EEVblog #373 - Multimeter Input Protection Tutorial

[EEVblog]

Dave.

[Excavatoree]

Great video, Dave.

[iloveelectronics]

Excellent stuff!!! Teaches me a lot as a newbie hobbyist in electronics. Thanks a bunch, Dave!

[robrenz]

Are there any variations of this on a LCR meter?

[nitro2k01]

13:26, idiot users? Aren't multimeters specced to accept negative currents of the same magnitude as positive currents?

[EEVblog]

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 

Dave.

[iloveelectronics]

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?

[cyteen]

They are actually specced for spark gaps (SG1, SG2).

see the question was asked here https://www.eevblog.com/forum/product-reviews-photos-and-discussion/uni-t-multimeter-ut71d-unboxing-and-teardown/msg89873/#msg89873

[Rick]

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?

[iloveelectronics]

They are actually specced for spark gaps (SG1, SG2).

see the question was asked here https://www.eevblog.com/forum/product-reviews-photos-and-discussion/uni-t-multimeter-ut71d-unboxing-and-teardown/msg89873/#msg89873

Thanks! I didn't even know what a spark gap is So after reading that other thread I guess it's ok to just put 1000V rated MOV's in those places?

[Excavatoree]

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?

Yes, 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.

[Excavatoree]

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.

[lcr1]

I have used the circuit in this application note on Charge Capacitor Protection.  Thought it might be helpful

The application note can be found at http://www.ietlabs.com/notes/lcr-meter

Scroll down to "LCR Meter, Standards and Stroboscope Application Note Library" and click on charged capacitor protection app note.

Sincerely,

BB

[Rick]

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.

Ok.

[Rick]

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?

Yes, 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.

And how about the UNI-T 71E (I don't have it yet) ?
People keep complaining about its glass fuses... 

[Wytnucls]

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:

[G7PSK]

Get a Fluke 110. There is no way you can blow the current range up on them. They don't have one.

[eV1Te]

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)?

[robrenz]

I have used the circuit in this application note on Charge Capacitor Protection.  Thought it might be helpful

The application note can be found at http://www.ietlabs.com/notes/lcr-meter

Scroll down to "LCR Meter, Standards and Stroboscope Application Note Library" and click on charged capacitor protection app note.

Sincerely,

BB

Thanks 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.

[Excavatoree]

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)?

Simple 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.

[ModemHead]

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)?

Remember 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.

[PSR B1257]

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 - sure 


Just 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

[Salas]

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 

Dave.

~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?

[Dread]

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 know

Thanks Dave.

BTW Dave please do more of these kinds of videos.

[Wytnucls]

~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.