Power delay on and SSR

Discussion in 'The Projects Forum' started by jm-a, Jan 8, 2011.

  1. jm-a

    Thread Starter Active Member

    Oct 20, 2010
    105
    2
    Hi all,

    I only need a power on delay circuit, roughly one second or two,to energize a SSR.

    The original board had a 2PST relay, and cut Live and Neutral , so i use also 2 SSR, in this schematic.

    I found 2 circuits on the web, and had questions about .

    LM555

    My choices:

    -Current :10 mA for the LS240D12 , sufficient to work and get a safety margin?

    - Delay: R1 =10 kOhm C1 = 100 µF T= R.C ≈ 1s

    - R2 : 10 kOhm 1/2 W

    2N3904

    I made a mistake for Ib !!!!

    Ic/ Ib = 10 so for Ic = 10 mA I got Ib = 1 mA Right?

    Math for Rb: Rb= ( Vs -0,7) (2.Ib) Rb= 4150 Ohm and 1/2 W

    R1 and C1 as above.


    As usual, could it work?

    Thanks a lot and happy new year to members and your families.

    Regards.

    jm
     
    Last edited: Jan 8, 2011
  2. #12

    Expert

    Nov 30, 2010
    16,345
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    The first drawing is so hard to read because it is too large. Try to make smaller in the future.
    No need to use 2 SSR.
    10k+100uf = ok

    second drawing not showing up here.
     
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  3. #12

    Expert

    Nov 30, 2010
    16,345
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    OK, got the second drawing.
     
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  4. #12

    Expert

    Nov 30, 2010
    16,345
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    This will work as long as you have 9 volts.
     
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  5. jm-a

    Thread Starter Active Member

    Oct 20, 2010
    105
    2
    Thanks for your replies first.

    I made a bad estimate for Ib, in fact it is not 10 mA, it's only 1 or 2 mA......

    10 mA is for Ic, to drive the SSR !!!!! see datasheet joined in previous post.

    In your last schematic, would you kind enough to tell me , what kind of transistors to use for this project?

    Best regards.

    jm
     
  6. #12

    Expert

    Nov 30, 2010
    16,345
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    2n3904....
     
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  7. jm-a

    Thread Starter Active Member

    Oct 20, 2010
    105
    2
    Hi,

    Thanks also for your quick and clear replies, i have still another , and last, question.

    About the capa, when 9V power supply is off, it's still charged.

    So, how to discharge it:

    - add a resistor and a diode in parallel with it
    - discharge through transistor base and 220 Ohm resistor

    Best regards.

    jm
     
  8. #12

    Expert

    Nov 30, 2010
    16,345
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    I thought about that later. (duh)

    That circuit needs another resistor. It goes across the capacitor and should be 56k to maybe a lot higher. The point is, you have to discharge the capacitor, but you have to keep it slower than the one second it takes to charge the capacitor. So...try 56k. That will leak out the charge enough that in 10 seconds, the circuit is ready to work correctly again.

    I offered this circuit because it is a compromise between the two circuits you offered. The single transistor just has too many jobs to do, and the 555 timer, I was too lazy to look it up while my instincts were saying a 2 transistor circuit will work. That's why I said, "Try this". If it's snappy enough to suit you, the job is done. If it acts slow and grumpy, post a question with 555 in the title. We have some amazing 555 guys here.
     
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  9. jm-a

    Thread Starter Active Member

    Oct 20, 2010
    105
    2
    Hi,

    Thanks of course, but with 340 V near LM555, i wasn't really convinced of good use for the IC.....

    Best regards.

    jm
     
  10. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,576
    231
    #1. You don't need diodes across SSR inputs. There is no inductive counter-EMF flyback from a resistor and LED.

    #2. An open question to anyone: Switching an electromechanical relay in these delay circuits are essentially like Schmitt triggered flop flop. As the voltage slowly increases across the coil, while the transistor is in the linear region, nothing happens until the pull-in voltage is reached. Then it stays on, because the drop-out voltage is significant lower. What happens as the LED in the SSR slowly turns on or off? Is there a possibility of intermittent switching for a short period, due to fluctuations in the LED or triac/SCR circuit voltages?

    Ken
     
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  11. jm-a

    Thread Starter Active Member

    Oct 20, 2010
    105
    2
    Hi,

    I agree, for diodes .....

    Next time, i'll watch schematic more closely before posting.

    Best regards.

    jm
     
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