but the forward voltage drop and power dissipation is not acceptable. So it is not the pull-in of the relay that is the problem, but the drop-out voltage. The voltage from the circuit could keep the relay active for several minutes. I need to get the relay to drop out as soon as the full source voltage disappears. I have very limited space to work with as well. I figure that a sugar-cube relay, with a snubber diode across the solenoid, connected to the source and grounded through a zener is the optimum solution. The zener would need to be rated for the full solenoid current power dissipation; approximately 500mW so I figure a 5 Watt zener is the best choice.

    I have only used a zener once in the past to limit the voltage from a multiplier circuit. I always seem to have trouble designing shut-off circuits.
     
  7. David Pate

    Thread Starter New Member

    Oct 29, 2013
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    Max & RB, thanks for your thoughts. The power supply is 12VDC. I measure the back-EMF at a little over 2 volts initially, but to be safe I think a 6V zener will permit sufficient safety margin; considering a 1N5340. Pair that with a 6 volt relay in the G5LE family and I should be good.
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You could use a transistor and zener to control the relay coil like this, set the drop out with the R1, remove R3 and Led1, and put your relay in their place with back emf diode across the coil.
     
  9. David Pate

    Thread Starter New Member

    Oct 29, 2013
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    So I ordered the parts, built & bench tested the prototype circuit, and have now tested the prototype in an actual situation. It works wonderfully. It is a simple, and elegant solution. It turns on at about 11 volts and shuts off at about 7 volts. Cost is under $2. I learned a bit more about Zeners from this project.
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That is pretty much identical to the method I had posted but for AC, in this case a high value cap is needed across the coil.
    Worked pretty good also.
    Max.
     
  11. David Pate

    Thread Starter New Member

    Oct 29, 2013
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    Thanks Max. It took a bit of digging to find the post you mentioned. Your post, as did many others, was related to turning on a relay. My problem, as I saw it, was turning off a relay.
     
  12. DickCappels

    Moderator

    Aug 21, 2008
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    This is very minor, but unless there is something you are trying to protect from the voltage spike across the relay solenoid when the current is switched off, the 1N4001 is probably of no use.
     
  13. David Pate

    Thread Starter New Member

    Oct 29, 2013
    25
    2
    Yes, there is an electronic display on a connected circuit that needs protecting from the spike; it is the same device that needed the back-feed shut off by the relay that was the reason for this circuit. Generally I find that I always use the diode across the solenoid; sort of a design rule that I picked up 40 years ago and it is a habit now.
     
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