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calculating shunt

Discussion in 'General Technical Questions and Answers' started by VA7AAX, Sep 15, 2007.

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  1. VA7AAX

    VA7AAX Ham Member QRZ Page

    the project specifies a 200uA meter with a 6.8k shunt. it also 100k pot for sensitivity. i have a 100uA meter how do i calculATE the shunt resistor required for my meter?
    BTW the project is the n1al gate dip oscillator in may 2003 qst
    thank you and 73
  2. K6UEY

    K6UEY Ham Member QRZ Page

    To calculate the shunt for a current meter you must know what the meter internal resistance is.
    You can measure the resistance by putting the meter in series with a pot (variable resistor ) and a low volatge source. Vary the pot to increase the current through the meter until the meter reads ½ full scale reading. Then disconnect the resistor and measure the pot resistance. That value is the value of the meter resistance. When the meter is ½ full scale the resistance of the pot and the meter are the same.
    Now with OHMs Law you can now caulate the shunt required to up scale the meter to the desired value.
    USE a low voltage source and BE CARFUL not to slam the movement against the pin.
  3. K9STH

    K9STH Ham Member QRZ Page


    You are "half right"!

    It takes 2 potentiometers to determine the internal resistance of the meter. You first insert one in series with the meter and a low voltage source (a 1.5 volt battery is often sufficient, especially with low current meters). Connect the positive voltage source to one side of the potentiometer. Then connect the "wiper" to the positive terminal on the meter. Connect the negative terminal of the meter to the negative side of the voltage source.

    Then you adjust the potentiometer so that the meter "just" reads the maximum current. Disconnect the power source. Then put a second potentiometer across the meter terminals with one side going to one terminal and the wiper going to the other terminal. Reconnect the voltage source and then adjust the second potentiometer until the meter reads half the value of the maximum reading of the meter.

    The value of the resistance between the wiper and the side of the potentiometer that was connected to the meter terminal is the value of the internal resistance of the meter.

    What UEY suggests will usually not work. It will only work when the voltage source produces exactly the amount of current that it takes for the maximum reading on the meter. Under those circumstances you don't even need the potentiometer because you can use Ohm's law to calculate the resistance of the meter. Of course having this happen is going to be a VERY rare occurance.

    Glen, K9STH
  4. K6UEY

    K6UEY Ham Member QRZ Page

    Glenn I don't dispute that your method will work but if you have a series circuit and you insert a pot in series with the meter half of the voltage is dropped across the meter and half across the pot,they being in series,so since the current is the same the voltage is divided in 2 ie the resistance must be the same since the meter is adjusted to 1/2 scale.
  5. VK2TIL

    VK2TIL Ham Member QRZ Page

    In this particular case the shunt would probably not be critical.

    I don't have the circuit but I imagine it's like any other dip meter where the meter is just an indicator.

    Do you really mean "shunt"? A shunt goes across the meter terminals whilst a multiplier goes in series; I would have thought it unusual to shunt the meter in a GDO.

    If you use the given values with a 100 uA meter instead of a 200 uA one, it will just mean that you will back the sensitivity pot off a little more.

    So I would just use the 100 uA meter with no circuit changes.

    If you want to know about meters & shunts, this is good;
  6. VK2TIL

    VK2TIL Ham Member QRZ Page

    I did find a little information on the N1AL design here;

    Interpreting the PCB, there is no "shunt" (unless it's off-board); R5 looks like a multiplier in series with the meter so that's probably your 6K8.

    My previous comment stands; using 6K8 with a 100 uA meter will give you a meter that's a little more sensitive, that's all.

    10K or so will give about the same sensitivity as the original.
  7. WB7DMX

    WB7DMX Ham Member QRZ Page

    thats a good lil gadget to build.
    I have built 4 of them so far as gifts to new ham friends.
    but I use a jack for a frequency counter for the display instead of the dial.
    got enough parts to build 50 more.
  8. VK2TIL

    VK2TIL Ham Member QRZ Page

    Bobby; is R5 (in series with the meter) the 6K8?
  9. K9STH

    K9STH Ham Member QRZ Page


    I had to stop and think for a moment with your "6K8" statement! The 6K8 is a vacuum tube that was used in many receivers before World War II.

    Had to "shift gears" to get my brain back into the resistor mode. Stateside a "6K8" resistor is more often called a "6.8K" resistor.

    Glen, K9STH
  10. VK2TIL

    VK2TIL Ham Member QRZ Page

    We had a good discussion here several months ago on this matter.

    My practice is to write resistor/capacitor values like that for clarity.

    You see it in European writings I think.

    To be pedantic it's 6k8 which I should have used.

    Dismal points can get lost in printing or on a monitor screen.

    I would also write it 6 800 ohms for that reason.
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