Beware Of Using Auto Detect Auto Range Multimeter – Case Of AHUJA Portable PA SYSTEM NBA-30WL
This Ahuja Portable PA System had come to me in May, 2025 for a complaint of ‘dead’, about which I have already submitted a report which is in queue. The re-visit was in August, 2025 with the complaint that battery backup was not good. I found the battery to be weak and needed replacement.
As I suspected there was a charge control circuit in the battery pack, though difficult to detect it from its size at a glance, I peeled the covering like a banana and found the following:
The charger circuit was using mainly two SMD ICs, one of which was a Mosfet. Let us have a closer look of it:
The charge control IC had a mark of G3R and Mosfet 8205. Let us have a look of its internal diagrams:
Since exact replacement of the battery was not available in the market and space indeed was a constraint in this device, I checked for any change of value or defect in the only few components used. I did not find any.
I used the following auto detect, auto range multimeter for this purpose as it was very easy.
This instrument was very convenient as it never required turning of knobs and selecting the mode & range. It will detect resistance, AC or DC Voltage or AC or DC Ampere (when red probe is connected to 10A) on its own. On switch on it would be in resistance range and as soon as a voltage is detected, we can listen to a relay clicking and the meter shifting to voltage measurement. The selection on top right is for NCV-Capacitance-Mhz. Let us keep this info in mind.
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I bought a new 3.7V Li-ion battery and charged it to its peak using a different charger externally. Then welded the positive and negative rails of the charger board and covered it with insulation tapes like how it was. Then used this multimeter to check the output before connecting it to the device. The multimeter clicked and shifted to voltage measurement mode, but the output was showing zero! I was wondering what went wrong! The battery was showing full charge when I had checked it. Then I remembered one incident when I went to a home for installation of their TV after repair. As a routine, I checked up their electrical fittings and used this multimeter for the AC measurements on the power outlet. It was showing ok. Since the TV was having three pin connections, I wanted to check the N to E and L to E voltage measurements. The moment I touched the Earth point, the ELCB tripped in their house. As we all know, the multimeters used DC voltage with controlled current and voltage when measuring resistance. So, that momentary feeding of a few mV and mA to the earth line tripped the ELCB! More or less was the same thing happened here! The momentary shorting caused built in protection activation and the Mosfet remained switched off! Look at the disadvantages we have to face when using such sophisticated, advanced technology instruments!
Anyhow, I connected the battery to the device and it was found working very well. As the battery was fully charged, there was no display of battery level indication. I fixed the covers and used all modes and found it to be ok. Mission accomplished with lessons learnt in the process that caused induction of satisfaction into the bag!
This article was prepared for you by Parasuraman Subramanian from India. He is 76 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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Arun Kumar
March 21, 2026 at 3:35 pm
Thanks for the information
I had faced the problem of tripping ELCB,
But could not analise it.
Now I understand
Thank you once again for clarification.
Parasuraman S
March 21, 2026 at 7:58 pm
Happy to hear that it was useful! Many thanks for your comments!
Waleed Rishmawi
March 21, 2026 at 4:28 pm
a very interesting article and analysis information. I never believed in such devices that has auto detect although we see the information provided on such devices and we get motivated to buy them to make our lives a little bit easier when it comes to testing but actually the opposite happens because we end up trusting the device and think it is the best device ever...thanks for sharing my friend and have a blessed day
Parasuraman S
March 21, 2026 at 8:00 pm
Yes, dear friend! Many times, the advanced technology itself makes things very confusing! Thanks for your comments!
Yogesh Panchal
March 21, 2026 at 4:30 pm
Good Job!Sir
You can use Li Ion Battery 18650 (Mostly found in old laptop batteries) with using external BMS.
Parasuraman S
March 21, 2026 at 8:01 pm
Size matters, my dear Yogesh Bai! Many thanks!
Albert van Bemmelen
March 21, 2026 at 7:18 pm
Thanks for sharing this interesting LPD178 battery charger article with G3R and Mosfet 8205 components. And for the warning when using auto detect and autorange DMM's that easily may trip the earth wire protection at home.
Parasuraman S
March 21, 2026 at 11:28 pm
Many thanks for your expert comments, dear Albert! I value it very much!
Albert van Bemmelen
March 22, 2026 at 2:59 am
Note: It looks like the G3R charger chip is almost identical or at least pincompatible with the well known much used DW01 ic. The datasheet shows practical the same circuit!
Parasuraman S
March 22, 2026 at 2:19 pm
Oh, I see! Many thanks for this input, dear Albert!
Albert van Bemmelen
March 22, 2026 at 5:02 pm
PS: The mentioned 8205 Mosfet is a Dual Mosfet.
Parasuraman S
March 22, 2026 at 6:00 pm
Yes, thanks, dear Albert! Your involvement in reading an article in minute detail and making comments, amends and additions, is really to be much appreciated!
Albert van Bemmelen
March 25, 2026 at 12:41 am
Often the mentioned dual mosfet and the DW01 controller are both placed on a small charger board called the TP4056 lithium battery charger module for max 1A charging.
See for more info this circuit here in detail : https://www.facebook.com/groups/821778979586987/permalink/1248112946953586/?rdid=1pNRdN6GMBYzOwhJ#
And to prevent overcharging smaller (like ~100mAh) Lithium 3.7V batteries the max 1A current is to be limited by changing resistor value of the prog resistor named Rprog in formule Ibattery=1200/Rprog (Where RPROG=R3 is set by the value of the resistor on pin 2 of the TP4056 Module. Components: Most generic blue TP4056 modules come with a 1.2kOhm (labeled 122 or 1201) resistor pre-soldered, setting it to 1000mA.
Thermal Regulation: The TP4056 reduces the current if the chip temperature becomes too high (>145°C), which can happen if input voltage is high while charging at 1A.
A small 0.4 ohm resistor is sometimes used at the VCC pin to reduce power dissipation and lower the chip's temperature.
Battery Safety: Ensure the programmed current does not exceed the maximum allowed charging rate (usually 0.5C to 1C) specified in your battery datasheet.
I've used this module already several times successfully in my some of my DIY make ic & transistor tester projects.
Parasuraman S
March 25, 2026 at 7:06 pm
Many thanks for this wonderful and educational post!
Albert van Bemmelen
April 2, 2026 at 7:22 pm
Additional note and warning about the TP4056 module and its temperature protection that apparently is disabled and not used in most Aliexpress sold modules! See this link: https://www.youtube.com/shorts/ZGzOkQPUfDI?feature=share
Andy Shiekh
March 21, 2026 at 10:19 pm
Very interesting about the auto meter. I have one but it stays in the car for emergency use.
Parasuraman S
March 21, 2026 at 11:29 pm
Many thanks for your comments!
Mark J
March 22, 2026 at 3:34 am
Parasuraman interesting article. Thank you again for sharing theses.
Parasuraman S
March 22, 2026 at 2:19 pm
Many thanks, dear Mark!
AdamS
March 23, 2026 at 5:06 pm
I've never seen a meter like that which automatically detects what to measure. It wouldn't be something I'd ever want to use. I like control over what I'm measuring - I'll stick to my good old Fluke!
Parasuraman S
March 23, 2026 at 8:53 pm
Yes, Fluke is the most reliable multimeter, and I have got a new one as a replacement for almost 20-year-old one. Many thanks for your comments!
Albert van Bemmelen
March 25, 2026 at 1:00 am
About mentioning Fluke. Fluke probably still exists because they probably have long life contracts with the military to keep servicing their often older digital test equipment models of which the defense industry wants them operational and documented. Even when they are very often heavy cased and often becoming obsolete in limited capabilities and options compared to newer modern devices that are sold today. And for any engineer who wants his old Fluke device calibrated or even fixed, their repair bills are very huge! I know this from a friend who tried to get his old heavy 'portable' Fluke oscilloscope fixed but passed after hearing their pricing. And they probably even refuse to fix older equipment that already was replaced by newer models. Just mentioning a Manufacturer like Fnirsi who these days presents great affordable oscilloscopes and all other interesting devices that not only work much faster but also with many more options and also in bright color!
Parasuraman S
March 25, 2026 at 7:05 pm
Yes, absolutely true! Fluke has these shortcomings and the story is not different in India too. I do have an SMD tester from Fnirsi and it damn good and accurate! The capacitance measurement in the new Fluke is wrong and found accurate in Fnirsi! Many thanks for your expert comments and feedback!