Connect with us

TRANSISTOR SWITCH DELAY

Discussion in 'General Electronics Discussion' started by vinod chandran, Jul 7, 2012.

Scroll to continue with content
  1. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi all,
    I just built an emergency lamp. I have made this lamp automatic by a switching transistor. but there is a latency in switching. (probably 1 second or nearer) Is that natural ? or just because of the resistance in base of the transistor ?. I copied this transistor switching idea from a 6v emergency lamp circuit. there the resistance in base was 1k and voltage coming to resistor was 9v. I decided to add a few kilo ohms in order to maintain my high voltage (27v dc). and connected a 6.7k to the base. Now the LED array starts to light after 1 second when the mains power goes. I think a relay will give faster switching than this . Do i need to add a low resistance like 2k to avoid this delay ?. This is the moment i think about a circuit simulation tool which can give me an idea about the base voltage of BD 140 in that 6v emergency lamp circuit. Plz help me .
    -vinod chandran.
     

    Attached Files:

    • DER.JPG
      DER.JPG
      File size:
      15.7 KB
      Views:
      321
  2. poor mystic

    poor mystic

    1,067
    31
    Apr 8, 2011
    :)
    Hi Vinod
    I'm sorry to say that your questions betray a completely wrong idea of how transistors work. Resistance cannot be responsible for the "latency" in the transistor.
    Transistors have the characteristic that they pass a collector current which is proportional to the base current. This characteristic is exploited in switching circuits, where base current is held at 0 for the "off" condition of the switch, and current allowed to flow into the base to switch the transistor on.
    A capacitance (which stores, and can temporarily divert electric charge) could be responsible for the switching delay. There may be a capacitor in the charger circuit.
    Your circuit diagram is not complete - it does not show where the led array draws its power from.
     
    Last edited: Jul 7, 2012
  3. davenn

    davenn Moderator

    13,674
    1,892
    Sep 5, 2009
    from the SLA battery ;)

    tho he has forgotten to show the negative side going back to the battery,
    rather an earth connection that no other negative supply is connected to

    Vinod, you need to edit your diagram ;)


    Dave
     
    Last edited: Jul 7, 2012
  4. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi poor mystic and davenn,
    I am sorry for the mistake . Anyway the information which poor mystic gave is very helpful.
     
  5. weird_dave

    weird_dave

    36
    0
    May 9, 2012
    I suspect the charger circuit has enough capacitance to keep the power rail active for a while, a diode in-line would stop the capacitance backfeeding to the transistor
     
  6. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi all, this is the circuit. The main problem is the two LED's are glowing all the time. My battery shows 13v . According to the author of this circuit charger stops when battery gets 13.7v. I hope then the charge indicating LED (green) will be off.
     

    Attached Files:

    Last edited: Jul 8, 2012
  7. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi weird_dave,
    Please indicate in this circuit that where to connect that diode.
     
  8. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    The voltage is dependent upon where VR1 is set, it you set it too low the battery will never get to the 13.5v or so it requires to switch over...

    You do have the proper zener diode installed in the circuit, right?

    This page will give give you details on how that charging circuit works...

    http://dmohankumar.wordpress.com/2011/08/03/4653/
     
  9. poor mystic

    poor mystic

    1,067
    31
    Apr 8, 2011
    I think that Weird Dave suspects that a capacitor in the charger circuit is storing charge which is being fed back to the base of the transistor. (I agree that it's likely.)
    So the diode he suggests will stop current flowing back to the base from the charger circuit.





    Hey I used to know a "Crazy Dave" who went up Bethunes Gully and tried to steal a helicopter, but he was crazy.
     

    Attached Files:

    Last edited: Jul 9, 2012
  10. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi cocacola,
    That VR1 is still confusing me. When i turn that to one side the red led will be off. and when i turn that to another side (most probably midway) red led starts glowing. Anyway i am planning to connect a LM 3914 monitor cum cut-off circuit to this charger. Last day i got the 3914 circuit that energize a relay when battery gets full charge. But the problem is there is no space in that old UPS cabin which i selected for this project .
     
  11. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi poormystic ,
    I just removed the cap (2200uF 72v) from this circuit and now there is only one cap (470uF 40v). Now the light starts right after the power goes off.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-