Author Topic: Question about VC8145-Newbie  (Read 8012 times)

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Offline Wytnucls

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Re: Question about VC8145-Newbie
« Reply #25 on: January 22, 2017, 01:28:44 pm »
I never said a bench meter should have at least 6.5 digits, but these days most do, as the technology gets cheaper. Accuracy goes hand in hand, whether you need it or not.
20A on a bench? What do you deal with? Hadron colliders? Equipped with a 13A medium blow British BS1632 fuse, a limit of 15 seconds and an accuracy of a few %? You must know that a lot of cheap DMM manufacturers often boast about that feature.
I am surprised you even brought it up, as most professionals would rather connect a safer current shunt to measure anything above a few amps.

That said, if on a budget and can get by with portable DMM features in a stackable box, the Vici is for you, but be aware of all its warts.
« Last Edit: January 22, 2017, 03:01:27 pm by Wytnucls »
 

Offline nctnico

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Re: Question about VC8145-Newbie
« Reply #26 on: January 22, 2017, 03:58:10 pm »
Every now and then I get projects involving power electronics and having some headroom over 10A (which is a nice round number to write in a specification) has proven to be beneficial. I'm aware of the time limits of the high current measurements capabilities but it is a convenience thing. Mostly I need to measure the current to check if the current read back of a circuit is within tolerance (usually worse than 1% accurate) or to measure efficiency. Both these measurements don't take longer than a few seconds.

I wouldn't call a current shunt (typically without enclosure) safer. Putting a shunt in the 0V (low side) prevents me from hooking the ground clip of a scope in various places or connect other grounded equipment (like a PC). Also a shunt may become open (due to a fuse or whatever) which could wreck a whole lot of equipment if it turns out there is a ground loop after all (*). Therefore in my test setups I generally want one central grounding point which is 0V so I (typically) measure currents in the supply line (high side) which in some cases can be (way) over 48V. A shunt which is out in the open would be dangerous to touch and have an extra bunch of leads coming from it to a DMM. The DMM would also need to be able to convert the voltage reading into a current reading. All in all a DMM with a high current capability brings everything conveniently together in a safe-to-touch box and if the shunt burns out then it does so inside a box and not out in the open.

(*) Some of the testing I do involve power supplies capable of delivering several kW of power which can do a lot of damage. This relay contact for example just melted away because of arcing:
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Wytnucls

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Re: Question about VC8145-Newbie
« Reply #27 on: January 22, 2017, 09:55:41 pm »
Most people won't need them, but there are enclosed shunts, like the one below, made by Fluke (30$ second-hand).
Direct connection is more convenient, but the problem with high current through the meter is that it could damage  the internal shunt permanently, if the shunt temperature exceeds 120 degrees C, compromising further readings.
The feature Y=mX+b is fairly common on bench meters. Both of my cheapish bench meters can easily do the V/I conversion (Rigol 3058E and Keithley 2000)
You're still left with the ground loop problem though.

http://www.ebay.ca/itm/Fluke-80J-10-10-Amp-100-mV-20-Amps-25-Accuracy-DC-to-10-kHz-Current-Shunt-/222031485558?hash=item33b21b9a76:g:9ysAAOSwezVWy3Ny
« Last Edit: January 24, 2017, 05:48:47 am by Wytnucls »
 


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