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Circuit is not working

Discussion in 'General Electronics Discussion' started by vinod chandran, Jul 4, 2012.

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  1. vinod chandran

    vinod chandran

    Jun 21, 2011
    Hi all,
    Please look this attached image. This is an automatic SLA charger circuit. I built this in breadboard. I didn't connect the battery to this circuit. Just tested the output voltage with DMM. MY input voltage is 27V. The problem is, the output is only 10.4 while turning the pot fully clockwise. And in counter clock wise, the output voltage is 8.3 . The author of this circuit says that the green LED will indicate the charging process and the red will indicate the battery full. But when i turn the pot clockwise the red LED too starts to light. I think this 10.4v is not enough to charge my 12v SLA. . My assumption is either the LM317T is damaged or the zener diode is below 8.2v or this circuit needs a 12v zener. Anybody plz help me to find a solution.

    Attached Files:

    • SLA.JPG
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  2. john monks

    john monks

    Mar 9, 2012
    I suggest that you place a 5K potentiometer in the circuit with the wiper on the base of the transistor and one end to the negative line so that you can adjust the threshold voltage to the proper place. The circuit as designed is counting on the Zener and the transistor to not have any leakage. This is not very realistic. It also is relying on the LED to have a particular forward voltage drop. I see no pull down circuit for the base of the transistor. You may need to adjust your 1K resistor value up or down to get the proper range of adjustment.
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Yes, it sounds like the 8.2V zener might be leaky. You should probably use a 400~500 mW device; 1W zeners have a higher leakage current. Also you could try adding a trimpot from the transistor's base to ground, perhaps try 10K.
    This circuit seems a bit odd. There are two voltage feedback paths (the trimpot and the transistor) and no current detection (it's limited by the 100 ohm resistor). It's also a bit odd to rely on the forward voltage of the LED to control one of the voltage thresholds. Maybe you can find a better design?
  4. MrEE


    Apr 13, 2012
    This circuit automatically stops charging when the output reaches a set voltage which I believe should be somewhere around 13.8 V. This voltage is set by the Zener diode plus Vbe (base- emitter voltage of T1, and Vled (Red led's forward voltage) plus Vr, the voltage drop across the 1K base resistor.. In addition to the suggestions by other members, also measure each one of these voltages and make sure they add up to 13.8V or whatever voltage the fully charged battery should be. Also try a 9V Zener diode.
  5. vinod chandran

    vinod chandran

    Jun 21, 2011
    Thanks friends. your replies helped me a lot. I've changed the zener and everything seems to be ok. now the output voltage is 13.4. My SLA shows a voltage of 12.9. (This is amazing because this battery is not in use for last 3 months) Now i am going to discharge this battery to 11v or 10 v. Then i connect this battery to charging circuit and charge for first time. I hope a 60 % charged battery won't light up the red LED. Another headache is the led array for my emergency lamp. I need to light up two rooms with this project. all i want a small light to read books when there is no mains power. Do i need a 12 LED array (8 mm 0.5W) for each room ?. (i forget to ask salseman about the LED's voltage and current. Unfortunately i only ask him to give 1w LED and he gave me 0.5w.)
    Last edited: Jul 6, 2012
  6. vinod chandran

    vinod chandran

    Jun 21, 2011
    Hi john and Kris,
    I am very happy if you draw the changes to this image, because i am confused.
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