EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: electr_peter on April 10, 2014, 04:53:05 pm
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Hello, EEVblog forum members,
Although this is my first post on the forum, I have read forum post and watched Dave's videos for a while. I am also proud owner of µRuler and µCurrent (still waiting). I decided to post a few posts myself.
Anyway, I got an Agilent U1272A multimeter with low Z mode. I used other multimeter to measured low Z mode impedance, I got 1.76kOhm (I suspect that is normal value). DC/AC modes reads as 10-11MOhm, all meter functions work well.
I was surprised to find that even with knob turned to off position, impedance of U1272A was still 1.76kOhm (same as low Z). This is pretty low value. For me it reminds to always disconnect probes before turning off the meter (most DMM manufacturers recommend first to choose measurement/range, only then connect the probes). I would not leave my probes connected to battery or mains(mostly due to safety issues, of course) and turn off the meter - impedance is a bit low. Manual doesn't mention this feature in off mode.
My questions for forum users:
1) Is it normal for Agilent U1272A multimeter to be in low Z mode when turned off? Can someone check their meter?
2) What purpose does low Z mode serve in off mode? Multimeter voltage inputs are protected by various other devices anyway. This seems a bit overkill to lower voltage build up inside the meter.
3) Some other meters (like Fluke 11X series) also have low Z mode. Can someone check off mode impedance on these meters?
4) What would you expect for off mode to do in typical multimeter?
a) disconnect voltage input + turn off power
a) high Z (~10MOhm) voltage input + turn off power
a) low Z (1-3kOhm) voltage input + turn off power
I appreciate your answers.
Edit:
Suggestion for Dave - when testing multimeters, add these 2 tests:
1) check input impedance on all modes and ranges, especially off mode
2) put your ears near the multimeter and listen - does it produce sounds it should or shouldn't produce?
[sarcasm]
I do hope 2) suggestion won't add too much audiophoolery to the blog.
[/sarcasm]
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Hi,
I measured this on my u1272a aswell :
-lowZ mode has 1.83kOhm input impedance
-normal V mode has about 11Mohm impedance
-turned off, it also has 1.83kOhm input impedance!! That sounds dangerous!
compared to an Agilent u1232a :
-V mode has 11Mohm
-turned off, it has infinite impedance
-lowZ mode has 2.66kOhm
-> the u1232a seems much safer compared to u1272a!
compared to a Fluke 187 (that does not have LowZ) :
- input is always 11MOhm (both off and in V mode)
compared to a Fluke 289 (now it's getting interesting), the 289 does not have off on the rotary dial, but a separate on/off switch :
- dial in V mode, turned on : 11MOhm
- dial in V mode, turned off : 13MOhm (huh?)
- dial in LowZ mode, turned on : 3.3kOhm
- dial in LowZ mode, turned off : 3.3kOhm
-> also not very safe, but still better than u1272a
Anyway, it's not a good thing that the U1272a reverts to 1.8kOhm when turned off! The u1232a seems to do it right, the Fluke 289 gets it mostly right.
I also have a question for you : I like to work without noises, so I turned the beeper off on my u1272a. Good! But it still makes 'clicking' noises where it would have beeped before. So when I turn it on, it goes 'click click'. I don't think it's a relay switching, but instead a DC voltage being applied to the buzzer (instead of a sine). Could you do me a favor and check if your's also clicks twice when turning it on after turning the beeper off?
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Reading the manual, it does actually say on p134 that the impedance is 1.67kOhm in off mode for the u1272a (protected by a PTcoef. resistor).
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WVL_KsZeN, thanks for measuring and confirming low Z mode in off setting for U1272A. It is dangerous in my opinion - some cheap pocket meters even have similar value resistor for current range.
Reading the manual, it does actually say on p134 that the impedance is 1.67kOhm in off mode for the u1272a (protected by a PTcoef. resistor).
I rechecked Agilent u1272A manual just to be sure http://cp.literature.agilent.com/litweb/pdf/U1271-90010.pdf (http://cp.literature.agilent.com/litweb/pdf/U1271-90010.pdf), p134/p.154
INPUT IMPEDANCE AT OFF MODE (U1272A only)
1.67 kOhm (protected by positive temperature coefficient resistor)
I also have a question for you : I like to work without noises, so I turned the beeper off on my u1272a. Good! But it still makes 'clicking' noises where it would have beeped before. So when I turn it on, it goes 'click click'. I don't think it's a relay switching, but instead a DC voltage being applied to the buzzer (instead of a sine). Could you do me a favor and check if your's also clicks twice when turning it on after turning the beeper off?
I turned off beeper in menu (by the way, I would like to have 3 beeper settings - one for on-off, one for buttons and one for beeper. U1272A allows only 1 global option). With beeper turned off, I can clearly hear 2 clicks during turn on! Clicks are heard when knob is turned from off to on or when you exit setup menu (hold setup to exit menu -> auto reset, turn off and and turn on).
I will try to describe sound:
knob is turned from off to on,
~0.5-1s passes, silence
first faint click is heard,
~0.3s passes, silence
second faint click is heard (same sound as first)
faint buzz starts (harder to hear, high pitch) and continues till turn off. I suspect it is crystal or dc/dc converter in action
2 clicks may have something to do with buzzer not fully of.
I think changes to firmware could eliminate those clicks.
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I also have a question for you : I like to work without noises, so I turned the beeper off on my u1272a. Good! But it still makes 'clicking' noises where it would have beeped before. So when I turn it on, it goes 'click click'. I don't think it's a relay switching, but instead a DC voltage being applied to the buzzer (instead of a sine). Could you do me a favor and check if your's also clicks twice when turning it on after turning the beeper off?
I have a U1273A. There is no "clicking" if you turn off the beeper.
Edit:
Sitting here with my meter at the ear... :D
There is realy no clicking. But i can here a veeeery quiet "feeeeeeeeeep" when the meter is on. Doesnt matter if the beep is on or not. Its always there.
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I also noticed that you still can hear extra faint clicks during continuity/current jack warning. On-off beeps (clicks that are) are much louder than these continuity ones. Thus it seems that buzzer is not isolated/turned off fully, most likely PCB/schematics + firmware nuance.
Upgraded firmware for U1272A could held here. Why do they drive buzzer if it is turned off?
I have a U1273A. There is no "clicking" if you turn off the beeper.
Click is not very loud or pronounced on U1272A, you have to put your ears near it to know for sure.
U1273A has different screen - maybe firmware is a bit different and they do not drive turned off buzzer in this case.
What is off mode impedance on your U1273A - the same as U1272A, ~2kOhm?
There is realy no clicking. But i can here a veeeery quiet "feeeeeeeeeep" when the meter is on. Doesnt matter if the beep is on or not. Its always there.
I would call that "high frequency buzzing" sound - it starts right after two turn on clicks(which you don't hear - so there aren't any because these are a bit louder) and continue till turn off.
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I was recently informed by one of my cohorts that he has observed this low impedence behavior when the U1272A is switched to OFF. This was noticed while troubleshooting a problem. When the meter automatically turned off, his habit is to just roll the knob to off and then back to the volt setting. This applied the low impedence input to the circuit and caused the circuit to begin misbehaving.
Not sure why you would design a meter to go to the impedence level of the low impedence setting. Seems like a bad design decision to do this, even if the manual does tell you the meter acts this way. By the way, I think they called Agilent and whoever answered the phone seemed to be surprised that the meter acted this way. Maybe Agilent didn't read the manual either.
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Hello guys ! I just bought an Agilent 1272A and it seems that has an issue with zlow voltage. it is showing 6.3 V AC without any input. if i short the probes i have the same reading (6.3V). Could you tell me what is the reading of your meter in this case ? thanks !
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Hello guys ! I just bought an Agilent 1272A and it seems that has an issue with zlow voltage. it is showing 6.3 V AC without any input. if i short the probes i have the same reading (6.3V). Could you tell me what is the reading of your meter in this case ? thanks !
Hi, spin9
Don't worry, your meter is perfectly fine, my meter also shows similar voltage on low Z. This is not a fault.
U1272A manual states that for low Z mode, DC specs are 1% + 20, AC specs are 2% + 40 - pretty awful in comparison to normal Z mode, but you get additional and very important functionality.
low Z mode is not meant to test low voltage batteries or be very accurate, it is designed and meant for 120/240/400V mains measurement (and measuring it in a safe way). This measurement should tell you if there is dangerous voltage on the line or not (binary answer, no need for high accuracy at all). Thus you get 1000V (high voltage) range with 0.1V resolution, clearly electrician type stuff.
low Z is also not meant to be accurate at the same level as low level DC volts (do you care if mains in your house is 233.5 or 233.4?).
So, if you think that your meter is "defective", I will be more than happy to buy this "defective" meter of yours at lower price.
EDIT: My U1272 show 3.4V on AC and 0.0V on DC for low Z, so everything is in spec.
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I was recently informed by one of my cohorts that he has observed this low impedence behavior when the U1272A is switched to OFF. This was noticed while troubleshooting a problem. When the meter automatically turned off, his habit is to just roll the knob to off and then back to the volt setting. This applied the low impedence input to the circuit and caused the circuit to begin misbehaving.
Not sure why you would design a meter to go to the impedence level of the low impedence setting. Seems like a bad design decision to do this, even if the manual does tell you the meter acts this way. By the way, I think they called Agilent and whoever answered the phone seemed to be surprised that the meter acted this way. Maybe Agilent didn't read the manual either.
U1272A behaviour on in off mode with low impedance is kinda strange, see posts above. It is mentioned in the manual though, but if even Agilent acts surprised...
I can think the only possible reason Agilent put low Z in off mode is the range switch design. Either it is easier to it this way or there was some mistake at early design stage. By the way, significant force has to be applied to switch low Z or off mode in comparison to other ranges.