EEVblog #931 - Designing A Better Multimeter PART 2

Dave takes a look at how to optimise the low burden voltage multimeter design from PART 2 to eliminate some parts.
And explains the µCurrent shunt circuit design and how that applies to this new optimised design.
It's basically a practical tutorial in circuit design optimisation.
Then another design variation is presented using MOSFET switching that eliminates ones of the HRC fuses and the amplifier, and still lowers the burden voltage yet again!

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[코더-gw]

I made powersupply thanks to your videos and I successfully took a job interview with this psu. sincerely thanks a lot

[mikeselectricstuff]

You'd also need to be careful about leakage current of the big MOSFET affecting accuracy of the uA range - I doubt low-level leakage is well specified on most big MOSFETs as most users of them won't care about it.

[Eo_Tunun]

Arr! This be pleasin' to me ears! Dave and a whiteboard are a dream team, really. I learn the most from these videos.
Awesome!

[mikeselectricstuff]

I wonder about the safety approvals aspect of putting a MOSFET in series with the 10A fuse. The point of the big-ass HRC fuse is so it doesn't explode when a high energy source is connected.
What's to stop the MOSFET exploding, creating just the same safety hazard the fuse was designed to protect against. Especially if it's turned off, so you have the energy dissipated in the body diode rather than the low on resistance.
And if it doesn't explode, it might well fail before the fuse opens.

[shomonercy]

The beauty is strong in this one:

6:19 bingo! too easy.
7:15 beauty!
7:44 trap for young players
9:16 beauty!
10:14 bingo! bob's your uncle.
10:41 trap for young players
17:31 everything's "hunky dory"
19:17 beauty!
19:42 bingo! .. no worries.
21:14 trap for young players
22:23 beauty!
23:36 beauty!
26:15 beauty!
30:05 beauty!
30:46 bingo!
31:42 winner, winner: chicken dinner.

[jessstuart]

Here are a few other ideas...

1. Measure the burden voltage across your DMM, then increase your supply voltage (if adjustable) by that amount to compensate for the burden voltage. That way your DUT will have the desired voltage drop across it. This will not work if the DUT current has large current spikes or significant non-DC components.

2. Use a precision 1-ohm shunt resistor, and a good auto-zeroing instrumentation amplifier (MAX4209, gain=100) and build up a little external battery-powered current-sense amplifier circuit. Then just use a regular DMM volt-meter to read off the output voltage, which is proportional to the current. Or watch the output voltage waveform on an oscilloscope for a DIY precision current probe!

[voltlog]

this part where you optimized the circuit seemed more interesting than the original video where you presented the idea.

[DigitalHomebrew]

Looks more elegant than the design in part #1, especially because as a user I only have to remember one burden voltage for all ranges. I personally would pay the extra $4 (plus markup) for the chopper amplifier to get 5mV burden voltage, even if it introduces 1LSB of error on a 60, 000 count meter. NEXT UP: A video on sexy input protection circuitry!

[johnnyprimavera]

Friggin 'osome!! I learnt more than in the last ten videos. Facts about mosfets, op amp nuissances, multimeter topologies... Thumbs up on a MOSFET selection video or related.

[IDoNotLikeHandlesOnYT]

My first thought when I saw you put two MOSFETs in inverse parallel was "you can't turn that off!" It took me a minute to realize that the forward voltage drop of the body diodes is what turns it off when you're using lower ranges.

[txescientist]

We need more design videos! :-) Keep up the good work!

[hlavaatch]

I may be weird but I like this ;) More design content please! Design livestream with viewer participation maybe?

[Feedback_Jack]

This was a really good video. Held my attention the whole time.

[tomy]

I really hope you will continue with the multimeter design all the way! Great video!

[worroSfOretsevraH]

Everyone's favorite segment: Mai... WHAT?!?!
This one IS!

[thegreatgreenarkleseizure]

Love seeing different ways to solve problems.

[antalz]

When you put the MOSFETs back-to-back for protection, doesn't that also cut the 25mv burden voltage in half to 12.5mv?

[alextrofimov]

Great video Dave!
I love the MEH concept) I think I will use it in my work)

[theantipope]

Dave: On the Amp range, why is it better to use that 2nd beefy MOSFET for protection instead of a generic rectifier diode? I get the advantage of not adding another type of component to the BOM, but wouldn't a diode be enough cheaper than a heavy MOSFET to outweigh that benefit?
PS: As a former digital design guy, I really appreciate the analogue design tutes like this one - please keep 'em coming! :)

[NaudioElectronics]

Great video, Dave!
IMHO cheaper solution is to replace the extra protection mosfet with a diode. Also the diode is smaller compared to the mosfet.