Voltage Divider - Safety

Discussion in 'General Electronics Chat' started by TheDag, Mar 28, 2012.

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  1. TheDag

    Thread Starter Member

    Jan 1, 2010
    94
    1
    For divide the voltage in half,
    What's more safe ?
    2 x 1 ohm
    or
    2 x 10 ohm
    or
    2 x 200 ohm

    just an examples to know what is more safety, bigger value or lower value...
     
  2. nuckollsr

    Member

    Dec 17, 2009
    16
    1
    It's not so much a matter of 'safety', i.e. reduction of personal hazard as it is of performance. Using a resistive divider to achieve some new, lower voltage from a power source is an art that goes back to the earliest days of electronics. "Bleeder" resistors on high voltage power supplies often had taps at one or more places down from the top to provide lower voltages for some lower current loads. The operative word here is "LOWER". Assuming that the circuit you wish to power at 1/2 of supply voltage draws a significant current, the voltage presented at the junction of your two resistors will be LESS than the calculated value of 1/2.

    Further, the degree by which the voltage sags from expected is a function of the size of your resistors. For example, using a pair of one ohm resistors to develop a 5v source from a 10v supply would cause 10/2=5 Amps of waste or parasitic current to flow in your divider. The equivalent source impedance of your new 5v source is the parallel value of the two resistors . . . or 1/2 ohm. So, if you attempt to draw 100 milliamps off the 5v tap, the 1/2 ohm equivalent source impedance will drop by 0.1 amps x 0.5 ohms = 0.05 volts; or 50 millivolts. Not too bad . . . but your 10v supply is suffering a 5A load to produce a high quality 5v source at 100 mA.

    Okay, raise the resistors to 200 ohms for a total of 400. Now the no-load current through the divider is 10V/400=.025A or 25 milliamps. MUCH better from the perspective of reducing wasted energy. How much current can we take from the divider tap and still keep the voltage drop at 0.05 volts? With a pair of 200 ohm resistors, the source impedance is 100 ohms. 0.05 volts divided by 100 ohms yields 0.0005 amps; or 0.5 milliamps!

    Comparing these two extremes illustrates the frustrating fact that voltage dividers are exceedingly wasteful of energy and/or unstable sources of voltage. This is why you never see them used to supply significant POWER to circuits. They are generally limited to setting gains and establishing reference voltages where the currents involved are exceedingly small.
     
  3. Stuntman

    Active Member

    Mar 28, 2011
    181
    47
    Well said nuckollsr!

    Not much that one can add to that. But I will mention that a general rule of thumb, is to use a voltage divider (or potentiometer) which has an impedance at least one order of magnitude lower than the load impedance.

    Example: If you want to design a divider that will feed voltage to a 10K ohm load, the total divider resistance should not exceed more than 1K (500Ω + 500Ω, 250Ω + 750Ω, etc).

    This is simply a guideline to minimize the issues nuckollsr describes above.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,452
    3,371
    I second that.
    Use a total divider circuit that draws 10 times your load current.
     
  5. TheDag

    Thread Starter Member

    Jan 1, 2010
    94
    1
    For automotive purposes...
    In this case, which resistor is better?
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    For automotive purposes this thread is toast.
     
  7. TheDag

    Thread Starter Member

    Jan 1, 2010
    94
    1
    What do you mean?
     
  8. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    It's against the rules of this site to discuss automotive applications. Look at the top of the "chat" page and see, "No more Automotive..."

    There is also a "Terms of Service" document somewhere.
     
  9. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Thus the membership has spoken truly.

    The All About Circuits forum Administrative Team has elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

    This thread is against the AAC forum rules, Chapter 6, as seen here:

    http://www.allaboutcircuits.com/l_tos.html


    Automotive modifications of any kind are strictly forbidden. Therefore, this thread will be closed.

    Please try to understand the reasons behind this action, and feel free to browse and use the forums.

    You might find answers to your questions in one of these forums:

    http://forum.allaboutcircuits.com/sh...ad.php?t=54400
     
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