Quick and Dirty for Loaded Voltage Dividers

Discussion in 'General Electronics Chat' started by TEX, Apr 8, 2007.

  1. TEX

    Thread Starter New Member

    Apr 7, 2007
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    Hello Everyone,

    I will be using an 18V (Vs) and R1...R4 to supply 9V (for LM311N & 741 OP amp), 6V (for a PIC16F628) and then I will be needing the other for a 3V reference.

    I've looked through all my stuff from tech school (1984) but must have lost the notes for calculating loaded voltage dividers.

    All the exaples I find lately on the web have been based on a R1,R2 scenario.

    Can anyone steer me in the right direction or just refresh my memory?

    Tex
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    Why on Earth would you want to use voltage dividers to supply power to your circuits? It has to be the least useful method, to say nothing of the fact that the regulation is non-existant and any change in loading produces a change in voltage. You should use three terminal regulators without giving the matter a second thought. At the currents low enough for a voltage divider to work, a 3-terminal regulator won't generate enough heat to even need a heatsink.

    Where did this notion of using voltage dividers come from anyway? It is one of the most pernicious myths on the internet forums.
     
  3. TEX

    Thread Starter New Member

    Apr 7, 2007
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    Basicaly, my circuitry has to fit on a board 1"X1". The pcb itself is dual layered and the smd's are taking up quite a bit of space themselves.

    Is your answer to the Voltage Divider question coming?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Papabravo is correct in his assessment of the viability of using voltage dividers to obtain voltages for the purpose of powering your ICs.

    Maybe you can answer a couple of questions that would help us better assist you.

    Have you considered powering those devices like the 741 and the 311 from 18 volts and then using the voltage dividers at their respective output to achieve the signal level you need at the final output?

    As for the PIC, I think you will need to provide a three terminal regulator to generate the voltage it requires.

    I will need to know more about what your plans are for the 3 reference in order to make a recommendation.

    With more information there is surely a way to get you where you need to be.

    hgmjr
     
  5. TEX

    Thread Starter New Member

    Apr 7, 2007
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    Ok thanks for the inquiry,

    On a 1x1 board I have the following: 3 individual 741's and 3 individual 311's, 3LED's and finally a pic for control, plus all the rest of the components to drive the circuitry. So far this is possible, but when I utilize the 7812 the boundaries change significantly enough that my board size is outside the 1x1 limitation. I have considered design of a board stack configuration but didn't go there yet.

    The idea here is to pulse the 741 and then both time the return pulse (around a closed loop) and then provide a nominal voltage ref for the 311 to compare what it receives, delivering a hi/lo to the pic which in turn lights LED's to alert the end user of the status of the loop.

    This circuit app, as fundamental as it seems, will be used to save thousands of dollars during a normal production day (oil exploration), but it will be used by, well lets just say people that throw things around quite regularly. So, the design spec cannot be any larger, and needs to be very rugged.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    Were you considering a 7812 in a surface mount package?

    hgmjr
     
  7. TEX

    Thread Starter New Member

    Apr 7, 2007
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    Yes. I am planning to use the MC7800 series.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    Do you think you could get away with using a dedicated 78LXX in a TO-92 package at the site of each of the devices you need to power?

    You can even form the leads and install it on surface mount solder pads.

    I suspect you will be potting this module anyway to ruggedize it.

    hgmjr
     
  9. TEX

    Thread Starter New Member

    Apr 7, 2007
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    I will probably use the DPAK-3 of the MC7800 series to save space. It has a 1amp pos capability, not that the circuit will require this much though. I guess I will use the voltage dividers (5V and ref voltage) in seperate configurations.
    I was just trying to both save some money and reduce the amount of space used(in case of future expansion).
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    If you are comfortable doing so, You can post your schematic and we can take a look at it. Maybe we can come up with suggestion on ways to simplify your circuit.

    hgmjr
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    I recommend the 78Lxx in a TO-92 package. As has already been mentioned it has a very small footprint, your circuit does not reuire much current, and you'll probably be potting the circuit anyway.

    Are you mounting active components on both sides? Most assembly houses can do this nowadays.

    There is one other option, and that would be to supply regulated voltages with sense lines to the board.
     
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