Circuit help

Discussion in 'General Electronics Chat' started by oidium45, May 9, 2010.

  1. oidium45

    Thread Starter Active Member

    Apr 24, 2010
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    I am thinking about setting up this circuit but have a few questions. It is a bit more complicated then the circuit that I originally intended to use.

    They may seem simple questions but I am a still rookie here.
    1. What is the -10 on Q1 and Q2. Does that mean that I will have two -10v supplies?
    2. Can someone recommend a resistor rating or part number for Rsc? Is a "current sense resistor" the same as a "current shunt resistor"?
    3. Do I have to use electrolytic caps for the 0.1uF and 10uF or is there a better option?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    At some point, everyone was a rookie ;)
    Nope. Those two points can be considered to both be connected to -10v.

    Current sense resistor. You should really look in the datasheets for the LM117/LM317. There are example schematics and much more detailed information in there. National Semiconductor has a good datasheet on the LM117/LM317.
    For the 1uF, use metallized poly caps or ceramics. You might use tantalum caps, but they tend to explode every once in awhile. I've never had a ceramic or metal poly cap explode on me.

    The idea is that the 1uF cap is placed as close as possible to the regulator to "swamp" any tendencies for it to oscillate.
     
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  3. oidium45

    Thread Starter Active Member

    Apr 24, 2010
    130
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    Yes, there is good information and schematics on the data sheets. I pulled this schematic from the the link below. Figure 22.
    http://www.datasheetcatalog.org/datasheet2/a/0s922lq6p7gwsc2hhzx6z4e3pl3y.pdf

    There were several "simpler" circuits that I did understand, however the -10v and Rcs is something new to me. I like this circuit because it has both a voltage "and" current adjustment with vary few components (most of which I already have at home). The other circuits provided by the data sheets only have a voltage adjustment.

    So now I have another question.
    How do I obtain -10v for the connections at Q1 and Q2? Can i use a few rectifier diodes to obtain this from the transformer? If so, how would I go about connecting them to the circuit? And, do i need a transformer capable of supplying both negative and positive outputs?

    I am still a bit hazy on the Rsc. I have found information online about calculating resistance values for Rsc. And I have found resistors being sold as "current sense resistors". Unfortunately even these resistors require that you choose a value.
     
    Last edited: May 9, 2010
  4. oidium45

    Thread Starter Active Member

    Apr 24, 2010
    130
    8
    I found this diagram on one of the data sheets (figure 26). Can I use a similar formula to determine Rsc? Perhaps I am looking at this wrong but in order to get a close estimate I should be able to use this right?

    Vref(1.25v)/Rsc =Iout (=1.5A)

    Vref- Is listed as 1.25 average on the data sheet.
    Rsc- In the diagram would be the resistor value that I need to determine.
    Iout- I would like to be around 1-1.5A.

    So I am working with:
    1.25v/(x-ohms)=1.5A
    (x)=0.83 ohms +/- 0.01

    Like I said, I could be completely mistaken. If so, does anyone have a formula that I can use?
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Your LM317 voltage regulator will get too hot then it will shut-down when the output voltage is set fairly low and the current is fairly high.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    People wanting to build a wide-range power supply using an LM317 etc. is a recurring theme on the board.

    You really need to narrow down the range of output voltage that you require.

    Let's say you built a 0v to 30v 1.5A supply using a linear regulator. Your unregulated input is a magically rock-steady 40v.

    Let's now say you have a load that requires 30v @ 1.5A.
    Power dissipation in the load will be 30v*1.5A = 45 Watts. What about the regulator?
    Vin = 40, Vout=30, I=1.5, so (40-30)*1.5=15 Watts. That's a lot of power to dissipate.

    Let's see what happens with a 1A load that requires 5v.
    Power in the load is 5 Watts; 5v*1a=5W
    Regulator: (40V-5v)*1A = 35 Watts - Yeow! Our poor regulator is mighty toasty.

    A LM317T regulator comes in a TO-220 package; thermal resistance from junction to package is 4°C per 1 Watt. It will shut down if it reaches 125°C.

    If you have an imaginary ideal heat sink (impossible, but just play along), at 35W, the junction will be 4°x35W = 140°C hotter than the package. This means in order to keep the package from shutting down at 125°C, you would have to keep the regulator at 125-140=-15°C; freezing it into a glacier would not be enough cooling.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    When the input to output voltage of an LM317 is more than only 15V then it reduces its max output current to protect itself. The datasheet lists a guaranteed output current of only 150mA when its input to output voltage is 40V.
     
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