100 Replies to “EEVblog #373 – Multimeter Input Protection Tutorial”

  1. It's kind of scary some of the crazy unsafe stuff out there that sometimes even falsely labels itself as having some kind of rating, when in reality, it probably would blow up or start on fire just connected to an electrical outlet.

  2. This is one of the best EEVblog episodes. A lot of this stuff in the previousones, but now I can understand everything finally! Thanks for that!

  3. Dave, how about just 2 diodes back to back instead of the bridge+extra diodes. I suppose the manufacturer decided the ADC doesn't need to sample more than 0.7V so that's why they just put 2. I have a couple of meters protected like that and it seems safe.

  4. My DMM got a combined V,Ohm, Hz,Capacitance, mA and µA jack. and it was 250 bucks! 😛 So there are some midrange DMMs that still got combined jacks. I'm not a HUGE fan of the combined jacks. But by now i'm used to it. So it's ok. The DMM is a APPA 82 btw. Very nice and solid DMM for the money.

  5. I was referring to my own meter, 200mA scale. I'd have to measure it's impedance on mA, but if it's below 0.7V / 200mA = 3.5ohm, than it looks ok.

  6. that particular ADC can handle voltages up to ( 0.5~0.7V x 6 ) = 3~4.2V on the input, so it clamps at that level. If it couldn't (needn't) handle more than 2V, they would have put 3 diodes (bridge+1 extra).

  7. Awesome! And just in time too: I need a new multimeter for current measurements… I'll be scanning eBay for those old yellow Flukes (; Also, could anyone suggest a cheap (or better yet DIY) inductor meter?

  8. So by that chart of cat ratings, is cat III better than cat 4? It said it handles 1000v working voltage and the peak impulse transient is 8000v (same as cat 4). Or am i missing something?

  9. Hi Dave, interesting video, thank you.
    A few weeks back I needed to come up with transient-proof voltage input stage, and what did I use for reference? Fluke 87 schematic of course from the service manual 😛

  10. you're videos are addicting Dave, I work from home and just have your stuff running all day, love the new stuff, and the old videos are great to re-review.

  11. Hi Dave, just wanted to say this video looks great and very informative. Just subscribed and look forward to seeing more of your videos.

  12. Really good one Dave! … And reading the automated transcript is hillarious poetry…
    8:16isolation slots in the typically between input jacks between components
    28:20southern white physically have a barrier between the vote is jack
    28:24and ground all the at current jackson might physically have
    29:56dammit sucked westphal at a certain and say will get his elbow teaching put
    30:01ceremony gently jack is going to get through a three point five kind while

  13. Great video one of my faves yet. I take it that one must also look at the I^2T rating of the fuse and make sure that it's guaranteed to pop first before the diode bridge goes up in flames?

  14. Well, there is the recommended fuse of course. If you don't buy the "approved" one, then you are of course on your own and should consider stuff like that. But all quality brand HRC fuses should have a very similar rating in that respect.

  15. How does the meter measure ohms accurately if there is a 1k PTC and 3.5k wirewound resistor in series with the input?

  16. Hi Dave, does a Mov act like a Surge Arrester ? (without reset functionality)
    can i assume that a Mov is doing the opposite function of a fuse ?
    great Video btw 🙂

  17. Hm… Very interesting! But… I don't understand how come the ambient temperature doesn't affect the PTC and thus also the measurment… 

    It will certainly help out the overall safety of the meter as a whole, but if the meter originally came with a glass fuse in it, chances are it's a lower quality meter anyways. If the fuse goes on a cheap meter because you connected it across the mains, chances are the meter is going to be toast. Using an HRC fuse instead of a glass fuse won't prevent that, but it will prevent that fuse from throwing arc plasma and/or shrapnel around the inside of the meter case. That could mean the difference between a startled but unharmed meter user, and a crispy critter!

  19. Just to convince the kid in us, it would be nice to have a demonstration of a multimeter actually blowing/exploding/catching fire and all like you say….. Just for the show… Hum ok next clip.

  20. 5:06
    Been there, done that.
    Was measuring a 3F engine current (with a primitive switch per phase arrangement) and shunted 400V mains with the meter by mistake (NEVER TRY TO DO YOUR WORK TOO FAST!). Awful bang, lots of fear and a dead meter. Thankfully it didn't catch fire. And thankfully I didn't catch any amps.

  21. My genuine onehunglow DMMs dont use fuses. I am not joking. Additionally, there is almost no components other than a blobbed down IC. Derpy piece of crap.

  22. I like how you've manage to keep the same content quality over +700 videos!
    This was really informative without being too simplified or complicated!
    I've been watching your videos since I started my studies. I've learned so much from watching your videos over 5 years, there's only so much you can learn at school and there's only so many cases you can get at work 🙂

  23. EEVblog , question for you. Can you build the circuit at 22.11 for the ptc and mov's etc in a seperate box with leads to go into your DMM that you can then plug your leads into externaly and still be protected so it is between your leads and the DMM ?. Retro fitting is ok I guess if you want extra protection and surety IF it would work.

  24. Thanks for this. I knew my meter was only good for CAT I (not the claimed CAT IV) but now I can see that despite the fuses, rectifier and PTCs, there's no MOVs or isolation, and fairly wimpy PCB tracks. Good to know exactly what's wrong.

  25. @EEVblog, how about this?
    1. Build an external uber-safe protection circuit
    2. Use it with my cheap crappy meters.
    3. You stop bitching about them.
    4. Profit. The meter is as good or bad as it was, but has protection of way more expensive models. Money saved.
    Not everyone shits money like in Australia to afford a Fluke or Agilent :>

  26. l have several pieces of junk that have served me well thank you very much and have never blown anything up but did like the video in your teardowns you point things out put do not always explain in detail why they are there as you did here

  27. Anyone having any experience with Sanwa DMM, particularity PC510a model? My chinese 50 bucks DMM died on me , now im blind in a middle of a project . I was wondering if anyone has any experience with it , Sanwa DMM , perticulary PC 510a model , apparently it has PC logging , but the cable for that is like 100bucks , that a lot of money , maybe it can be home built . Also another adapter for it , current clamp , thats also expensive , but im sure i can make some , its just an inductor on leads if im correct …so yeah anyone with some experience on those , grateful

  28. Dave, if I am only working on low voltage circuits, like automotive and residential AC, can I just short-circuit the 10 amp fuse/holder inside my meter and install a common glass fuse/holder in line with my meter lead? Those HRC fuses are damn pricey!! Thank You

  29. Hi,
    Is there any way to upgrade input protection on UNI-T UT61E?
    Maybe by upgrading some of components, or adding som extra?
    If there is a way please guide me to it.

  30. Muy bureno sus videos!!! una pregunta que proteccion tiene un multimetro al medir continuidad a un fusible presumiblemente quemado y con tension??? gracias y saludos

  31. A fellow engineer had borrowed my DMM to calibrate an transducer of 4-20ma. He left the leads plugged into the milliamp jacks, I used this same meter later without checking the lead configuration,  across the mains of a 575VAC buss. …Yeah  B O O M !!!  The meter exploded in my hands. This was a Greenlee meter, not the highest quality but a decent meter.

  32. Once I threw a 10A unfused range right across the mains… I kicked off two 16A breakers in series. Fun stuff.

  33. How does the protection work when in resistance measuring mode? In resistance measuring mode it has a little current source, how is this current source protected against high voltages?

  34. Hello
    I want to ask, i have measured current by connect multimeter with wall outlet in parallel instead of series, so i replaced one fuse and still not working 😐😐

    Wens 52 multimeter

  35. Why can't a zener diodes in back to back configuration be used instead of the diode bridge for the current protection. Is it because the diode bridges have a greater PIV(Peak Inverse Voltage) than the Zener Diodes?
    Great Tutorial by the way !

  36. I love your videos Dave… i have learned alot from them! Could you do a tutorial on how to build a surge protection circuit?

  37. I would shit my pants if I put a meter on 2kvolts and the meter blew up in my face. Serious stuff.
    I'd love to see a HV non contact meter integrated into the smaller meters. Or some cheap alternative. Too many faults happen this way from conventional meter touching neon signs or other specialty things.

  38. I have shorted my 789 fluke process meter via the mA or mA source jacks to mains, 120vac only and have been just fine. 10$ fuse every time but works like a charm. 1k$ meter but for process industry is mandatory.

  39. I didn't understand the bridge explanation. If it's there to take some load off of the the resistor, why not just use another resistor instead of a bridge?

  40. Chinese meters are so cheap the greatest loss would be the inconvenience of going to a store to buy a new one, or waiting for mail order.

  41. Hi! I'm little confused about this monster protection diode bridge with many diodes… Why they can't simply connect bidirection TVS , couple of power zeners or something like that? OK TVS might be not a very good idea for long time pulses but it looks like very cheap way to solve the same problem…

  42. If you want to measure current in the negative/grounded output wire from an isolated dc/dc converter, but by misstake puts the range switch to volt instead of current… The fluke will burn… My friend tried that on a 1 Amp, 100 kV dc/dc 😀

  43. How does the ohms range protection work agains external voltage? How can it measure low resistance values like 0.1ohm across this several kilo-ohm input circuit?

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