LM555 and power delay

Discussion in 'The Projects Forum' started by jm-a, Jan 9, 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.

    If someone could tell me why this solution was choosen, thanks also !!!

    I found this circuit 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


    Should it work ?

    In fact I need to get this delay, in order to power V bus of my bldc card,after Vge voltage of igbt module, hence roughly 1 or 2 seconds.




    In a previous post, i was told that LM555 gurus could reply me.


    Thanks a lot.

    jm
     
  2. #12

    Expert

    Nov 30, 2010
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    6,743
    You have a connection from the SSR's to pins 2&6 which is wrong. That connection should be to +9 volts. All the the diodes are not needed. The second SSR is not needed.

    R2 is not in your drawing. The SSR limits its current. All you have to do is apply voltage.

    2 pole relay might have been used for safety reasons, but there is not enough information here to show why 2 SSR are needed.
     
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  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    To add to #12...R1 is not needed in series with the SSR's LEDs because there are already internal resistors in the SSRs. That is what the SSR's "impedance" spec refers to. Is the 9v supply a battery? Most people use that value when they intend to use batteries in a final circuit.

    ken
     
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  4. Wendy

    Moderator

    Mar 24, 2008
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    Pin 4 MUST be connected to Vcc (9V).

    SSR stands for Solid State Relay, so you don't need the diodes, except maybe to drop voltage to the SSRs. The "coil" of the SSRs are actually LEDs and resistors. The LEDs trigger photo TRIACs.

    You don't want the SSRs connected to R1 they way you have it.

    A more typical delayed power on would look like this...

    [​IMG]

    Use a CMOS 555, replace the relay and transistor with the SSR and you'll get good results. A regular 555 (like yours) may work, but put two diodes in series with both SSRs, they are parallel.

    A regular 555 will not output 9V, the max is around 7.8VDC. See Chapter 4, The 555 Integrated Circuit, of LEDs, 555s, Flashers, and Light Chasers.
     
    Last edited: Jan 9, 2011
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  5. #12

    Expert

    Nov 30, 2010
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    I wasn't sure about pin 4 needing to be tied high, but I am sure the SSR is labeled "4-10 volts". A chip that will not put out 9 volts will not cause a problem as long as it can put out at least 4 volts. Right?
     
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  6. Wendy

    Moderator

    Mar 24, 2008
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    The problem is the 1.4 volts differential when the chip is high. The reason I recommended a couple of regular diodes is you don't want to partly turn the SSR on. A CMOS 555 sidesteps that issue, it doesn't have the problem.
     
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  7. jm-a

    Thread Starter Active Member

    Oct 20, 2010
    105
    2
    Hi all,

    Thanks a lot for your clear and fast replies.

    Part of previous board schematic joined, to see why i plannned to use 2 SSR.

    Best regards.


    jm
     
    Last edited: Jan 10, 2011
  8. #12

    Expert

    Nov 30, 2010
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    I still don't think you need 2 SSR's.

    and a "thank you" to Bill.
     
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  9. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    What's the bridge circuit feeding? The double-contact relay will completely cut power to that circuit, isolating it from the line voltage. However, SSR's are not perfect switches like relays, and can leak a small amount of current when in the off-state.

    "Maximum Off-State Leakage Current @ Rated Voltage [mArms].......0.1 " for your SSR. If there are filter capacitors in the bridge circuit, they can be charging up to peak line voltage levels over time, when the circuit is supposed to be off.

    Ken
     
    Last edited: Jan 10, 2011
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  10. Wendy

    Moderator

    Mar 24, 2008
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    Generally all you need to switch is the hot line, this is what a wall switch does. Neutral does not generally need switched.
     
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  11. jm-a

    Thread Starter Active Member

    Oct 20, 2010
    105
    2
    Hi,

    Thanks you for all guys of this helpful forum:Bill, ≠ 12 , KMoffett .....

    Now, i know what to do with these SSR .

    I wanted to avoid electromechanical parts, coil, contacts,fwd and so on, but i'm surely going to use them !!!!

    My power supply card, has already 7815 so adding 7812 to drive a relay is not too difficult.

    For bill, i forgot , last time to click on "Thanks" and now, it's done. Thanks for your advices on 555.

    Best regards.

    jm
     
    Last edited: Jan 10, 2011
  12. Wendy

    Moderator

    Mar 24, 2008
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    I don't use them as much as relays due to cost, but I really like SSRs. I used to use a lot of them to make temperature controllers.
     
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