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Designing a Charger ? help

Discussion in 'Homework Help' started by Freeza, Sep 7, 2012.

  1. Freeza

    Freeza New Member

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    can you help me with a schematic for this

    Design a charger with constant current regulated at 500mA andvoltage max at 5.2V. Input varying between 10.8V to 13V

    by using zener diodes
    NO IC
     
    Last edited: Sep 7, 2012
  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Sounds like we would be doing your homework
     
  3. Freeza

    Freeza New Member

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    nope sir

    View attachment 67027

    can you explain to me why the output voltage is tap in the emitter instead of the collector?

    im just a newbie here in electronics ;)

    thanks
     
  4. dave

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    The transistor is in the "emitter follower" configuration. It has current gain, but no voltage gain.

    This circuit will sort-of work. The regulation will not be very good, but probably adequate for charging a battery.

    Be advised that if the input is 13V, and the output is ~5V, the voltage drop across the NPN transistor is 8V. At 0.5A, the power dissipation in the transistor will be 8X0.5=4W. It will need to be a power transistor bolted to a several square inch heat sink.

    Much better ways to do this using an IC. Why dont you want to use one?
     
    Last edited: Sep 7, 2012
  6. BioniC187

    BioniC187 Member

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    Mike, you mention it would be better to do this using an IC. What IC would accomplish this?
     
  7. Mr RB

    Mr RB Well-Known Member

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    I'm not Mike but you could just use two LM317 regulators, the first one with a resistor to make it a 500mA constant current source, the second one with two resistors to make it a 5.2v voltage regulator.

    That's a total of two 3-pin regulators, a heatsink and three resistors and job done. ;)
     
    Last edited: Sep 7, 2012
  8. 3v0

    3v0 Coop Build Coordinator Forum Supporter

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    Totally agree with RB.

    If this is not homework why limit the design to using zener diodes and no ICs.
     
  9. Freeza

    Freeza New Member

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    thanks

    but we are not allowed to use IC only zener,bjt,resistors only

    @mikeml thanks well said

    1 one more Q

    what if the output voltage is tap in the collector side?

    im just wondering
     
  10. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Look at the schematic you posted. What node does the collector connect to?

    Isn't that where the unregulated input comes from????
     
  11. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello there,

    It's good to wonder. That's how we learn things.

    Your question about taking the output from the collector is a good one. We dont do it that way however because doing it that way the transistor would exhibit voltage gain as well as current gain. We dont want voltage gain, we only want current gain, so we use the transistor as what is called a "voltage follower". This means the emitter will "follow" the zener voltage.
    In reality it is one diode drop lower (around 0.7v lower than the zener) but that just means we make the zener 0.7 volts higher and that way we get the intended output voltage at the emitter.

    We can do a design with a collector output, but because of the voltage gain we'd have to provide some feedback and that might require another transistor or other parts. We dont need voltage gain anyway though.

    Designs that do in fact use the collector as output are done that way to provide a lower "drop out" voltage. That's the difference between the input voltage and output voltage. When we need very low drop out we have to use the collector because we always have that base emitter diode drop when we use the emitter, unless of course we provide a voltage doubler but that's not usually the way it is done because it complicates the circuit even more.

    So if you dont need very low dropout voltage then use the emitter. It's a much simpler circuit and works pretty well considering it's just a zener, resistor, and transistor.
     
    Last edited: Sep 8, 2012
  12. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Who is setting those rules? You say this isn't homework?
     
  13. Freeza

    Freeza New Member

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    thanks again MrAl
     
  14. darica

    darica New Member

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    where the circuit have just said?:confused:
     
  15. meowth08

    meowth08 Member

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    I ALSO WANT TO LEARN FROM THIS THREAD AND I'M CONFUSED!!!:confused::confused::confused:

    I tried tracing and if we tap to the collector, the output would be the same as the unregulated voltage.

    But from MrAl's post:

    Question: How can there be a voltage and current gain if we tap to the unregulated input???

    meowth08
     
  16. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    That's a good question. If we have say 10v input how can we get a voltage gain like 2x as that would give us 20v output?

    The answer is that when we talk about transistor amplifier circuits and we mention voltage gain, we dont mean that we boost the supply voltage itself. We mean we boost some smaller signal somewhere else in the circuit usually the signal input not the power supply voltage input. For this circuit that would mean the voltage at the top of the zener diode. We dont want to boost that, we just want to increase the current ability of the zener so we can use a load the draws a lot of current.

    So if we want a 5v output and we use a 5.6 or 5.7v zener diode we dont want to boost that to 10v or more, we just want a little less to get the output to 5v. If we used the collector instead of the emitter as output, we'd end up with the full power supply voltage at the output because we would see voltage gain (an increase in the voltage caused by the zener alone) and that would be unregulated too. This means that if we wanted to use the collector we'd have to use some sort of special feedback to control that voltage gain as the voltage gain is harder to control.
     
    Last edited: Sep 11, 2012
  17. ikelectro

    ikelectro Member

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    CAN YOU POST the circuit here????
     
  18. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,


    Here is a typical circuit using a collector output. It has a 5v output.
     
    Last edited: Sep 11, 2012
  19. Freeza

    Freeza New Member

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    Question :D

    constant output voltage and current in the circuit even though the input voltage is changing? is this possible?


    :confused:
     
  20. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,


    Sure, a voltage regulator does that. If the input is 12v, you get 5v out, if the input is 11v, you get 5v out, if the input is 10v you get 5v out, you always get 5v output UNLESS the input goes below some minimum requirement, and the difference between the input and output at that point is called the "dropout voltage" of the regulator. For example, if the dropout voltage is 2 volts and the output is 5v, then the min input voltage is 7 volts or else the output will start to drop.
     
  21. Freeza

    Freeza New Member

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    how about the current sir .. it will remain constant even if you change the input supply ? or there are current regulators ? :p

    :confused:
     

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