78xx ic regulator query

Discussion in 'The Projects Forum' started by 007trains, May 20, 2011.

  1. 007trains

    Thread Starter Member

    May 20, 2011
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    I have a small project running where i am building a small wind turbine producing up to 30volts for a remote shed running about 14-15volts. I have two 7.2volt ni-cd battery packs that i will charge. I was wondering rather than using a 7815 ic regulator to charge the two battery packs could i put one 7808 ic on each battery pack whilst drawing power from the battery packs. And could i run both ic's from the power same source.

    Any tips or suggestions welcome. :)
     
  2. debjit625

    Well-Known Member

    Apr 17, 2010
    790
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    Many times we charge rechargable battery with a bit higher voltage but only if it is specified for the battery,I am not sure for your ni-cd battery check out any document for the battery if available.

    You can use LM317 voltage regulator to get 7.2 volt.

    I didnt understood,anyway you can regulate the turbine's voltage using the IC and charge the battery using that regulated voltage.

    Good Luck
     
  3. 007trains

    Thread Starter Member

    May 20, 2011
    13
    0
    i have a wind turbine that is supplying the power and i was wondering if i could run both ic's off the one turbine rather than making another or rewiring it.

    nicd's can charge at that voltage safely, i think it is 1.47volts per cell while charging.
    .
    my real query was if i could charge the batteries separately without causing any faults with the ic's.
     
    Last edited: May 20, 2011
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    Please ignore everything here as it was intended for another thread.

    There's nothing to see here. Move along.



    The LM regulators from back then were tough little beasties, very hard to kill. They self-protect for over current and over temperature issues.

    When running normally (fet full on) I would assume you get close to 12V out for those lamps; with 19V in you have (19-12)*2.7A = 18.9 Watts in the LM338; that is a lot of power!

    Also, when you ramp the fet off you are asking the fet to see the full current supplied to the lamps, as this is a constant current driver. I would expect the voltage to the lamps to initially increase in pace with the fet's voltage until the LM338 starves out (is giving it's max voltage output but at minimum power dissipated) and only then will the fet be able to "steal" voltage from the lamps and dim them. Ultimately the circuit will have the full starve out voltage on the fet, nothing across the lamps, and no current.

    Assuming the LM can put out a max of 15 V when in current regulation the power is being dissipated in the fet will go up to 15V * 2.7A = 40 W just before it starves out the LM. That's not a constant power and does depend on how long and how frequent the transition occures. But there is a ton of current in the fet too.
     
    Last edited: May 21, 2011
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    NiCD's really should be charged at a constant current until they are nearly fully charged, and then charged at a constant voltage. The 78xx series regulators have fixed voltage output, so they are not really suitable for use as battery chargers.

    Charging 7.2v batteries from a 30v source using linear regulators will be very inefficient, as for every Watt of power dissipation in the battery, you will have roughly 3.3 Watts or more power dissipation in the regulator. That's less than 24% efficient.
     
  6. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    Quick answer: Yes. You can run as many regulator chips as you want, in parallel, each one charging a battery.

    You can select a low voltage chip like a 317 and use resistors to get XActly the voltage you need.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Well, to fully recharge a 7.2v NiCD battery, you would need to get it up to around 8.4v. As I mentioned before, you should not try to just use a fixed voltage regulator, as charging current would be maximum that the regulator can supply until it reached close to 8.4v.

    See the attached circuit and simulation. The circuit is designed to limit charge current to 150mA until the battery is nearly charged to 8.4v; then current decreases until it is only a trickle to keep the battery maintained at 8.4v.

    You need to check the manufacturers' specifications for the batteries that you want to charge, and adjust R4's resistance to suit the required charging current - but beware of power dissipation in the regulator; it will be rather high due to the large difference in input and output voltages (30v in vs ~7v out); 150mA charge rate will cause ~3.45 Watts power dissipation in the regulator; and you will need a good heat sink for your regulator.

    You set the terminal charge voltage using a capacitor (basically anything will work; suggest 10uF to 1,000uF) and a resistor (1k to 10k) in parallel across B+ and B-, and adjust R2 until you get exactly 8.4v out.

    Perhaps it isn't obvious, but the rectangle on the right enclosing Rbat and BAT1 is a rough simulation of a battery; your real 7.2v battery takes the place of that.
     
  8. 007trains

    Thread Starter Member

    May 20, 2011
    13
    0
    Okay so i know how i will set up the ic's but one problem using a fixed current to charge the batteries is that the batteries will have a varying load (short periods of high power draw) thats the reason i wanted to use a 7808 ic or a similar setup
     
  9. 007trains

    Thread Starter Member

    May 20, 2011
    13
    0
    also i am less worried about efficiency when the voltage rises, the wind turbine produces more power than i really need but i would like to be able to use the low voltages it sometimes produces (6-10v in low wind)
     
    Last edited: May 21, 2011
  10. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    No fair! Now you're talking about adapting to every condition that can happen to a wind turbine. Oh...too much...head hurts.
     
  11. 007trains

    Thread Starter Member

    May 20, 2011
    13
    0
    yeah tell me about it. :confused:

    How about we keep it simple and see if it works. if i use two 7808 ic's like i originally planned to charge the two battery packs i should be able to use a decent range in voltage and hope like hell i don't blow up the batteries in the process. :D

    If anything does go wrong i'll drop down to either a 7806 or a LM317 and just have to deal with the lower voltage some other way.

    So thats the plan; Any objections... :cool:
     
  12. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    I know that some noobs, think LM317 will be too complicated to bias compared to 78xx but its not,you just need to calculate two resistors.Here is the datasheet you will find all details in it... http://www.national.com/ds/LM/LM117.pdf

    Their are many charger circuit using LM317 ,you may google them..

    Good Luck
     
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