Test/Review of USB meter: Juwei J7-B

6 posts / 0 new
Last post
HKJ
HKJ's picture
Offline
Last seen: 8 hours 36 min ago
Joined: 05/24/2011 - 12:23
Posts: 7425
Location: Copenhagen, Denmark
Test/Review of USB meter: Juwei J7-B

USB meter: Juwei J7-B

DSC_7502

Official specifications:

  • USB safety monitor,protect your valuable electronic products from charging damage
  • With mode key, short press to change display mode
  • Long press to reset all the data
  • Built-in 18 bit micro processor,automatic cutout when meet high volatage, low voltage
  • With warning speaker
  • Net weight: 30 g / 1.06 oz
  • Size: 77 x 45 x 16 mm / 3.03 x 1.77 x 0.63 inch
  • Input voltage: DC3.5-7.4V
  • Current test range: 0-3A
  • Voltage: 3.5-7.4V
  • Electric quantity: 0-99999mAH
  • Voltage resolution: 0.1V
  • Current resolution: 0.01A
  • Power: <20mW
  • Consumed current: <0.02A
  • Over-current warning: Yes
  • Over-voltage warning: Yes
  • Low-power warning: Yes
  • Low-voltage warming: Yes
  • Leakage warning: Yes


I found this unit on Ebay at a dealer called directlinkz.

The above unit is supposed to have 3 current digit and 2 voltage digit, but the one I got had 3 voltage digit and 2 current digit.



How does it look

DSC_7504

This meter looks rather fancy.

DSC_7508

DSC_7506 DSC_7507

DSC_7510 DSC_7512

The meter has a button to select between the 3 displays and a buzzer for alarm conditions.

DSC_7515

Main display with voltage and current, my version has 3 voltage digits and 2 current digits.

DSC_7516 DSC_7517

The Wh and Ah display, there is no indication of what display is shown, this must be deduced from the reading or the number of presses on the button.

DSC_7812

I could just pry the box open.

DSC_7813

On the topside we have the buzzer, the switch, the display and the 50mOhm resistor.

DSC_7815

DSC_7816

The resistor is much easier to see here.

DSC_7817

On the bottom side there is a memory chip (Atmel 2402) to remember the readings and Q1/Q2 is probably the switch that is used to disconnect the output and also the reason that the device get hot and has a large resistance.

DSC_7818
DSC_7819
DSC_7820



Measurements

Test

This device is very critial with voltage and will disconnect output when it is outside 4.4 to 5.8 volt range. For this reason I have not included the current measurements at different voltages.

  • Own current consumption is 30mA
  • Internal resistance is about 0.27ohm including connection resistance.
  • Remembers Wh and Ah when power is off
  • Voltage display is within 0.02 volt in the 4.0 to 6.0 volt range.
  • Current display is within 0.1 ampere in the 0 to 3.0 ampere range with 5 volt supply.
  • Over current alarm is at 4.9A
  • Over voltage alarm is at 5.8 volt
  • Under voltage alarm is at 4.4 volt
  • USB data works fine.



Temp1348

M1: 53,1°C, M2: 41,7°C, M3: 47,4°C, HS1: 61,6°C


Temp1349

M1: 55,7°C, M2: 66,2°C, HS1: 70,4°C
It get rather hot



Conclusion

With the ability to remember Ah and Wh this device can be used to check capacity of power banks. The safety cutoff can also be useful, but I would have liked a wider input voltage range.
The fairly high internal resistance limits its usefulness at high current, it is best used at 0.5A.
With a total of 5 digits, either current or voltage is limited to 1 decimal.

Due to the high resistance, problems at low voltages and the limit on decimals, I will only call it acceptable.



Notes

How do I make the test

My website with reviews of many chargers and batteries (More than 1000): https://lygte-info.dk/

Edited by: HKJ on 06/25/2015 - 13:18
MRsDNF
MRsDNF's picture
Offline
Last seen: 1 month 6 days ago
Joined: 12/22/2011 - 21:18
Posts: 13473
Location: A light beam away from the missus in the land of Aus.

Thanks HKJ. I got a little excited with the looks and official specs and then a little unexcited when I read the review.

 

djozz quotes, "it came with chinese lettering that is chinese to me".

                      "My man mousehole needs one too"

old4570 said "I'm not an expert , so don't suffer from any such technical restrictions".

Old-Lumens. Highly admired and cherished member of Budget Light Forum. 11.5.2011 - 20.12.16. RIP.

 

musicmagic
Offline
Last seen: 6 years 6 months ago
Joined: 10/28/2013 - 09:53
Posts: 984
Location: CT

interesting to see how the paint show up in the thermal images.

If you can’t blind them with your brilliance, baffle them with your bullcrap.

The real currency in the world is not money, it’s trust.

HKJ
HKJ's picture
Offline
Last seen: 8 hours 36 min ago
Joined: 05/24/2011 - 12:23
Posts: 7425
Location: Copenhagen, Denmark

musicmagic wrote:
interesting to see how the paint show up in the thermal images.

A thermal image captures radiated heat, white does not radiate as well as black, i.e. it will look cooler.

Thermal image measuring equipment has a setting to adjust for what surface they are looking on, to get the best precision.

My website with reviews of many chargers and batteries (More than 1000): https://lygte-info.dk/

nay
nay's picture
Offline
Last seen: 1 year 2 months ago
Joined: 07/08/2020 - 00:21
Posts: 6
Location: earth

do you have an index for only usb meters? Smile

yes

HKJ
HKJ's picture
Offline
Last seen: 8 hours 36 min ago
Joined: 05/24/2011 - 12:23
Posts: 7425
Location: Copenhagen, Denmark
nay wrote:
do you have an index for only usb meters? Smile

No, it is on my todo list, but there is a lot of stuff on it and not everything get done.

My website with reviews of many chargers and batteries (More than 1000): https://lygte-info.dk/