A Regulator???

Discussion in 'General Electronics Chat' started by Raam, Apr 22, 2008.

  1. Raam

    Thread Starter New Member

    Apr 22, 2008
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    Hi- I am in need of a circuit that takes a 5Hz 8V(Peak to peak) sine wave input, whose load(s) can draw a maximum current of 10A (but not always).The circuit mustbe able to supply 10A to the loads, without a significant voltage drop.Loads are resistive, but stray reactances(especially inductive) cannot be avoided.

    Can the load be simply connected to the collector of a suitable BJT/MOSFET? (The supply voltage that is,Vcc of the BJT/MOSFET,cannot possibly exceed 14V), and the input be connected to the base of the Transistor? Will it work???

    If yes, then I wud proceed looking for the transistor with sutable ratings...
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Transistors tend not to survive long at this curent level, even on massive heatsinks (yes you would need them).

    Can the load be connected across a diode bridge with the AC as the bridge input?
    You can control the supply using scrs in the bridge then with much better efficiency.
     
  3. Raam

    Thread Starter New Member

    Apr 22, 2008
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    The loads are supposed to draw DC current only( though not strictly a pure DC).

    I am actually going to use this sine wave generating circuit to test the tolerance of the loads to low freq sine wave( by making it act as an unstable RPS). So, I thought a transistor wud do.( get the picture of what I am trying to do???)

    I have several loads, and if all of them are switched on, they wud collectively draw a current of not more than 10A.

    Do I still need an SCR??? I also wish to limit the circuit to cheap and easily available components. wud I still need SCR???
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    First mention of SCR. Where is the SCR in the circuit? Remember, SCR's turn off when there is no current. John
     
  5. Raam

    Thread Starter New Member

    Apr 22, 2008
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    I sent the quote by mistake. Could u kindly refer to my previous post, John?
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Well yes the output of a diode bridge is DC.

    I'm just trying to get some details to test the feasibility of my proposal before detailing circuits. There are several ways of using controlled rectifiers in conjuction with or as part of bridge circuits.

    You say the current draw is ' up to 10 amps '

    Firstly are we talking about using the 5Hz 8Vpkpk to supply the 10 amps curent?

    Or is there another DC supply you haven't mentioned?
     
  7. mrmeval

    Distinguished Member

    Jun 30, 2006
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    What is generating the AC? What are the loads specifically. You can find a transistor that can handle 5hz at 8v and 10 amps current, that's approximately 80 watts.

    It sounds like an audio amplifier.
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Yes of course you can, but one transistor will either operate in class A and be less than 50% efficient or you need at least two of them.
     
  9. Raam

    Thread Starter New Member

    Apr 22, 2008
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    I will be using another power supply. The 5v 8hz pk to pk signal will be generated by means of a function generator.It cannot supply more than 50mA. The regulating/ switching transistor has to supply the 10A current needed by the loads.

    I am still confused. I too came up with the idea of power amplifiers, but 5hz is too small a freq to fall under audio range. Moreover, power(audio) amplifiers are rated for 4,8 and 16 ohm (speakers) generally. Since the loads are not fixed, (there may be upto 6 loads!), I will not be knowing the exact impedance. How can I go for a power ampliier like class A, Class AB, etc.?
     
  10. Raam

    Thread Starter New Member

    Apr 22, 2008
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    I think that purchasing a new AC voltage source (with 10A max current capability) will be uneconomical, since I will be using it only for testing purposes. That is why, I am simulating the 5Hz 8V p-p signal by means of a function generator, and letting the active device(BJT/ MOSFET or whatever) regulate the current demanded by the load. Sinking the heat would not be a problem,I think, since I will operate the circuit in open condition(meaning without a box or anything), and the circuit will not be continuously supplying 10A current.

    Am I making any sense? Is my idea feasible???
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Well, National Semiconductor makes power operational amplifiers.
    The LM675 is one of these ICs; it has a 3A output and a broad bandwidth.

    More suitable for your requirements might be the LM12CL (13A, 80W)
    http://www.national.com/mpf/LM/LM12CL.html
    At roughly $45 each, they're not cheap.

    There's the LM3886, which is actually a 11A/68W audio amp IC.
    http://www.national.com/mpf/LM/LM3886.html
    At $3/each, they're pretty reasonable. You'd need a mondo heat sink.
    But, I don't see why you couldn't use one in a voltage follower mode. Connect the output to the inverting input and your load(s), the noninverting input to your signal generator output, supply pins, and use the mute pin for your emergency off or whatever.

    You could use a left-over ATX-form factor power supply from a computer for your 10a supply. Converting them to a bench supply unit isn't hard - a quick Google search on "ATX bench supply" will find you quite a few how-to's. You might need to use a couple of them in parallel. I have a 250w unit that puts out just 8A on the +12v; you might find that a 400W or so unit has more current on the +12v.
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    OK let's summarise.

    You want a module to supply a 5Hz, 10amp (peak) sine wave 8Volts peak to peak to a resisitive load.

    You have or will have suitable power supply for this module.

    Your 5Hz source is a standard signal generator.

    ***************

    Your figures imply 2.83 Volts rms and 7 amps rms.
    This implies a load of 0.4 ohms.

    This puts your module in the realms of DC servo amplifiers. These operate in the range DC to a few hundred Hz.
    You do not want AC servoamps they operate at much higher frequencies and you are quite right in shying away from most audio amps here, they operate into a higher load impedance.

    Your choices are to look for a standard module or to 'roll your own'.

    I don't know where in the world you are or what your budget is but neither option is very cheap. I've seen Sanyo modules on American Ebay in the $30 to $100 range.

    I doubt that the components for a homebru would come much cheaper.

    A final thought. A very high power audio amp could be used if you were prepared to waste most of the power passing the curent also through a high power series resistor.
    Some PA amps are designed to operate into 2 ohms.

    Let us have your thoughts.
     
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