Switching a TIP41 Directly from the output of a 741 IC

Discussion in 'The Projects Forum' started by RodneyB, May 28, 2013.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I have made a daynight switch for my LED solar lights and have been switching a relay through a BC547 transistor.
    I have included a time delay by doing the following
    From the output (pin 6) through a 30K resistor a 470μF capacitor is charged. A 100K resistor feeds the base of a BC547 which turns on a 12 Volt relay.
    For the sake of Space I want to take out the 12Volt relay and substitute it for a TIP41.
    My question is will i be able to substitute the BC547 with the TIP41 ( Naturally observing the pin spacings are not the same) Or will I have to have the BC547 to turn on the TIP41.
    Many thanks in advance for the help.
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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    741 can provide only small current and since tip41 has low gain. you need another transistor to drive it
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,963
    743
    I would use a darlington configuration with a BFY50 switching the Tip41 on.
     
    Last edited: May 28, 2013
  4. ramancini8

    Member

    Jul 18, 2012
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    Other transistors in the TIP series are Darlington configuration, or you could use a FET.
     
  5. panic mode

    Senior Member

    Oct 10, 2011
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    can you post entire circuit? 100k base resistor is huge for BJT.
    how was the relay contact used? do you need high or low side switching?
    what is the voltage swing of the output? do you need isolation?
     
  6. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    As requested here is the complete circuit diagram. What op amp can I use in place of the 741. Also I want to build this circuit with SMD components. Are SMD LDR's available
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    have the bc547 turn on the TIP 41. However, you will have to re-configure your LED arrays because the bc547/TIP41 pair will form a darlington and reduce your voltage by 1.2 volts (approximately) for the base-emitter voltage. I am not sure you will be able to light 4 white LEDs (~3.3 volts each) with 1.2 volts less than your current relay allows.

    Your time delay may need some help if you want smaller components, sharper on/off of LEDs, more accurate timing.

    Finally, your opAmp may as well be open gain. You have neither an inverting or non-inverting circuit here. The 1Meg resistor can be eliminated.
     
  8. panic mode

    Senior Member

    Oct 10, 2011
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    do not make a Darlington, keep collectors apart:
     
  9. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Thank you for the reply I have been off line for awhile internet in my country erratic at times.

    The Value of the resistor on the collector of the BC547 what should it be.

    Thanks

    Rodney
     
  10. wayneh

    Expert

    Sep 9, 2010
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    Everything to the right of the op-amp could be replaced by a MOSFET. There is plenty of voltage available to drive the gate of a normal, n-type MOSFET such as IRF540N operated as a low-side switch. No darlington, no relay.

    The 741 is the oldest op-amp available and I would never choose it for any project. Nevertheless, it will work for this one.
     
  11. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Thank you for the advice the resistors as they are to the base do they just connect to the Gate.

    I would be very interested in an optional op amp to the 741.

    Many thanks

    Rodney
     
  12. odinhg

    Active Member

    Jul 22, 2009
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    I think TL071 is a good replacement.
     
  13. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Thank You !
     
  14. wayneh

    Expert

    Sep 9, 2010
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    There is no need for a resistor between the op-amp and the gate of the MOSFET, although it wouldn't hurt to place one there with any value <1kΩ. It slows switching time, which could make your MOSFET heat up if you are switching a large load at high frequency, but it would also serve to limit the current from the op-amp into the capacitive load of the gate. That's not really much of a concern I think.
     
  15. BobTPH

    Active Member

    Jun 5, 2013
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    Since you are using the op amp as a comparator, I would substitute a comparator for the 741.

    Bob
     
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