Differences in current drain for LED volt meters

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station240

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Oct 9, 2016
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177
As per HBPowerwall's recent video, where he raised concern about the power drawn by the tiny LED volt meters you get from ebay, aliexpress, amazon etc.

If the values displayed are different for each series pack, due to voltages differences, then the number of led segments lit is also different. Hence the current drawn by each LED volt meter varies depending on what numbers it displays.

Lets assume worse case for the last digit.
Pack 1 displays 3.51V
Pack 2 displays 3.58V
Each segment draws ~ 20mA each,
Pack 1 is drawing 100mA+100mA+40mA = 240mA
Pack 2 is drawing 100mA+100mA+140mA = 340mA +42% extra power

breakdown of value vs segments lit and current draw.
1 2s 40mA
2-5 5s 100mA
6 6s 120mA
7 3s 60mA
8 7s 140mA
9-0 6s 120mA

Aside from value 1 and 7, the current drawaverages between 100 to 120mA.
I've got one of these voltmeters myself so I can do further test if anyone wants.
I don't have the version with the extra wires that has a separate 5V input to power the device, but obviously it drains very little from batteries.
 

daromer

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Oct 8, 2016
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Its a valid concern and i would say the best is the ones you can turn on when you want to take a peak and then turn off afterwards.


I have a couple of meters here as well. If i do get time i will just quickly take a look at them.
 

hbpowerwall

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Oct 7, 2016
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Your contributions are invaluable. THANK YOU GUYS!
 

station240

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Oct 9, 2016
Messages
177
Did a quick test using the actual volt meter and a multimeter to measure the current it draws.
Got something around 8.9mA, but two interesting points.
1. The voltage dropped 0.5V with the multimeter inline to measure current. Entirely expected but a trap for those unaware.
2. The voltage wasn't stable, and neither was the current, both changed at the same rate, up and down.
With an extra segment lit extra current was drawn, which lowers the battery voltage, which then causes that segment to go out.

The lower than expected current is the result of display multiplexing, only a few segments (one ?) are on at any one time, but they switch rapidly so you cannot see the effect. Also saves power. Bar taking the volt meter to pieces, there is no easy way to measure the current only drawn by the display, and not the electronics that drives it.
 

Korishan

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Jan 7, 2017
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6,522
Curious, would you be able to put a capacitor to help smooth out fluctuations? Or would that kind of defeat the purpose of the volt/amp-meter?
 

alfu

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Jan 12, 2017
Messages
35
I bet a cheap multimeter like Harbor Freight sells (gives away sometimes) draws less current than one of these, and the LCD display is bigger. They operate for years (intermittently) from a single 9V battery. Not sure if they would work from the same source they are measuring, though. But 2 18650 in series dedicated to power one would run it a long, long time.
 

alfu

Member
Joined
Jan 12, 2017
Messages
35
I bet a cheap multimeter like Harbor Freight sells (gives away sometimes) draws less current than one of these, and the LCD display is bigger. They operate for years (intermittently) from a single 9V battery. Not sure if they would work from the same source they are measuring, though. But 2 18650 in series dedicated to power one would run it a long, long time.
 
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