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Affordable 4-digit panel meters?

Discussion in 'Electronic Components' started by Usual Suspect, Mar 26, 2007.

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  1. I can find lots of LCD & LED 3-1/2 panel meters for under US$20. But look for
    4 digits and the price triples.

    Is this just my experience, or is this pretty standard pricing?

    I'm flexible re. type (prefer LCD), background lighting (not necessary),
    color (prefer green), and size (have ~25mm x 50mm panel openings waiting to
    be filled).

    I've looked at Mouser & Digikey. Spendy!

    Where else to look?

    Thanks,
     
  2. Guest

    I'd appreciate knowing your sources on the 3 1/2 digit panel meters
    that you have first hand experience. Basically overall quality of
    construction, did they ship any duds, stuff like that.
     
  3. Tim Shoppa

    Tim Shoppa Guest

    I would say that the $20 meters you found were probably not "standard
    pricing" but were probably no-names.

    Very very few real-world applications can use the extra digits
    usefully. There are some customers that really need it, but most
    customers demanding more digits are on an ego-trip :).

    The demand for standalone meters seems to be getting smaller and
    smaller as everyone moves to microcontrollers and commodity A/D that
    actually matches the resolution/accuracy needed. If you need isolation
    then you have to think a bit. In a lot of one-off applications it
    doesn't pay to think, you just buy the standalone meter!

    Tim.
     
  4. Very very few real-world applications can use the extra digits
    Want to replace the analog meters in my bench PS with digits. It goes beyond
    20, so I'll need 4-digit meters, yes? I'd like more than 1 decimal place.

    Maybe I've got it wrong... How do these meters work? Do they show the max
    decimal places possible (<2v, 3 digits; <20v, 2 digits; <200v; 1 digits)?
    Maybe I presumed incorrectly that the decimal is fixed...

    Or am I letting my ego go rampant...? (c:

    Thanks,
     
  5. Tim Shoppa

    Tim Shoppa Guest

    Typically the surplus 3.5 or 4.5 digit DPM unit itself is either 0-2V
    full scale, or 0-200mV full scale.

    Some units can be jumpered for either, and some other ranges
    (including current ranges) are available. If you buy from a
    distributor you will have much better choice of ranges compared to
    buying a surplus unit.

    You are responsible for adding the dividers/shunts if you want, and
    you are responsible for putting the decimal point where you want.

    If you want 0.1V resolution, then a 3.5 digit meter gets 0.1V through
    199.9V. Or at 0.01V resolution, a 3.5 digit meter gets 0.01V through
    19.99V. As commonly incorporated into low-end power supplies, you
    might find a slide switch that changes the divider and the decimal
    point.

    I would seriously question the need (other than "ego trip") of going
    to a 4 digit meter. Especially if (for example) you aren't buying 0.1%
    or better resistors for your divider or shunt already.

    You also may have to look at where the power to the DPM module is
    coming from if you do not have a separate isolated power source just
    for the meter and you need to float the meter reference above ground.
    One often-seen option is a battery.

    Tim.
     
  6. If you are powering from the same regulator that powers a sensitive circuit
    that is outputting millvolts for measurement, take care, especially if
    you're using an LED meter. The meter can change the load, the load
    regulation might allow the other circuit's output to change, in turn
    changing the meter reading...

    The easiest way to avoid this really vicious cycle is to use separate
    regulation for the meter and the measured, or even to isolate them with
    separate transformer windings, if battery power is not important.

    There may be other, better advise to come on this subject. I hope so, cos
    I'll be watching it avidly if there is.
     
  7. Tim Shoppa

    Tim Shoppa Guest

    All the DPM meters I've ever dealt with have separate connections for
    power ground and signal ground. (On Modutec brand meters, they're "V-"
    and "IN LO", respectively.) Tying them together at the meter is (as
    you pointed out) a Bad Thing - so you run them back to your common
    ground. Right?

    Tim.
     
  8. That's how I did it, problem still happened. DPM is made by 'Summit', and
    specifies a direct (or via 10K) connection between Gnd and in- if the meter
    and measured are otherwise isolated from ech other, but it doesn't specify
    what to do if they aren't. I put it down to an LM2904 LDO 5V regulator not
    having the load regulation I was hoping for. I'd had the circuit on a
    pinboard previously and powered by a simple smoothed supply and LM317 to
    regulate for 5V, and I guess the large caps in the external supply and the
    LM317 together prevented the problem that arose with later supplies based
    on a small 12V SMPU and the LM2905.

    If the meter and measured share a power supply, and the measured is making
    a local ground after being supplied by small DC-DC power converter, I guess
    it complicates things. I ruled out the power converter as being the cause
    though, I think. What I found was small shifts in voltages all over the
    system in sync with the LED meter's updates, if the value changed.

    It was actually interesting, in a perverse way, sometimes steady, sometimes
    oscillating between two values, or two different values, or three, or a
    random series, it was like an illustration of chaos theory.
     
  9. Rich Grise

    Rich Grise Guest

    You'd have to look up the meter's common-mode rejection, but I'd rather
    find a signal common for "IN LO", and not even tie it to power ground,
    just signal ground (which might or might not be in common with power
    ground - it depends on the circuit). If it's a decent meter, they "should
    be" totally isolated (up to its CMR, of course.)

    If it's a cheap crapola meter, then all bets are off. :)

    Cheers!
    Rich
     
  10. If you are powering from the same regulator that powers a sensitive circuit
    I plan on installing completely separate supplies (xfmr, regs, etc.) for each
    meter.

    I've not decided on LED or LCD.

    Each meter will do double duty as V and I meter (via toggle switch).

    My problem is understanding my need for accuracy.

    The outputs are 0-25/0-25/0-16. So my presumptions are:
    - a 3-1/2 digit meter that is wired for 20v max will give 2 decimal places
    of accuracy; wired for 200v max it will give 1 decimal place of accuracy.
    - a 4-plus digit meter will give an additional digit of accuracy under these
    conditions.

    So I must decide if 0.1v accuracy is OK for 2 of the outputs.

    Comments welcome.

    Thanks,
     
  11. Two. You'd be feeding it through a potential divider to get the 2V or 200
    mV the meter wants. The accuracy would be the same if your dividers were
    good. You'd move the decimal point when selecting your divider for range.

    A 200 mV input meter might accept 20V while indicating over-range, so a
    200V line could safely be connected to a /10 divider. Just don't let it
    switch to the meter input directly in error. >:)

    The dividers are important, that's how you control the accuracy, the meter
    then shows the most significant digits, and your dividers can be hand-
    picked resistors giving better accuracy than the meter's own.
    That's true, even with the significant digits thing solved, 25.00 wants 4
    full digits. Don't forget that there's no point in the extra digit unless
    the meter's native accuracy supports it. The accuracy and maximum count of
    99.99 is 5 fold higher than 19.99, so a tripling of cost isn't a bad spend.
     
  12. Tim Shoppa

    Tim Shoppa Guest

    It's also possibly conversion noise from the meter getting back into
    the circuit being measured, not through the meter power lines, but
    through the meter sensing line. Typically the conversion cycle is
    exactly tied to the LED update rate.

    I have seen some perverse cases where crystal oscillators in my
    circuit intermittently lock to EMI coming from switching power
    supplies etc in prefab modules I was using. In that case it was
    radiated, not conducted, EMI.

    Tim.
     
  13. Tim Shoppa

    Tim Shoppa Guest

    Way, way overkill, man. It sounds to me like you're going to be
    building 3 power supplies to support your upgraded existing power
    supply.
    Watch out for voltage drop across toggle switch and shunt, and now
    you'll have to do current shunts too.
    Yeah, you're the one who has to decide that.

    What's the resolution/accuracy you got from the old analog meters?

    0.1V is 0.4% of your 25V full-scale, and that's pretty good for a big
    old analog meter with a mirrored scale. More typical accuracies for a
    small front-of-power-supply analog meter is 2 or 3%.

    And don't confuse resolution with accuracy. You'll have to be using
    better-than-1-percent resistors (probably just go to 0.1% resistors)
    in your dividers and shunts etc.

    This is beginning to sound more like an exercise in specmanship (well,
    not even that, more like an exercise in how many digits can we
    display) than any actual utility.

    If you're getting the impression that I actually prefer analog meters
    for many quantities, you'd be correct! Do you really need to know the
    current to a fraction of a percent, or just see if it's drawing any at
    all or pegging the needle? Some of us know exactly what sort of
    overload it is by not looking at the analog meter, but by the noise it
    makes when it hits the peg!

    Tim.
     
  14. Interesting, but doubtful, it would have to go back through a /10 divider
    pair of resistors, then into the output of an op-amp, and it never happened
    on the pinboard layout with different supplies. I'll look into it with that
    in mind next time I try stuff, but I still think poor load regulation is
    the usual suspect. Sorry, that phrase seems to be haunting me, lately. :)
    If the problem turns out to be particularly vicious and mysterious, I might
    even start calling it Keyser Soze...
     
  15. I like 1%, it's cheap, metal film is stable. I hand-pick mine. I buy 10 of
    910R, ten of 8K2, I measure each 910R with a Fluke 79 (or whatever is the
    best meter in reach). I note the values in a list, then repeat for the 8K2.
    I multiply the fist real '910R' value by 9, and if there is a perfect match
    in the 8K2 list, I have my pair. If not, I move to the second 910R value...

    Doing this is a cheap way to get at least a couple of resistors matched for
    a /10 network. If the resistors were tighter in tolerance it might be
    harder if not impossible to find pairs this way. 1% similarly allows enough
    likely excursions to give you a /100 pair of 100R and 10K. Both pairs can
    have accuracies of around 0.1% or better. They're unlikely to be the weak
    link in a chain of accuracy.
     
  16. You also need to remember that a DC meter will read the average, and not
    the true RMS (AC+DC). When you have a a power supply that has significant
    ripple and noise, the reading may not be accurate for your purposes, and
    the additional digit of resolution will not be meaningful. 25.0 volts out
    of 199.9 will still be better than 1/2% of reading for 1 count.

    Paul
     
  17. DaveM

    DaveM Guest


    Another problem, in addition to the metering resolution and accuracy, is the
    resolution of the power supply's adjustment controls. Can you actually set the
    output to 10mv resolution on the 0-25V range? If not, then there's no reason to
    expect that a higher resolution meter will make it happen.
    Connect a 4- or 4-1/2 digit DMM to the output and see if you can accurately and
    repeatedly set the output to within 10mv of your desired voltage. Unless your
    power supply has good coarse and fine voltage setting controls, I bet you can't.

    --
    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
    address)

    Life is like a roll of toilet paper; the closer to the end, the faster it goes.
     
  18. Another problem, in addition to the metering resolution and accuracy, is the
    All very good points I hadn't considered. Many thanks to all who contributed.


    Truly grateful for this forum.

    Thanks,
     
  19. Ed

    Ed Guest

    I have some PM128 3 1/2 digit panel meters for sale @ $12.00 +
    shipping.

    Listing on my web site at:

    http://mysite.verizon.net/topossibilities/id1.html

    Ed, W1AAZ
     
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