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Unstable latching circuit`

Discussion in 'General Electronics Discussion' started by Rob_K, Oct 2, 2013.

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  1. Rob_K

    Rob_K

    59
    0
    Sep 20, 2013
    Hi there,

    This is a latching circuit that is eventually to be used to activate a relay that is running a 12V, 30A power supply. The idea is that you switch the power on but require a push to make button press to energise the circuit, and if the power is cut it will not work again till the push to make switch is used again.

    Right then, i have been struggling with this circuit now for a number of weeks. I am using a 9V battery and instead of a relay as the load, I am using an LED and a 150Ω resistor. Every time I put this onto a breadboard it works as expected. As soon as I transfer it to a bit of strip board and solder it, it no longer works and the light just comes on when the main power supply is switched on.

    So now, I have had this circuit on a breadboard for about 24 hours now, and I have been testing it over time, I wanted to know if it would drain the battery if the main switch was left on. This test worked wonderfully and all was going well. So now I come to switch it on after the main switch has been off for 6 hours or so, only for the light to just come on without the trigger being used. I replaced the battery in case it was something to do with voltage supply, but no change. Then I removed the transistors one at a time and tested them with a multimeter on diode setting - ground probe to base and then probe the collector and emitter then swapped probes and tested again. Both transistors came out fine, with equally high or equally low readings. When I replaced the same transistors back to their original positions, suddenly the circuit works again.

    My suspicions are that the resistor values may be incorrect, so that the transistors don't reach saturation and so don't switch properly or that there is something wrong with the capacitor.

    If anyone could shed some light on this I would be very grateful.

    Kind regards

    Rob K
     

    Attached Files:

  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    This is a common problem and is used with motor contactors.

    You do not need any transistors. Use a 12V relay which is switched in with your push button. Use a spare set of contacts to supply power to the relay when it is pulled in. The output will not come on until the button is pressed and the relay will drop out if the input power is lost.

    A break push button can be used in series with the relay coil to stop the output if required.
     
  3. Rob_K

    Rob_K

    59
    0
    Sep 20, 2013
    Thanks for your reply Duke. I originally had a relay marked out for this circuit, and it seems to me that it would be a little more stable. I was just hoping that this little circuit might be a nice solution whilst also being able to learn about transistors.

    So is the nature of this circuit unstable?
     
  4. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    464
    105
    Aug 27, 2013
    If you want to use a semi-conductor to latch the relay on, use an SCR with a holding current < the relay current.

    Fish
     
  5. Rob_K

    Rob_K

    59
    0
    Sep 20, 2013
    Brilliant, thanks Fish. Is an SCR as reliable as a relay. I have to admit, I would like to get this down as small as possible.
     
  6. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Some comments about your circuit.
    1. The base of the BC547 should have a resistor to ground , say 10k, to ensure that it turns off.
    2. R3 could be dropped to 10k.
    3. The load of a resistor and led is not suitable since the led will not pass significant current until its turn on voltage is reached. A small amount of mains hum will turn on the circuit. A 1k resistor across the led will sort this.
    4. The circuit is effectively what an SCR is.

    I do not see how you can make anything smaller and cheaper than than a couple of wires to the relay.

    The reliability of components will depend on the stress they are subjected to. I have used relays and SCRs in electric fencers. The SCR is usually more reliable than a relay since it does not wear out (switching once a second) and there are no contacts to get dirty.. It can however get zapped by a lightning strike.
     
  7. Rob_K

    Rob_K

    59
    0
    Sep 20, 2013
    Thanks Duke, this is really very relevant and valuable. i am basically building an ignition switch for a motorcycle, but as a thief just rips them off anyway, I am playing with electronics make it difficult to even work out how to switch the bike on let alone ride off with it. I have my complex trigger system, now I believe I have the switching system in my grubby little hands. I will post back what I have achieved once I have it working. Obviously can't give details as it defeats the object of a secure ignition if I boast how it works on a forum.

    Thanks again, this was just what I needed.
     
  8. Rob_K

    Rob_K

    59
    0
    Sep 20, 2013
    A well known high street electronics shop in the UK has kindly given me 2 voltage regulators instead of thyristors it seems well they gave me 2 x TS78M12.

    Googled it to find the pin out and the data sheet is for Voltage regulator. I am assuming that a voltage regulator is not thyristor is that right?
     
  9. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    A voltage regulator is nothing like a thyristor. I assume that what you have is a 12V regulator and will need over 14V in. You will not have this on a bike.

    Maplin sell a CP106D SCR (400V, 2A) UM74R. This should do to drive any relay coil that you have.
     
  10. Rob_K

    Rob_K

    59
    0
    Sep 20, 2013
    Thanks duke, yes, this is what I had suspected, unfortunately their product code system is a little out there, so you can't be sure of what you get if they have popped them in the wrong box, not to worry, I am going to learn patience and get a rapid online account.

    Not sure what to do with two voltage regulators, but maybe they will be hand for another project at some time.

    Just means another week without a result.
     
  11. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    464
    105
    Aug 27, 2013
    Latching a thyristor in a DC circuit is about as difficult as dropping an egg. The outcome is highly predictable and you have to go to extraordinary efforts to avoid it.

    Fish
     
  12. Rob_K

    Rob_K

    59
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    Sep 20, 2013
    Does the diode across the coil of a relay still need to be there, or are thyristors sturdy chaps. If so, will any diode do, I have an IN4001G here, which, oh, says it's a rectifier. Darn it.
     
  13. duke37

    duke37

    5,364
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    Jan 9, 2011
    The diode (rectifier) is not necessary. They are fitted to reduce the voltage when an active device is turned off. In the case of the SCR it will only turn off when the power supply is removed i.e. it cannot be turned off.

    A diode is a rectifier but 'diode' is often used for fast, small devices such as a 1N4148. The 1N4001 is a 1A device and either word may be used. If four diodes are encapsulated in a block, it is always described as a bridge rectifier.
     
  14. Rob_K

    Rob_K

    59
    0
    Sep 20, 2013
    Yes, but the SCR is being used as the latch for a relay, I assume I will require a diode across that to protect the SCR when the power is cut?
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    It can't hurt.

    However the current trying to continue flowing will flow in the same direction, so it sill keep the SCR conducting, The SCR will not be at risk.

    The diode might protect other parts of your circuit from a brief reversal of voltage, and if that is a possibility then a diode across the relay would help (as would a reverse biased diode across the power supply connections).
     
  16. Rob_K

    Rob_K

    59
    0
    Sep 20, 2013
    This is getting better and better. I think it might be an idea to put one in as I don't want to blow my ignition system now I think about it. but all it is for is to enable a hidden push to make switch to turn ignition on without a key. However after looking at my wiring earlier today, the ignition switch wires do seem to splatter throughout the wiring loom, so better safe than sorry I guess.

    Thanks for all your help, I didn't think I would be able to get this far so easily, great forum.
     
  17. Rob_K

    Rob_K

    59
    0
    Sep 20, 2013
    Ok, This is not working for me, I have said thyristors and they work on a breadboard, but not soldered to a strip board.

    I removed it from the board and back onto breadboard and it works again. I have taken a fine saw down along the groves of the strips on the board cleaned them with contact spray, and ran a pin down them to make sure they are clean and not shorting out, but still, the thyristor just passes current without being triggered.

    What could be going on here and how can I fix this.

    Regards

    Rob
     
  18. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    As Fish4Fun says it is easy to trigger a thyristor. Some are more sensitive than others.

    It is normally recommended to put a low value resistor from gate to ground. I have seen values of 100 to 1000 used. This means that any slight internal leakage does not trigger it on.
     
  19. Rob_K

    Rob_K

    59
    0
    Sep 20, 2013
    Perfect Duke, thanks for that, I put a 150Ω resistor in, as that was what was handy and it has solved the problem at the moment.

    Should it happen again in situ, do I just up the resistor value more?
     
  20. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Raising the resistance will make the gate more sensitive. You should be OK where you are. I could not find what the SCR is that you are using. The C106 has a sensitive gate.
     
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