Will instructions be provided for replacing the 4007 diodes with the TVS for current 121gw owners in the future?
There is also a resistor that Dr. Frank has identified that may need replacing to enable a higher current for stability reasons.
Ground, Earth, Static Field... whatever.
I propose simply to calculate the equivalent voltage on the last digit in 50mOhm range, that is about 0.4 mA times 1mOhm = 400nV.
That's simply extremely sensitive, so shielding is also difficult in a handheld DMM.
The reference current should have been 5mA, to get a stable reading as in 500 Ohm range.
Instead both ranges are derived from the same 1k reference resistor, but the 50 Ohm range uses an amplified AD range, which is the culprit
Frank
The 50 Mohm range uses 20nA reference current by its internal R9 = 10MOhm reference resistor, which causes a susceptibility to leakage currents (I will demonstrate that later). Maybe it's possible to increase the Vref (by firmware) from about 0.2V to 2V, to get 200nA test current, but probably that's not possible due to supply/reference voltage limitation, as in total >4V would be required. See HY3131 Confirguration document, page 31.
The 50 Ohm configuration also follows the HY3131 document, see page 33, so R26 = 1k is used also for 50 Ohm, giving 470µA test current only, and a sensitivity of 470nV on the last digit, causing the noise trouble. The analog multiplex bus of the HY3131 is completely occupied, so it's not possible to add another 100 Ohm reference resistor for 5mA test current.
Frank
The voltage reference is connected to AGND, not AVSS which is the battery negative. The full battery voltage is not across the reference, as AGND is a referenced voltage rail lifted above ground.
If you actually measure the voltage across the reference I think you'll find it's fine.
The 50 Ohm configuration also follows the HY3131 document, see page 33, so R26 = 1k is used also for 50 Ohm, giving 470µA test current only, and a sensitivity of 470nV on the last digit, causing the noise trouble. The analog multiplex bus of the HY3131 is completely occupied, so it's not possible to add another 100 Ohm reference resistor for 5mA test current.
The voltage reference is connected to AGND, not AVSS which is the battery negative. The full battery voltage is not across the reference, as AGND is a referenced voltage rail lifted above ground.
If you actually measure the voltage across the reference I think you'll find it's fine.
Doh...
i was sure I made some mistake.
I have mixed up AVSS with AGND, sorry.
I must subract 1.8V from B+ of course.
So it is clear that the 121GW can be used with lithium batteries.
In case that the lithium cells are brandnew the voltage at the reference chip is above the operation voltage for a short time, but never above the absolute maximum voltage.
Thanks for clarifying.
The replacement of the 1N4007s by a TVS (SM6T22CA) probably is much better for ESD / overvoltage protection, as these TVS react much faster than ordinary power diodes.
A TVS can't be used in that application, though, because they have leakage currents of up to 200nA @ 25C
The voltage reference is connected to AGND, not AVSS which is the battery negative. The full battery voltage is not across the reference, as AGND is a referenced voltage rail lifted above ground.
If you actually measure the voltage across the reference I think you'll find it's fine.
Doh...
i was sure I made some mistake.
I have mixed up AVSS with AGND, sorry.
I must subract 1.8V from B+ of course.
So it is clear that the 121GW can be used with lithium batteries.
In case that the lithium cells are brandnew the voltage at the reference chip is above the operation voltage for a short time, but never above the absolute maximum voltage.
Thanks for clarifying.
Beware that the AGND voltage seems to change depending on measurement mode. It's i.e. 1.09V in Ohms mode and 0.38V in Diode mode - and that seems to put more voltage over the ADR3412 than it perhaps was designed for - all depending on how fresh the batteries are.
Or am I also missing something? I measured the voltage between the batteries (-) which = VSS and the COM terminal which = AGND.
The replacement of the 1N4007s by a TVS (SM6T22CA) probably is much better for ESD / overvoltage protection, as these TVS react much faster than ordinary power diodes.
A TVS can't be used in that application, though, because they have leakage currents of up to 200nA @ 25C
That's not correct.
Not only is the 200nA a maximum figure (not typical), it's at the rated voltage.
It drops drastically with lower voltage.
The voltage reference is connected to AGND, not AVSS which is the battery negative. The full battery voltage is not across the reference, as AGND is a referenced voltage rail lifted above ground.
If you actually measure the voltage across the reference I think you'll find it's fine.
Doh...
i was sure I made some mistake.
I have mixed up AVSS with AGND, sorry.
I must subract 1.8V from B+ of course.
So it is clear that the 121GW can be used with lithium batteries.
In case that the lithium cells are brandnew the voltage at the reference chip is above the operation voltage for a short time, but never above the absolute maximum voltage.
Thanks for clarifying.
Beware that the AGND voltage seems to change depending on measurement mode. It's i.e. 1.09V in Ohms mode and 0.38V in Diode mode - and that seems to put more voltage over the ADR3412 than it perhaps was designed for - all depending on how fresh the batteries are.
Or am I also missing something? I measured the voltage between the batteries (-) which = VSS and the COM terminal which = AGND.
I was not aware that the virtual ground is changing with the measurement mode. With 0.38V in Diode mode my doubts would be true, event though I made a mistake with AGND. That needs further investigations. But I think like you, we maybe overlook something. i cannot believe that UEI has overlooked such a problem.
Update:
Meanwhile I made the same measurement, the voltage between com terminal and - battery. It is -1.8V as expected in all modes, except Diode test. In 15V Diode test mode there are indeed only 0.38V. I assume because of the high 15V level they had to shift the virtual gnd level in Diode mode, prevent the adc to overrange.
And now the problem is back again, with lithium batteries the voltage at the reference element is over 6 V in 15V Diode test mode.
Update 2:
Actually there is no problem.
With alkaline batteries the voltage at the reference chip will never exceed 6 V, even in the 15V Diode mode. According to the manual there are only alkaline batteries mentioned.
This meter cannot be used with lithium batteries. In the 15V Diode test mode is a high risk to damage it.
So: Do not use the meter with lithium batteries!
That is the point we have overlooked.
It should be a warning in the manual.
I put Lithium in mine the first day I got it - wanting to avoid any potential leakage problems with the usual chemistries.
Now I am worried about the Lithiums...
Hi,
I noticed that the meter is locked in a auto-ranging loop in the DC+AC mode in some conditions (Ver. 1.26).
For example: Genrate this signal with a function generator:
6.4Vss, sinus, 800Hz together with a DC offset of +6.6V.
I've seen this very same auto-ranging loop in AC+DC simply measuring the mains 50 Hz, 230 V.
Regards.
can one use Lithium or LSD NiMH batteries?
can one use Lithium or LSD NiMH batteries?
Looks like the problem is when the batteries voltage goes above 6V. Full Lithium batteries are at 1,7V and they stay above 1,5V until they are done:
http://data.energizer.com/pdfs/l92.pdf
So it's 1,7x4=6,8V
LSD NiMH are more relaxed:
https://www.kronium.cz/uploads/BK-3MCCE.pdf
they start at 1,5V and they drop to 1,2V.
So LSD NiMH are just fine, Lithium are dangerous, see posts above.
PS: In my future projects the MAX AA Battery Voltage MUST be 1,8V to be safe.
My 121GW showed 7.7V (!!) with brandnew lithium cells...
About leaking batteries, just replace them every a year or two, that's all it takes, even ig they are not fully depleated, and use them till they die in the mouse, tv remote or torch.