Power supply circuit...

Discussion in 'The Projects Forum' started by tofts, Aug 15, 2010.

  1. tofts

    Thread Starter New Member

    Aug 15, 2010
    2
    0
    Just a quick query, I have designed a quick schematic (see attached) for a power supply.

    I needed it to deliver somthign like 1a at 9v output total with the ability to charge a 1.5ah 12 battery so that when AC is not around the battery will take over. I supose it could eb classed as a UPS circuit. I woudl just like your expertise on the circuit before I go ahead and order the relevant components.

    My one concern was chargign current, with a 47r resistor, will this be too much/too little to charge the battery. i wanted a current of around 250-300mah and with an aprox charge voltage of 15v, 47 ohms seemed like a good idea.

    The BC549 is to ground out the adjust pin of the 317, so that when the battery reached 15v the zener woudl conduct and reduce the output of the first 317 to about 2v, thus stopping charge. This would obviously mean the battery wodul take over powering the output.

    The TIP127 is so that when the battery voltage reached just over 10v, the second zener woudl stop conducting and mzke the PNP base positive and cutting the output preventing deep discharge.

    Do you think this circuit will suffice or shoudl I make some alterations any where?

    Thanks for your aproval,

    Jody
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    I'm not going to comment too deeply on this circuit, but you are missing a couple of minor points. The LM317 datasheet shows capacitors on both the input and output of the device. Electrolytics on the input along with something in the 0.1µF range on both the input and output of the device (s). This is to prevent the regulator from oscillating, and is required.
     
  3. tofts

    Thread Starter New Member

    Aug 15, 2010
    2
    0
    Thanks for the comment bill, I have omitted the rectifier circuit for clarity which of course will have a filter capacitor before the first 317. According to the datasheets, the second capacitor is to improve transient response but is optional. Seeing as my application is non crital I didn't think it was especially important. However, I have just wondered if I woudl need to impliment a small 0.1uf filter capacitor before the second 317?

    What did you mean by not commenting too deeply upon the circuit, is that implied there are further faults here or is it generally ok?

    I "thought" the circuit was ok, my largest concern was if I had calculated the charging current correctly.

    I know my way around the odd circuit, but I don't profess to be any sort of expert in these matters. i am mearly learning the ropes and thus any insightful comments would be most apreciated.

    With thanks again,

    jody
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    It means I would have to spend an hour or so studying it to fully understand it. I am currently up to my neck is other projects, both mine and others.

    Have you ever used a SPICE simulator? They are ideal for this kind of work.

    I don't think the 0.1µF is optional on the outputs. I've had bad experiences with oscillations, and caps are cheap.

    Speaking of which, I've got to follow my own advice on this circuit. It was something I designed almost a decade ago.

    SLA Battery float charger w/ LM317T

    It has some features in common with yours.
     
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