1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Free Voltmeters!

Discussion in 'TiVo Coffee House - TiVo Discussion' started by unitron, Oct 20, 2013.

  1. unitron

    unitron Active Member

    16,389
    2
    Apr 28, 2006
    semi-coastal NC
    Just in time to aid in diagnosing your suddenly sick Series 3, you can get a free digital multimeter.

    Look in your Sunday paper today (October 20, 2013) for the Harbor Freight Tools circular.

    (It probably helps if there's a Harbor Freight retail outlet in or near your community.)

    There's a coupon in there for a free meter.

    (You do have to actually go into the store with the coupon, but you don't have to buy anything else)

    Remember when calculators that only did the basic 4 functions (+, -, *, /) sold for several hundred bucks and now you can get one for $2.95 or maybe free inside your breakfast cereal?

    Well, same trend has happened with digital multimeters.

    So cheap doesn't necessarily mean worthless.


    Suggestion:

    Get up with someone else getting one of these and put one on DC Volts and the other on resistance/continuity and see what the polarity is of the current the second one uses to test resistance and continuity, so that if you ever need to know, you already will.

    In the old days of "analog" moving needle meters, the red lead, usually thought of as the "positive" lead because that's the one you used to make contact with the positive terminal of a direct current source, became the source of "negative" electrons when switched to the Ohms function, and the black lead, usually "negative" when measuring current or voltage, became the return path for those electrons, so that the negative lead was positive and the positive lead was negative.

    Not doing it that way would have made switching from Volt/Amp functions to Ohm functions more complicated.

    Most digital meters these days, when putting out current to measure resistance and continuity, make the black lead the source of electrons and the red lead the more positive return path for them.

    But I can't swear that they all do, so it's a good idea to check it in advance so that you'll know if you ever need to.
     
  2. JohnnyO

    JohnnyO Crimper

    325
    0
    Nov 3, 2002
    Twin...
    Thanks for the pointer!
     
  3. Arcady

    Arcady Stargate Fan

    3,960
    0
    Oct 14, 2004
    Philadelphia...
    Sunday paper? People still get those?
     
  4. Worf

    Worf Active Member

    1,990
    4
    Sep 15, 2000
    Unfortunately, for multimeters, cheap does mean worthless. Less than worthless - potentially life-threatening because these POS don't have input protection worth crap. At best, if you happen to touch a live wire, something just smokes.

    In the worst case, the meter could literally explode - showering you not just with shrapnel, but potentially electrocuting you from dangerously designed leads and all that.

    Dave Jones often gets many $5-20 meters given to him to blow up. Even the good guys sometimes release crap too.
    http://www.eevblog.com/2010/06/15/eevblog-94-near-death-multimeter-experience/
    http://www.eevblog.com/2010/05/05/eevblog-84-high-energy-multimeter-destruction/

    And if you're probing around power supplies like the TiVo, there are some nasty voltages in there.
    And yes, one of his famous tests is to use it on the Phms range and plug it into mains because on most multimeters, it's trivially easy to switch between DC/AC Volts and Ohms. Especially since practically all meters use the same jacks for it, too.

    The worst meters are ones that use the same jacks for current because an accidental flip can easily cause real damage.

    Unlike a calculator, using a cheap multimeter can severely shorten your life. (Especially since there are decent ones that cost $50 that won't cost you your life if they fail).

    This crap is pretty much only good for measuring 12V car electrical systems.
     
  5. unitron

    unitron Active Member

    16,389
    2
    Apr 28, 2006
    semi-coastal NC
    If you are careless, reckless, and ignorant, even a $200 Fluke or Simpson or B&K is only going to protect you from yourself just so much.

    There isn't an exact translation into electron wrangling of the old carpenter's adage
    "Measure twice, cut once", but the philosophy of "think ahead about what you're doing, think about what could go wrong and how to avoid it, and double check settings" applies there as well.
     
  6. Worf

    Worf Active Member

    1,990
    4
    Sep 15, 2000
    Actually, the Fluke or Simpson will likely survive just fine. Remember those links to the EEV Blog? Dave Jones tests multimeters by doing just that. He switches them to the Ohms range across 240V. A VERY common accidentally occurrence. Guess what? The Agilents, Flukes, Teks, and others? They survive. They even keep the user safe too.

    Those meters are generally properly insulated, with cases that are made of strong plastic and with strong isolation and proper sealing so when a capacitor does blow inside them, the damage is contained well within the unit without compromising safety of the user.

    And they do it by proper double insulated terminal jacks, HRC (high rupture capacity) fuses, full CAT III or better protection (600V or higher). At the very worst of these meters, if you really do manage to kill them, you'd end up with a dead meter that sacrificed itself to protect you.

    With the cheap $20 units, forget it. The free one here? Probably a $10 unit that's fake rated. Like I said, good for 12V automotive systems (but stay away from electric cars or hybrids), but if you're poking around a TiVo where the exposed power supply can have a big 300V surface, those cheap ass wimpy probes really are no match.

    And yes, proper deliberate handling is the best defense. But if you're going to be that careful, you're probably going to invest in good equipment. And even then accidents still happen. If you know you're going to work with a table saw all the time, you might even seek out a SawStop unit for the one time some idiot distracts you at the wrong moment.

    If the EEVBlog's site search was better, you could search for mailbag episodes where he's sent piles of $20 and under multimeters to rip apart. Usually they're really only good for the battery. They're so dodgy and unsafe that even doing a perfectly normal test of applying mains voltage is questionable due to poor design. It does, however lead to amusing episodes where he does nothing but blows them up.
     
  7. steve614

    steve614 what ru lookin at?

    10,722
    0
    May 1, 2006
    Dallas, TX
    Reminds me of that saying (paraphrasing) - the cheap often turns out to be expensive.

    Or something like that.

    There's a reason people can afford to give away free stuff.
    BECAUSE IT'S CHEAP.

    I may be biased because I am an electrician, but I don't think I would trust this meter even if it wasn't free. And not because of the safety issue. It's quite possible this meter won't even be accurate in its readings.

    Although I do appreciate unitron for the effort, I would suggest people pass on this offer for the sake of safety.
     
  8. dlfl

    dlfl Cranky old novice

    7,002
    19
    Jul 6, 2006
    Near...
    I bet there are stunt motorcycle drivers that wouldn't be bothered by the safety issues at all! And we (apparently) tolerate them on our public roads.
     
  9. unitron

    unitron Active Member

    16,389
    2
    Apr 28, 2006
    semi-coastal NC

    If it were a D'Arsonval type I'd expect accuracy problems in anything that cheap if for no other reason than the input impedence, and I'm sure this thing isn't exactly directly traceable to NIST standards and calibration, but to see if the yellow wire is closer to 11 than 12 on a TiVo power supply it should be good enough.
     

Share This Page