Low Z Voltage Measurement - Page 1
Forum section: electronics / testgear

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 31, 2014, 07:24:57 am
content: I own a Keysight U1272A DMM which has a Z_Low voltage measurement, where the input impedance is as low as 2k ohm.I would simply like to know more about this function, what the values mean and how they differ from a regular voltage measurement? When measuring my 12V lead-acid battery, the normal mode measures 12.888 V, the Low Z measure 1.5 V (Which drops to 1.3V after a minute). What does this 1.5V mean?If I discharged a battery through this Low Z path and datalogged the values, would I be able to calculate the battery capacity?

author: Wytnucls
author_rank: Super Contributor
timestamp: August 31, 2014, 08:34:18 am
content: With an impedance of 2K Ohms, the meter should only draw 6mA from the battery and display the proper 12V, unless the battery cells are faulty or close to fully discharged.Read this Fluke article for a better understanding of the feature:http://www.fluke.com/fluke/uses/comunidad/fluke-news-plus/articlecategories/electrical/dualimpedance.htm

author: HighVoltage
author_rank: Super Contributor
timestamp: August 31, 2014, 08:59:29 am
content: Quote from: TheAmmoniacal on August 31, 2014, 07:24:57 amIt seems to me that your battery is defective.A good and charged battery that shows 12.8V in normal high impedance mode, will drop may be to 12.5 V in Low Z mode.This difference is caused by the internal resistance of your battery. And since you know the external resistor that you connected to the battery is 2k Ohm, you could calculate the internal resistance. Although, depending on the size of your battery, the 2k Ohm resistor may not provide sufficient load to the cells. But it gives a good first impression, if your battery is still good. In your case it has proven that your battery is dead.

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 31, 2014, 09:13:13 am
content: I think there's some misunderstandings here, this is what the Low Z mode reading looks like (from the manual):Up in the right corner is the voltage measurement of the battery, in my case that shows 12.8V on a new pack and 12.6V on an old one. While both show 12.900-12.888 on regular mode.I want to know what the number in the center means, the ~1.2 V

author: Wytnucls
author_rank: Super Contributor
timestamp: August 31, 2014, 09:23:49 am
content: It tells you in the multimeter manual:The AC component. Why you should have a 1.2V AC component is unclear to me at this stage.I suspect an artifact from the TRMS converter.

author: omgfire
author_rank: Regular Contributor
timestamp: August 31, 2014, 09:27:27 am
content: Quote from: Wytnucls on August 31, 2014, 09:23:49 amTheAmmoniacal, Is battery connected to something?

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 31, 2014, 09:28:34 am
content: Quote from: Wytnucls on August 31, 2014, 09:23:49 amThe AC component of a DC signal? Like if there's coupling in my leads? Or is that value only for Low Z measurements in AC circuits? (Making the value just noise?)

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 31, 2014, 09:35:01 am
content: Quote from: omgfire on August 31, 2014, 09:27:27 amNo, the battery is not connected to anything. And even if I keep the probes for several minutes the AC measurement value stops at ~ 1.2 V. It doesn't drop any further.

author: Wytnucls
author_rank: Super Contributor
timestamp: August 31, 2014, 09:35:53 am
content: Try the same with the low pass filter ON.

author: omgfire
author_rank: Regular Contributor
timestamp: August 31, 2014, 09:42:40 am
content: BTW, accuracy of U1272A Zlow is 1% + 20 for DC and 2% + 40 for AC.So, 4.0V AC reading for 0V AC will be within specification.

author: Wytnucls
author_rank: Super Contributor
timestamp: August 31, 2014, 09:43:17 am
content: In Low Z mode the meter is set to 1000V range, with an accuracy of 2%+40 digits. So, at O volt AC, any readings up to 4V error is normal.

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 31, 2014, 09:47:17 am
content: Yeah, I'm starting to understand this now. The 1.2 V that it's showing can be either within spec or internal stray voltages in the meter itself. And it turns out to be the AC measurement. Thanks for making this clear.At 2k Ohm a 12V battery would take a long long time to discharge at 6 mA, not really worth it.

author: Wytnucls
author_rank: Super Contributor
timestamp: August 31, 2014, 09:50:52 am
content: I think it has more to do with the true RMS converter limitations, than stray voltages.

author: commongrounder
author_rank: Frequent Contributor
timestamp: September 01, 2014, 09:40:06 pm
content: The low-Z mode on this, and other multimeters, is designed for reading AC line voltages in electrical systems. The purpose is to load down (and eliminate) "phantom", inductively coupled, voltages that might appear on disconnected circuits. This will help the electrician determine the true status of a circuit. I expect the secondary DC display would be useful for showing any (rare) DC offset that may be present on the line.

author: electr_peter
author_rank: Unknown
timestamp: November 27, 2014, 11:37:09 pm
content: Topic about U1272A low Z mode has been discussed beforeu1272a-low-z-in-off-modeYes, it is normal for U1272A on low Z mode AC voltage mode (or any AC voltage mode) to have up to ~40 counts of residual reading (this is due to true RMS converter). Manual for low Z mode says that AC specs are 2% + 40.One thing to be aware on U1272A series multimeters - off position is low Z mode as well. That is ~1.8kOhm with meter switched off across the inputs.