Author Topic: EEVblog #373 - Multimeter Input Protection Tutorial  (Read 17272 times)

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Offline Ed.Kloonk

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EEVblog #373 - Multimeter Input Protection Tutorial
« Reply #25 on: October 20, 2012, 08:37:06 pm »

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.

 :)

« Last Edit: October 20, 2012, 10:31:57 pm by Ed.Kloonk »
 

Offline nitro2k01

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Re: EEVblog #373 - Multimeter Input Protection Tutorial
« Reply #26 on: October 20, 2012, 09:31:56 pm »
13:26, idiot users? Aren't multimeters specced to accept negative currents of the same magnitude as positive currents?

Umm, yeah, that didn't make much sense did it?  :-[
Either way, when you connect the current range to a voltage source, you are an idiot  ;D

Dave.
I 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.
Whoa! How the hell did Dave know that Bob is my uncle? Amazing!
 

Offline JackOfVA

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Re: EEVblog #373 - Multimeter Input Protection Tutorial
« Reply #27 on: October 20, 2012, 11:04:08 pm »
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 at http://www.cliftonlaboratories.com/diode_turn-on_time.htm

2. 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.
 

Offline Salas

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Re: EEVblog #373 - Multimeter Input Protection Tutorial
« Reply #28 on: October 21, 2012, 09:34:47 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?
I 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.

True, 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?
 

alm

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Re: EEVblog #373 - Multimeter Input Protection Tutorial
« Reply #29 on: October 21, 2012, 10:02:06 am »
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 the 1N400x is specced to survive a non-repetitive 8.3 ms 30 A forward current. At 5A, a Siba 440 mA HRC fuse would 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.
 

HLA-27b

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Re: EEVblog #373 - Multimeter Input Protection Tutorial
« Reply #30 on: October 21, 2012, 11:54:36 am »
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 at http://www.cliftonlaboratories.com/diode_turn-on_time.htm

Very illuminating read, thanks for that.
 

Offline king.oslo

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Re: EEVblog #373 - Multimeter Input Protection Tutorial
« Reply #31 on: October 21, 2012, 11:56:25 am »
 

Offline ZekeD

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Re: EEVblog #373 - Multimeter Input Protection Tutorial
« Reply #32 on: October 24, 2012, 02:51:17 pm »
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
« Last Edit: November 01, 2012, 06:46:03 am by ZekeD »
 

Offline SeanB

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Re: EEVblog #373 - Multimeter Input Protection Tutorial
« Reply #33 on: October 24, 2012, 03:42:50 pm »
That is a spark gap capacitor. Designed that way to have a controlled breakdown of around 2kV when high voltage is applied across it.
 

Online Marco

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Re: EEVblog #373 - Multimeter Input Protection Tutorial
« Reply #34 on: October 05, 2013, 12:16:19 pm »
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 the service manual am 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?
 


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