please for any eng know ???

Discussion in 'General Electronics Chat' started by u-a-l, Jun 12, 2010.

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  1. u-a-l

    Thread Starter New Member

    Jun 12, 2010
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    please for any eng know ???
    i need to built digital multimeter using microcotroller (pic 18fxx) but i have proplem in hight voltage & current that pic can not take it
    how i can measure hight voltage & current by this pic

    :confused: :confused: :confused: :confused:
     
  2. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    You don't. You measure a fraction using resistance dividers (series for voltage and parallel for current) and then program the PIC to multiply your measurements and get the full values.
     
  3. u-a-l

    Thread Starter New Member

    Jun 12, 2010
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    I have circiut about what you said but the value of resistance is also about (250m ohm , ........,until 50micro ohm ) and this values in prctical life does not found if you have circiut please share it with me
    at the end thank you for fast response potato
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You can find 10 MEG Ohm resistors, right?

    Start by using that value for the high side resistor.
     
  5. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Use a pyramid of resistors.

    1990k and 10k, then another 490k and 10k in parallel with the first 10k and keep going out as far as you want to.

    All of the resistors in your pyramid should be connected at all times and you only switch which of the 10k resistors you use for taking your measurements from.

    I would also suggest some protection for the PIC. Make certain there is a Zener shunt in there that limits the voltage at the measurement input pin of the PIC and a fuse that limits current at some point closer to the probe. An Op Amp set up as a voltage follower buffer between the measurement range switch and the PIC might also help and with a few adjustments you can switch the Op Amps resistors to add some gain and take microvoltage measurements, just program in the proper multiplier to the PIC.
     
  6. u-a-l

    Thread Starter New Member

    Jun 12, 2010
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    sorry but may be u don not understand me in my circuit many resistors connect in series with one resistor in parallel that resistor i used it tow measure voltage by op-amp .
    for example if the scale of multi meter from (0-10kv) i need small resistors(for example 100m ohm) tow connect it in series .
    but if you have anther circuit that do same job i will be happy if you share it with me
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    10kV? :eek:
    What are you trying to do.
     
  8. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    You need at least 100 Megaohms to prevent loading down 10Kv. Are you trying to measure the voltage on large Cathode Ray Tubes or what exactly are you trying to do?

    A 2 Megaohm resistor across 10KV would need to be about 50W and could be very dangerous if it failed.

    High Voltage means 600V.

    You are talking some really stupidly dangerous territory.
    Ultrahigh voltages are a bad place to go if you have any questions in your mind. If you aren't certain what you are doing then just stop and don't start until you have everything figured out from every angle. Trying stuff until it works is not an option. You are easily dead from mistakes working with voltages considerably lower and killing power increase with the square of the voltage.

    Check your bodies resistance. There is a good chance it is less than 2 Megaohms. You would draw 50 Watts from 10KV. You would fail as a conductor in a very messy way. We are talking about things like your bones exploding.
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    This is becoming problematic. In general, meters do not measure 10 KV. A special probe is necessary, and the reading is made on a lower scale, like 0 - 10 volts.

    A person attempting to take high voltage measurements with homemade equipment may be endangering his life. This is not a first-time project.

    Would the OP please clarify his intent?
     
  10. u-a-l

    Thread Starter New Member

    Jun 12, 2010
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    sorry engineerings[SIZE=+0] i said for example not in fact life in know 10kv is so hight for measuring by normal multimeter [/SIZE]
    [SIZE=+0]now let as say the max scale of voltmeter is 300v how i can find suitable circuit to measure (AC,DC)voltage from zero to 300v [/SIZE]
    [SIZE=+0]in my comment you just care about (10kv) but you do not answer me about second part of my quotation that speak about any eng have circiut that i can used to complete my (Digital multimeter by using pic )[/SIZE]
    about my circiut i know it's not modern but that what i found until now
    in the end i happy from in one share with me my proplem and i hope to find solution soon ???
     
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Beenthere..
    Now I like ur approach concerning the well being of the OP.
    At least now we all know why this thread will be locked if it comes to that.
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    OP, if u do not clarify ur intention and how well you know high voltage ( minimum is more that 48V, which can cause shock or death in some circumstances), this thread will be locked.
     
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  13. retched

    AAC Fanatic!

    Dec 5, 2009
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    You want to build a PIC voltage meter, correct?

    Is this a general use meter or do you have a device you want to measure?

    Once you decide what range you want your meter to be used for, you can protect the cricuit (and yourself) by putting a fuse in the design.

    If you want to design a general purpose voltmeter that measures 0v - 300v, 0-5a you can use a 360v 5a fuse. This will ensure if someone tries to use your meter for a purpose OVER its designed use, the fuse will blow.

    Once you establish the range for the meter, you can start designing the circuit.

    If you want this meter to be auto-ranging, you can use a few relays or switches to select different resistors for use in measuring.

    The PIC can take a sample, determine if it is to high or low for the active resistor and switch to a higher or lower rated resistor setup.

    The PIC can be programmed to use the particular resistor to calculate voltage and current draw.

    You have to decide the limits of the meter you want to build. That is the next step.
     
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  14. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    A multi meter is probably too advanced a hobby project for anyone who needs help setting up voltage dividers. You can buy a useful multimeter for very few dollars, likely for less than the component costs to build one.

    If this is required as a homework assignment then I would suggest asking your instructors so that they can help supervise and keep you out of danger.
     
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  15. beenthere

    Retired Moderator

    Apr 20, 2004
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    This is probably a correct statement of the situation.

    Constructing a meter is one thing, but going immediately to lethal voltages (the OP is reminded that it was he who first mentioned 10 KV) is a big first step.

    The comment about purchasing a meter is well-made.
     
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