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Please help me find values for this simple circuit

Discussion in 'Electronic Design' started by Gary, Jun 15, 2007.

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  1. Gary

    Gary Guest

    Could someone tell me what the values of a,b,c,d,e,f are for this

    A supervisory alarm is defined as the loss of the part of the circuit
    that is outside of the thick black box.
    Vin is the voltage measure at the Input Module.
    The voltage shown is 28VDC, but I need to know what the values will be
    for a 24VDC scenario.

    Thanks very much!
  2. Robert Baer

    Robert Baer Guest

    Well, it would be useful to know (measure directly or indirectly) the
    input resistance of the module (and maybe if it is relatively constant
    WRT voltage)...
    Then one could use Ohms law, etc to calculate voltages using a 28V
    supply and then do another set of calculations with the persumed 24V supply.
    Most likely those resistors are 10 percent tolerance, and the ranges
    (a<V<b, c<V<d and e<V<f) are wide enough that the system will still be
    reliable and reasonably robust with a 24V supply.
    Naturally, it is useful to know the values of a, b, c, d, e and f.
    I find it strange that an alarm system is designed around a
    non-standard battery voltage.
  3. For 24V: R4= 10k, R5= 9.66k, and R6= 2.33k.
  4. What is non-standard about 24/28 volt batteries? Everything from Cessna to
    Boeing uses this as the primary dc battery voltage.

  5. Gary

    Gary Guest

    I meant to say I need the values of:

    voltage a
    voltage b
    voltage c
    voltage d
    voltage e
    voltage f

    based on using a 24V not a 28V source.
  6. Jasen

    Jasen Guest

    redraw the circuit for each case, then apply krichorff's laws and
    basic algebra.

    any further questions belong in sci.electronics.basics

  7. Fred Bloggs

    Fred Bloggs Guest

    The ratio of Vmodule/Vbatt runs at 1/11, 1/8, and 1/6 for the respective
    circuit conditions. Beyond this it is impossible to say without
    knowing the ratiometric extents of Vbatt and the resistors relative to
    their nominal values. Once *you* determine these parameters, simply sum
    and difference those errors from 1 and multiply by Vbatt to get your bounds.
  8. Robert Baer

    Robert Baer Guest

    24V is standard, 28V is not.
  9. In practice, they are used interchangeably to mean the same thing. 24 volts
    is comparable to an automotive "at rest" 12 volt system and 28 volts is the
    "under charge" 14 volt automotive value. If you had ever worked in the
    aviation environment you would know that.

  10. Gary

    Gary Guest

    That's right, indeed, that's what I'm asking for in my post.

    I used to have a program called Electronics Workbench. When I had a
    simple circuit like this, I would let the program use whatever its
    default values were for everything, I would wire the circuit, set the
    resistor and voltage values, and click "Run" with the switch open
    (Normal), and then closed (Alarm), and then in the supervisory alarm
    failure position (the 6.8k disconnected). The results were all I
    needed for my purposes.

    This is the same situation. I need to know what Va, Vb, Vc, Vd, Ve,
    Vf would be for this circuit. Doesn't anyone out there have something
    like Electronics Workbench they can plug in these values, and post of
    screen-shot of the circuit in the Normal, Alarm, and Supervisory Alarm
    conditions? Takes about 2 minutes. Do they still make Electronics
    Workbench? Is there a simple, cheap, easy, basic R/l/C circuit
    analysis shareware program out there I can download?

    I know how to do KVL, I'd just rather not dive into it that way,
    that's like doing long division instead of using a calculator.
  11. Robert Baer

    Robert Baer Guest

    Well, i do know that, but the batteries are labelled and specified at
    24V, and the circuits made to run on them are likewise labelled and
  12. Robert Baer

    Robert Baer Guest

    Do your own homework; get out the pencil and paper and do the
    *simple* math.
  13. Gary

    Gary Guest


    I found a pretty good free circuit modeling program called SwitcherCAD/
    LTspice by Linear Technologies. It displays voltage and current
    readings wherever you point the probe.

    Old fashioned military IC/SM alarm switchboard panels are required to
    be hooked up to various types (liquid level, pressure, temperature,
    whatever) of level switches that have a built-in 6.8k resistor, for
    supervisory circuit monitoring (current sensing). This circuit
    replaces the IC/SM panel, and it backwards compatible with the 6.8k
    switch combo. I want to use a 0 to 10 VDC analog input module, so I
    chose these R1 R2 R3 values to keep my measurement across R1 in that
    range. I don't want the 24VDC supply's current draw to be under 3mA.
    This circuit accomplishes this. It's just a simple voltage divider, of
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