LDR switch

Discussion in 'The Projects Forum' started by vinukc, Jul 2, 2011.

  1. vinukc

    Thread Starter New Member

    Jul 2, 2011
    i've an emerjency light circuit. i want a ldr switch for it to make it automatically turn on when power faiure. My input voltage is 52v dc. i attached the circuit here. plz help me
  2. simo_x


    Dec 23, 2010
    I don't understand very well your circuit.
    First of all, maximum voltage input for LM317 is 40V.
    This 52V are VRMS, right? You should also put a capacitor for the ripple voltage....

    Finally, where is the LDR?
    Last edited: Jul 2, 2011
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
    The bridge rectifier you are using is incorrect. If your input voltage is 52V DC then you do not need the rectifier anyway. The LM317 can only accept an input voltage up to 40V. An alternative part would be the LM2591HV which accepts voltages up to 63V. Your desired output voltage is not known, but is assumed to be around 36V as you want to supply 12 3V LED's in series. We also do not know the Vf or If of the LED's.

    Why a source voltage of 52V? You would do better to make 3 branches strands of 4 LED's each and have a supply of 12V. This way you will not need a voltage regulator at all, just a current limiting resistor on each branch. Ideally you want a constant current source as apposed to a constant voltage.

    I think the first thing is to understand the source voltage. Is it a Battery? If so what is the AH rating? Is it a DC wall adapter?
    If so what is the current output. Do you have any other Wall adapter with a lower voltage that could be used? Also What are the specs on the LED's. You mentioned 3V LED's. This would presumably be the Vf (Nominal Forward Voltage). What is If (the nominal forward current). 20mA?
    Last edited: Jul 2, 2011
  4. SgtWookie


    Jul 17, 2007
    From the input voltage, I'm thinking that our O.P. wants to run the LEDs from the telephone line. If so, the telephone company will disconnect your telephone service; they are not a power company, they are a telephone company.
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    The rectifier may be included to stop the ringing current reversing the supply. One might think that someone would have to be pretty desperate to want to abstract a few mA from the phone line to get a light during a power cut, but then again life is very difficult in some parts of the world.
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
    @ Ionic: in principle the maximum input voltage for such a scheme would be the sum of the 40V input-output differential fr the LM317, plus ~36V for the LEDs. In practice, this thing still looks likely to be blown up by the ringing current, and as for line surges, lightning etc...:eek:

    Perhaps the OP would be better advised to get a small rechargeable battery for his lamp. Much easer to engineer, safer, and legal.