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Multimeter woes

Discussion in 'Electronic Basics' started by Noodle, May 4, 2007.

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  1. Noodle

    Noodle Guest

    Hi, not sure if anyone can help me but I am just starting out in
    electronics. I have bought a multimeter so I can go through a book I
    bought 'Starting Electronics' by Keith Brindley. I am trying to
    measure amps on it but cannot see where I can do it. I didn't buy the
    cheapest multimeter and thought it would do all of the basic functions
    but cannot for the life of me find how to do it unless I measure the
    resistance and voltage seperately. Here is a link to the multimeter I
    bought.

    http://www.maplin.co.uk/module.aspx?ITAG=FAQ&ModuleNo=4404&doy=4m5#faq

    any help would be appreciated, even if it is just to tell me I bought
    a duff multimeter :(

    Headnoodle.
     
  2. I wouldn't have thought it possible, but sure enough that meter won't
    measure current. Sorry to be the bearer of bad news. Here's a couple of
    cheap ones that will do it. It's nice to have more than one meter anyway.
    :)

    http://www.maplin.co.uk/Module.aspx?TabID=1&ModuleNo=44679&doy=4m5
    http://www.maplin.co.uk/module.aspx?ITAG=SPEC&ModuleNo=37279&doy=4m5#spec

    If you can send the other one back and exchange it, you might like something
    like this, it has 4000 counts, better accuracy and a PC interface (if you
    care ;-)
    http://www.maplin.co.uk/Module.aspx?TabID=1&ModuleNo=46381&doy=4m5
     
  3. It looks like a decent meter, but it has no current ranges. It looks like
    the usual current functions were traded off for the capacitance meter and
    frequency counter functions. You have a 200 mV range, so you could insert
    a .1-ohm precision resistor into the circuit and measure the voltage
    across it, reading .199V as 1.99 amps.

    Reading DVM specs reminds me of the Fluke 8050A I abandoned to hurricane
    Katrina. I was too busy piling the computers, the insurance policies and
    the cats into the car to think of it.
     
  4. Noodle

    Noodle Guest

    Thanks for that Stephen, wish I understood what you was going on about
    hehe. I'm gonna bookmark this thread then read this in a few months
    time when I have learn't a bit more and will probably understand what
    you mean then hehe. :p
     
  5. Noodle

    Noodle Guest

    Unfortunatly I have had the multimeter too long to send back :( I
    might go out and buy one of thos £6 ones, I like an excuse to go to
    maplins.... :)
     
  6. Noodle

    Noodle Guest

    Went for this one in the end.

    http://www.maplin.co.uk/Module.aspx?TabID=1&ModuleNo=44679&doy=4m5.

    Thanks for your help :)
     
  7. Be very careful when measuring current as you will be passing that current
    thru the meter. Never touch the probes to a voltage source when the meter
    is in current measuring mode, or you will immediately blow the internal fuse
    since nothing will there to limit the current to a safe level. Be sure to
    read the documentation for the meter. Get some extra fuses, you will need
    them. ;-)
     
  8. I've had various meters for 35 years, and the reality is that as a hobbyist
    I've never really used the current range. I remember doing it, for curiosity
    sake, with that first VOM (an analog meter) circa 1972, and that was likely
    the time I pinned the needle so hard against the stop it bent. There was
    a period when I only had VTVM (vacuum tube voltmeter) and it didn't have
    the ability to directly measure current, and I never suffered.

    Michael
     

  9. And another side note about that is to always make sure you use the current
    jack instead of the voltage one or you will blow the fuse in almost al
    cases. Its a good idea to get into the habit of making sure you always look
    at which one your using before you try to measure anything. Sometimes I
    forget and end up blowing a fuse... and unfortunately the fuses my amp uses
    are not stock.
     
  10. ian field

    ian field Guest

  11. Noodle

    Noodle Guest

    yup, just done that hehe. better trot back to maplins for some spare
    fuses
     
  12. Noodle

    Noodle Guest

    hmmm, no I'm not. do i just sign up from their home page.
     
  13. ian field

    ian field Guest

    Like yourself I occasionally like an excuse to trot down to Maplins, so I
    rarely visit their website, IIRC they signed me up for the newsletter at the
    counter, you can also ask for it on their customer service number.

    Make it clear that you are asking for this, a while back I signed up for the
    Mailing Preference Service to stop all the junk mail I was getting - then
    the Maplin price crash letter stopped as well! This usually contains about
    £7 worth of vouchers so I had to phone them and get them to start sending it
    again.
     
  14. Circa 4 May 2007 03:11:26 -0700 recorded as
    <> looks like Noodle
    In your first post, it seemed clear that you know how to find amp values by
    combining voltage and resistance:

    "...cannot for the life of me find how to do it unless I measure the
    resistance and voltage separately."

    What Stephen is saying is the same thing, and many high-power circuits that
    have a need to know current conditions real-time, use these small,
    precision resistors. They are called shunt resistors.

    The value is chosen small enough so not to interfere in more than a
    negligible way with the current in the circuit. So, you build a circuit,
    get it wired up, and oh by the way put a 0.1 ohm resistor in the common
    path. Don't forget to put test terminals on each side. Get the thing
    running, and measure the voltage across the resistor. Multiply the voltage
    by ten (which is the same as division by 0.1: I = E / R), and you have the
    current.

    You can use this technique as a way to measure current in any circuit.
    Interrupt the output path, and insert the precision resistor in the path
    (if you put it in the return/common/ground path, exposure to high voltage
    will be avoided). Reconnect everything, and voltage measurements across
    the resistor will allow you to simply convert the measurement to amps.
    This is precisely how ammeters work, when you buy one that has the
    function. The big giveaway hint is that in addition to AMPS range on the
    meter selector, it will have a separate input jack for measuring amps.
     
  15. It looks like this meter has banana jacks set at the "standard" 3/4"
    spacing. You can get dual banana plugs with a built in set of holes where
    you can install a sense resistor of 0.1 ohms (for up to 10 amps), 1 ohm for
    1 amp, and 10 ohms for 100 mA. Being very obvious on the meter, it is not
    so easy to flip the range switch or forget you're plugged into the current
    hole when you go to read a voltage. You can get good enough resistors (at
    least 1 watt) for less than the multimeter fuse. However, without a fuse,
    an exposed resistor could explode if you apply mains voltage or a 12 volt
    battery directly, so *be careful*. Always wear safety glasses, and if you
    are really prone to wrong connections, don a welder's outfit and a flak
    jacket :)

    Paul
     
  16. JeffM

    JeffM Guest

    Add this:
    http://www.google.com/search?q=define:shunt+resistor

    It's probably a good thing that the meter doesn't have a current
    range.
    Newbies (and even old hands) are know to pop the fuse inside the meter
    --or worse, smoke the resistors--
    by leaving the meter set to a current range.

    An external shunt resistor (if it's right at the meter)
    is also a reminder to reconfigure the measuring setup.
    If the shunt is part of your circuit, things may be even easier.
    ..
    ..
    BTW, it's OK to strip out much of the old text from your post
    when the blockquotes get to be quite long.
    All you need is a *reminder* of what it is you are responding to.
     
  17. It's a nice meter but it won't measure amps -- or current of any sort.
     
  18. Guest

    Then I (an electronics novice and wannabe competent) use my DMMs often
    to check the actual battery charging current from my old, rather
    indeterminate power supply with analogue meters. I don't need to check
    any more on my whizz-bang, digital-display, power supply, once I
    assured myself that it was near enough. [Interesting to see the actual
    voltage drop from my PS to the larger batteries at the end of 15' of
    figure 8.] jack
     
  19. ian field

    ian field Guest

    Back when I started out meters were a bit expensive, so it didn't take long
    to get in the habit of switching to the highest voltage range as soon as I'd
    completed a measurement, in fact this was one of the first things drummed
    into every student at the college I attended.
     
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