Benchtop DMM with low voltage continunity test (controllable)? - Page 1
Forum section: electronics / testgear

author: forrestc
author_rank: Unknown
timestamp: July 08, 2017, 01:10:53 am
content: I just happened across the specs in the manual for a DMM I'm using for automated test purposes (E1411B) which specifies the open circuit voltage for resistance test. It's 11.5V. Considering that I'm using this for (among other things) making sure the Vcc rail isn't shorted to ground before power up, I'd rather use something which has a lower open circuit voltage to avoid any possible damage during test.Unfortunately this spec is fairly rare to actually see published. I've tested the Rigol DM3058E and the Hameg HMC8012 that I have on the bench here and it seems the rigol uses around 6.2V in all ranges including continuity, and the Hameg is over 8.5V, except in continunity where it is an acceptable 1V - at least when loaded with whatever the impedance of the DM3058E is.I found a spec on the Keithley 2000 which is also in the automatic test rack, and it states it is over 6V in all ranges (some even higher). I need to measure it to make sure this data is valid, but I have no reason to doubt it.Since none of the DMMs I have around here seem acceptable (except the HMC but that's an expensive continunity tester), Is anyone aware of a DMM with GPIB or LXI or serial control which uses a lower open circuit voltage?For those of you with a couple of computer-controllable DMM's on the bench, would you mind measuring the open circuit voltage of the resistance and continuity ranges? I'm asking for "ranges" since using a DMM to measure the open circuit voltage will likely be inaccurate at the higher resistance ranges, and it also seems that DMM's sometimes use different voltages for different ranges.

author: alm
author_rank: Super Contributor
timestamp: July 08, 2017, 01:52:31 am
content: In general I would not expect an issue because if it is shorted, it will obviously be pulled low, an if it is not, there is likely enough capacitance to absorb the energy. But if the rail capacitance is very low or your device is extremely sensitive, then I can see your concern. It might be interesting to connect a FET probe across the power rail, and then switch on the resistance/continuity test.I am not aware of any meter that has this configurable. Only dedicated low resistance meters like the Keithley 585 allow you to limit the maximum voltage a value like 100 mV (but not for damage, but to prevent breaking up any corrosion that may be on the contacts). Even in diode check the open circuit voltage will often be something like 6 V (just checked a Keithley 2000 and HP 34401A). Even if meter specify that the maximum voltage is 2 V, that is often only if the resistance is within range. The open circuit voltage will often be at least 5 V.The only suggestion I have is to connect a resistor in parallel that is near the top of the range you are using. This should limit the voltage as long as that resistor does not get disconnected. That, or use a programmable current source / SMU that can limit the compliance voltage to a low value (or a programmable voltage source that can limit the current). That is how you measure resistance whenever the canned DMM resistance test are not sufficient for whatever reason (voltage coefficient, very low resistance, very high resistance, speed, etc).

author: tautech
author_rank: Super Contributor
timestamp: July 08, 2017, 01:58:01 am
content: IIRC there's been a few discussions about this and bench units seem to have higher V values but they more correctly need be considered as a current source which is current limited, commonly @ 1mA.Wonder where those threads are.............Edit.One found so far:https://www.eevblog.com/forum/testgear/multimeter-with-low-continuity-test-voltage/

author: alm
author_rank: Super Contributor
timestamp: July 08, 2017, 02:13:45 am
content: Well, an ideal current source will develop an infinite voltage when open circuit, so I am not sure how reassuring that is. Especially with long cables (capacitance) in an ATE, I can definitely see the potential for damage for some very sensitive circuits. Probably not on power rails, however.

author: Someone
author_rank: Super Contributor
timestamp: July 08, 2017, 03:21:00 am
content: Quote from: alm on July 08, 2017, 01:52:31 amGreat advice, I use an SMU as a multimeter as it can do all the interesting things like capacitance at different bias voltages or zero burden voltage current measurements. All sorts of SMU capabilities are rapidly appearing in affordable "power supplies" while lacking 4 quadrant (or even 2 quadrant) operation the new Keithley 2280 has 6.5 digit measurements:http://www.tek.com/tektronix-and-keithley-dc-power-supplies/keithley-2280s-series

author: forrestc
author_rank: Unknown
timestamp: July 08, 2017, 05:26:50 am
content: Quote from: alm on July 08, 2017, 01:52:31 amI guess I should have been more clear: by controllable I really meant that I could operate the meter via GPIB or serial or LXI or similar. Not that the open circuit voltage is controllable.Quote from: alm on July 08, 2017, 01:52:31 amConsidering I have a current source available to me as part of the test routine, I will probably go that route. I may alternatively switch a resistor across the DMM to ensure the voltage is below a certain level - I'm using this for continunity so if I set the range such that as an example a 10K resistor drops the "open circuit" voltage below 3.3V I should be good.

author: 2N3055
author_rank: Super Contributor
timestamp: July 08, 2017, 09:03:32 am
content: On some meters that have separate diode and continuity test, you can also clamp continuity test with diode.On my Brymen 869 it works even with Schottky diode, so clamping at less than 0.2V..With diode it still shows like it is open circuit and functions correctly ...This doesn't work with Fluke meters, they have continuity/diode test combined...Of course, that's only continuity...

author: pigrew
author_rank: Frequent Contributor
timestamp: July 08, 2017, 01:27:44 pm
content: Keithley has dry-ohms mode on some of their units, which limits the output voltage. Seethis brochure. For example the 2010, 3706, and 2750 support dry ohms