EEVblog #373 - Multimeter Input Protection Tutorial - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: October 19, 2012, 01:15:15 am
content: Dave.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: October 19, 2012, 01:31:48 am
content: Great video, Dave.

author: iloveelectronics
author_rank: Frequent Contributor
timestamp: October 19, 2012, 02:21:06 am
content: Excellent stuff!!! Teaches me a lot as a newbie hobbyist in electronics. Thanks a bunch, Dave!

author: robrenz
author_rank: Super Contributor
timestamp: October 19, 2012, 02:39:53 am
content: Are there any variations of this on a LCR meter?

author: nitro2k01
author_rank: Frequent Contributor
timestamp: October 19, 2012, 03:52:16 am
content: 13:26, idiot users? Aren't multimeters specced to accept negative currents of the same magnitude as positive currents?

author: EEVblog
author_rank: Unknown
timestamp: October 19, 2012, 04:38:43 am
content: Quote from: nitro2k01 on October 19, 2012, 03:52:16 amUmm, yeah, that didn't make much sense did it?Either way, when you connect the current range to a voltage source, you are an idiotDave.

author: iloveelectronics
author_rank: Frequent Contributor
timestamp: October 19, 2012, 05:19:06 am
content: In my UNI-T UT-61E there seems to be footprints for a couple of MOVs but being a cheap meter of course they are not there. Will it help in any way if I add some MOVs there myself? If so what kind of specs should I look for in choosing the right MOVs?

author: cyteen
author_rank: Newbie
timestamp: October 19, 2012, 10:30:42 am
content: They are actually specced for spark gaps (SG1, SG2).see the question was asked herehttps://www.eevblog.com/forum/product-reviews-photos-and-discussion/uni-t-multimeter-ut71d-unboxing-and-teardown/msg89873/#msg89873

author: Rick
author_rank: Frequent Contributor
timestamp: October 19, 2012, 01:36:12 pm
content: So may be an obvious question for all of you experts: can I replace the glass fuses in my cheap multimeter with HRC fuses of the same rating to make the multimeter safer?

author: iloveelectronics
author_rank: Frequent Contributor
timestamp: October 19, 2012, 01:56:34 pm
content: Quote from: cyteen on October 19, 2012, 10:30:42 amThanks! I didn't even know what a spark gap isSo after reading that other thread I guess it's ok to just put 1000V rated MOV's in those places?

author: Excavatoree
author_rank: Frequent Contributor
timestamp: October 19, 2012, 01:56:54 pm
content: Quote from: Rick on October 19, 2012, 01:36:12 pmYes, I've done so with my 27 meters that I use. I installed the 650 (close) ma ceramic fuse that was specified for the older 70 series. (Before they moved to the newer, larger fuse.)Likewise, the newer 440 ma large size fuse can be installed to older 27s and 87s for more protection.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: October 19, 2012, 02:23:32 pm
content: I should add that if the meter has a weak link elsewhere (poor blast protection, etc.) then the new fuse really won't result in a safer meter overall. It's always better to use a better fuse, but I don't want to instill a false sense of confidence. A great fuse in a poor meter still results in a poor meter.

author: lcr1
author_rank: Newbie
timestamp: October 19, 2012, 02:31:40 pm
content: I have used the circuit in this application note on Charge Capacitor Protection. Thought it might be helpfulThe application note can be found athttp://www.ietlabs.com/notes/lcr-meterScroll down to "LCR Meter, Standards and Stroboscope Application Note Library" and click on charged capacitor protection app note.Sincerely,BB

author: Rick
author_rank: Frequent Contributor
timestamp: October 19, 2012, 02:38:55 pm
content: Quote from: Excavatoree on October 19, 2012, 02:23:32 pmOk.

author: Rick
author_rank: Frequent Contributor
timestamp: October 19, 2012, 02:44:42 pm
content: Quote from: Excavatoree on October 19, 2012, 01:56:54 pmAnd how about the UNI-T 71E (I don't have it yet) ?People keep complaining about its glass fuses...

author: Wytnucls
author_rank: Super Contributor
timestamp: October 19, 2012, 02:55:37 pm
content: On the UT-71, depending on your board revision, there might be a spark gap footprint next to the PTC. The 10A glass fuse and holding clips can be easily removed to install a HRC fuse, as there is plenty of space on the PCB. It is more difficult to find room for the ceramic 3AG 500mA fuse, as space is at a premium in that area.Of course, it may not improve safety all that much, as trace spacing cannot be modified.Standard protection is usually one PTC on the Volt/Ohm range, 2 glass fuses on the A/mA ranges and a diode bridge rectifier with a Zener diode across it, to clamp the voltage at 6.3V on the mA/uA ranges. The A range also has a big 3W wire resistor on the Rev6 board.Here is the 71D with Rev6 board and the 71B Rev4 PCB with 3AG replacement clips fitted:

author: G7PSK
author_rank: Super Contributor
timestamp: October 19, 2012, 02:58:10 pm
content: Get a Fluke 110. There is no way you can blow the current range up on them. They don't have one.

author: eV1Te
author_rank: Regular Contributor
timestamp: October 19, 2012, 03:29:21 pm
content: How come you can measure down to 0.1 ohm with a multimeter when there is 3.5kohms in series with the input (especially the thermistor that will change several ohms if the ambient temperature changes)?

author: robrenz
author_rank: Super Contributor
timestamp: October 19, 2012, 03:42:03 pm
content: Quote from: lcr1 on October 19, 2012, 02:31:40 pmThanks lcr1,That is what I was asking about in my post above. My DE5000 LCR does not have input protection and I want to add some. I looked at Daves teardown of the Agilent LCR to see what they use but it is not a 4 wire test connection like the IET. That circuit answers my question and it made me look at the IET circuit board and as in Daves video it has unpopulated spots that look like they would be for the diodes. I called IET about it and they are going to look into what that could be populated with.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: October 19, 2012, 03:50:06 pm
content: Quote from: eV1Te on October 19, 2012, 03:29:21 pmSimple answer - the meter can tell the difference between 3.5K ohm (zero ohms across leads) and 3.50001.Stated differently - the meter is designed to measure the resistance at the jacks, down to it's lowest resolution. The circuit inside, including the 3.5K resistor is accounted for in the design.

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 19, 2012, 04:46:04 pm
content: Quote from: eV1Te on October 19, 2012, 03:29:21 pmRemember the extra 1-meg path to the input jack? That's the (high-impedance) sense line for the ohms function. You'll find it runs straight to the input jack, like a Kelvin connection. The current source for ohms measurement comes through the other relatively low-impedance series protection components.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: October 19, 2012, 09:41:40 pm
content: Here an example of a very poor input "protection" circuit. Its the Voltcraft VC840, which is rated at, believe it or not, CAT IV 600V / CAT III 1000V - sureJust to compair to a proper meter, the Gossen METRA Hit 25s (Made in Germany of course), which is rated at CAT III 600V / CAT II 1000V

author: Salas
author_rank: Frequent Contributor
timestamp: October 20, 2012, 03:52:14 am
content: Quote from: EEVblog on October 19, 2012, 04:38:43 am~13:50, are the extra diodes circling the bridge just for the burden voltage threshold at mA or A spec limit, or it could be to share power in high overload that could blow one bridge's diode if alone?

author: Dread
author_rank: Regular Contributor
timestamp: October 20, 2012, 05:40:32 am
content: Great video Dave.There was something for everyone in that video, if you are new to electronics Dave broke it down into really nice Digestable nuggets of info. If you already know how all the components work you got an excellent view of them being used in a multimeter. Most of us use meters every single day but never examin how the inputs are designed, now we knowThanks Dave.BTW Dave please do more of these kinds of videos.

author: Wytnucls
author_rank: Super Contributor
timestamp: October 20, 2012, 06:14:51 am
content: Quote from: Salas on October 20, 2012, 03:52:14 amI think their main purpose is to clamp the voltage at 3.6V (6x 0.6V). They only protect the mA and uA range, not the amp range.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 20, 2012, 09:37:06 am
content: Hi Dave.Enjoyed that tute. Explained a lot. I had a few "a-hah's". Many things I sorta knew but I am too lazy to go investigate and confirm each thing myself. Nice to have it all in one place.And the new lighting looks great. All Nicely presented.Not many youtubers' shows can I sit and watch 40 continuous minutes, mate. I hate to think how much time it took you to produce it.Thanks for that.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: October 20, 2012, 10:31:56 am
content: Quote from: EEVblog on October 19, 2012, 04:38:43 amI paused the video right before that moment and then resumed, so that was a brainfart. My point still, was that you apparently mentioned idiot users in the context of reversing the polarity. If you connect a voltage source to the current jacks, and reverse the polarity, it's not the reversed polarity that makes you an idiot. Just wanted to clarify what I was getting at.

author: JackOfVA
author_rank: Unknown
timestamp: October 20, 2012, 12:04:08 pm
content: Excellent tutorial on DMM input protection.Couple points though.1. The 1N400x series diodes are usually thought of as "slow." That's true for turn-off ("reverse recovery time"), but not for turn-on. I've measured several diode types and found the 1N400x types have a turn-on ("forward recovery time") time in the few ns range. For overload protection, the turn-on time is the limiting factor. My measurements are athttp://www.cliftonlaboratories.com/diode_turn-on_time.htm2. When used in a high impedance circuit (that is to say above the few ohms or less in a typical power supply) the MOV capacitance should be considered. This can be a problem if MOVs are used to protect a data circuit for example, as hanging a 18V MOV across the data line adds perhaps 10,000 pF of shunt capacitance. MOV capacitance is less of a concern for multi-meter usage as a 430V MOV of 6 or 7 mm diameter has only 50pF or so capacitance.It would be interesting to see a similar review of the A/D input protection and isolation portion of the meter.

author: Salas
author_rank: Frequent Contributor
timestamp: October 20, 2012, 10:34:47 pm
content: Quote from: Wytnucls on October 20, 2012, 06:14:51 amTrue, only for the mA and uA. I was hasty not to look better. Still if someone sets the switch on mA and the leads too the day before, then another one grabs it without looking and sticks it to an AC gen because the lights are out, say its a military F27/FM and there is a hasty situation, won't possibly need some 5W diodes combined power before the smaller HRC goes out?

author: Unknown
author_rank: Unknown
timestamp: October 20, 2012, 11:02:06 pm
content: Note that the fuse will blow faster the higher the current is. I assume they specced the diodes so they would draw sufficient current without overheating to safely blow the fuse. Diodes often have much higher peak surge current, for example the1N400xis specced to survive a non-repetitive 8.3 ms 30 A forward current. At 5A, aSiba 440 mA HRC fusewould blow in less than 8 ms. They would have compared these specs with the time/current curve of the fuses and made sure they stayed within safe limits of the diodes for all allowable inputs / input transients.

author: Unknown
author_rank: Unknown
timestamp: October 21, 2012, 12:54:36 am
content: Quote from: JackOfVA on October 20, 2012, 12:04:08 pmVery illuminating read, thanks for that.

author: king.oslo
author_rank: Frequent Contributor
timestamp: October 21, 2012, 12:56:25 am
content: This was an excellent video. Is this relevant?https://www.eevblog.com/forum/beginners/esd-how-can-it-be-a-problem-when-the-component-isn't-grounded/Thanks.M

author: ZekeD
author_rank: Newbie
timestamp: October 24, 2012, 03:51:17 am
content: Ran across an older Fluke 87 at work that was blown up on the DC side. I was going to repair the front end, but ran across a strange modification on one of the MOV's. Threw it in a bin and haven't thought of it in a while.The MOV has a notch cut into the top that runs down about 1/5th of the diameter. Looks like it was done with a Dremel cutting wheel, very deliberate and clean.Just wondering if anyone has run across a MOV with a notch cut into it, and what is the purpose of such a modification, (blast direction, part derating etc.). Seems a wonky mod that I wouldn't expect from Fluke.Thanks.edit - added picture of spark gap capacitor (thanks Sean for ident on this beast) with notch, unit is a 85 III true RMS

author: SeanB
author_rank: Super Contributor
timestamp: October 24, 2012, 04:42:50 am
content: That is a spark gap capacitor. Designed that way to have a controlled breakdown of around 2kV when high voltage is applied across it.

author: Marco
author_rank: Super Contributor
timestamp: October 05, 2013, 02:16:19 am
content: I'd really like a short follow up to this video which handles the second half of the input protection going to the ADC.Looking at the schematic in theservice manualam I correct in saying that Q1 and Q2 act as rather bizarre voltage clamps, with R38 taking most of the remaining voltage after the circuit described in the video?