delay timer off relay 12volts

Discussion in 'The Projects Forum' started by nickmms, Jan 3, 2014.

  1. nickmms

    Thread Starter New Member

    Jan 2, 2014
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    Hi All
    First time post, I hope this is the right forum. I have very limited electronics experience. I will try to give as much detail as I can

    I have been designing a hydraulic system for our diamond drill, using solenoids to open hydraulic valves (3 on/off hydraulic valves and 1 proportional pressure relief valve.) The electronics for this are fairly simple. I have 3 switches that operate 30 amp horn relays (easy to get hold of) to power the 3amp solenoid coils. The PWM driver (bought) to control the pressure relief is activated by one of these switches as well. The drill control box containing the PWM board and switches is about 15m away from the solenoid valving.

    I would like to have a time delay off period (about 0.1 sec) to one of the relays so this hydraulic solenoid valve stays open for a split second to relieve the pressure in the line. I made a time delay circuit I got from the internet with very few components and works well… but in the event of a component failure (2n2222) the solenoid relay may stay on and keep the valve open without realising this as the switch is off. The circuit for the time delay powers the relay at all times. So my question is what is the best way of having the solenoid as fail safe. I have tried to attach some circuits I have already made
    thank you. I hope this is clear
    Nick
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
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    .1 sec is pretty small interval.

    Just put a large capacitor across the coil.
    That's about as simple and fail-safe as you can get.

    If capacitor gets to be too large. Use an auxiliary relay with high resistance coil to drive main relay.

    The time constant is related to coil resistance and capacitance.

    ps.
    Failure mode is blowing capacitor and losing td.
     
  3. nickmms

    Thread Starter New Member

    Jan 2, 2014
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    Thank you,.. so you mean a capacitor across the relay coil or the solenoid?
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    How about this with no transistor?

    When the switch is first closed, the relay L2 pulls in the Solenoid as before. While the switch is closed, C1 charges up to +12V through R1. R1 is there mostly to limit the inrush current to C1, but you have plenty of time to charge it.

    When S1 opens, the charge stored in C1 flows through D2 into the relay coil, holding it pulled-in about 100ms after the switch opens. To determine the size of C1, we need to know the coil resistance and the drop-out voltage of the relay L2. I'm guessing that C1 will be about 680uF.
     
  5. inwo

    Well-Known Member

    Nov 7, 2013
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    Across the relay coil.

    Would also work across the solenoid, but the higher current the device (lower resistance) the more capacity is required for delay.

    If you can't get enough delay using the horn relay, you may need a smaller relay (with cap on coil) in front of that.
     
  6. inwo

    Well-Known Member

    Nov 7, 2013
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    MikeML will do the same thing but will be more technically correct.

    It limits inrush etc. Which should make everything more reliable.

    And if you give him the values he will simulate it for rc value.

    ps.

    A quick test of cap only will probably not hurt anything.
     
  7. nickmms

    Thread Starter New Member

    Jan 2, 2014
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    thanks all I was hoping for an easy fail safe solution like the one you suggested. I have a switch to bypass the time delay circuit anyway if something blew... but as I said i cant have the relay energised all the time if something went wrong with the electronics

    I tied a few caps yesterday and blew them... probably had something wired wrong.

    measured a few things now only had 180uF cap left
    relay resistance 88ohms
    5.8Volts still operated relay

    In the circuit for the time delay provided I used a 47uF cap and 120ohm resistor and that gave me the right time delay. I am not seeing this with the 180uf cap.. i guess too small and not going through transistor

    Nick
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    You will need a much larger capacitor. In your circuit, the capacitor discharges into the base-emitter junction of the NPN transistor. Because of the transistor's current gain, the relay will remain pulled in until the base current decreases to less than 1mA.

    The relay you are using has a very low coil resistance (88Ω). I was hoping you were using a much more sensitive relay; one with a coil resistance upwards of 150Ω.

    With 88Ω, to get an aproximate delay of 0.1s:

    τ = RC, where τ is the time-constant in sec., R is the coil resistance in Ohms, and C is the capacitance in Farads.

    C = τ/R = 0.1/88 = 0.00114 Farads or 1114uF. Note the tolerance on the capacitor that you purchase, the delay may be longer based on the capacitor tolerance...

    Round that up to 1200uF. You need a capacitor rated at 15V to 25V.

    I set up the following simulation, during which I learned that 1200uF is a bit small, so I went to the next available standard capacitor size of 1500uF. Here are the results:

    These are standard capacitors that DigiKey stocks.
     
    Last edited: Jan 3, 2014
  9. nickmms

    Thread Starter New Member

    Jan 2, 2014
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    Thank you very much Mike for all your help.
    I bought some stuff today from Jaycar... new caps, higher resistance relays. I ran some tests using the existing horn relay (88ohms) with a 1000uF cap using your circuit to operate the hydraulic valve. The 1000uF cap reduced the pressure….enough time delay. I tried a 470uF cap but did not reduce pressure fully. So I guess with the 1000uF and the relay coil has about 0.07 sec delay
    Also attached pdf of one of the circuits if you could please advise on the flyback diodes etc
    With the original time delay circuit using the NPN222 transistor as mentioned it was not fail safe as the relay coil was always +12v energised and could potentially keep the valve open. When this happened I must have shorted something and the Base and collector were shorted?. I was wondering if it is common for this to happen and would not be possible if something was not shorted. Using your circuit it is fail safe and at worst the cap would blow and no power to solenoid

    thanks
    Nick
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The most likely failure mode of the Capacitor is loss of Capacity due to the electrolyte drying out, which will prevent the delay from working, but the solenoid will still be actuated by the switch. If the capacitor shorts, that also kills the delay, but the solenoid continues to follow the switch. It would take a double failure, shorted cap and shorted diode to prevent the solenoid from energizining. I dont see a failure mode that would cause the solenoid to stay on except welded contacts on the switch.
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    On the snubbers: there are two reasons for using them. One is to minimize radiated coupling to other sensistive circuits. If you have a computer or microcontroller nearby, using them might prevent hangups and spurious resets.

    The other reason is to minimize arcing at the relay contacts. As the relay contacts pull apart, the inductance of the solenoid cause an arc to form at the opening gap. Using a snubber prevents that arc from forming, making the contacts last longer.

    You dont need a snubber on the time-delayed relay because the current decays slowly due to the capacitor. On the other two relays, their inductance is likely low enough that the switches will last without them. On the solenoids, I would use some garden variety Silicon rectifiers, like 1n4001 thru 1n4007. 1A diodes are fine, because of the low duty cycle at turn-off. The piv rating only needs to be 25V (double 12 for good measure). High speed diodes are not needed.
     
    Last edited: Jan 4, 2014
    nickmms likes this.
  12. nickmms

    Thread Starter New Member

    Jan 2, 2014
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    Thanks Mike
    I thought that was the case
    If you dont mind When you get a chance if you could pls look at the pdf I sent
    and let me know about the snubber diodes for the solenoid coils
    Nick
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Did you try the delay caused just by the BEMF diode across the solenoid coil?
    3A is quite a bit of energy to dissipate coupled with the large hysteresis that a DC coil has, I would have though you would experience somewhere around .1sec for the SOL to drop out.
    Any of the IN400x series would suffice as a diode. I usually stock 1N4007 as this covers all voltage ratings.
    Max.
     
    Last edited: Jan 4, 2014
  14. nickmms

    Thread Starter New Member

    Jan 2, 2014
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    Thanks Max
    I put a 1N4004 diode across each of the 4 solenoid coils ( including the proportional PWM driven solenoid), so effectively would be same time delay on all coils from the effect of the diode. I need one coil to stay open for the time delay
    I found that the proportional solenoid was a bit erratic. I took the diode off this coil to see the decay time on the other solenoid with the diode on it. I believe the delay was in the order of about 0.04secs about the same when I had a 470uF cap on the delay circuit. THe 1000uF cap gives me the exact time delay I need about 0.08secs... hard to measure the exact time delay.
     
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