Using Zener Diode for Voltage reference - Page 1
Forum section: electronics / beginners

author: praveen_khm
author_rank: Contributor
timestamp: March 31, 2018, 01:06:30 pm
content: Hi,I have two Lithium Ion batteries which outputs 7.4v. I need some way to make sure the battery is completely charged. Would the setup in the image work? I mean, can I use a Zener Diode in reverse to check if LED turns on when battery is fully charged at 8.2v? When I flip the switch, it starts charging. When flipped, the zener diode turns on LED if it crosses breakdown voltage. Kindly suggest.Thanks,Praveen

author: MosherIV
author_rank: Super Contributor
timestamp: March 31, 2018, 04:12:32 pm
content: Nope.The LED will drop volts across it so there is not enough volt from the LiIon batteries to provide 9.4V (8.4 + 1.2) to turn on the zener and overcome the LED turn on voltage.Why you you think there will be 1A going through 1K resistor? Ohms law!

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2018, 05:00:59 pm
content: Yes the LED's forward voltage adds to the zener's breakdown voltage.The voltage across the 1k resistor, will need to be 1500V for 1.5A to flow and the power dissipated will be 2250W.

author: praveen_khm
author_rank: Contributor
timestamp: April 01, 2018, 03:33:44 am
content: Thanks. I was assuming it's simpler to check if battery is charged or not, but seems no. I rather use a 2$ LED voltmeter I guess.The 1.5A is a load along with the resistor which includes motors and other circuitry. To keep it simple, I mentioned load.

author: not1xor1
author_rank: Frequent Contributor
timestamp: April 01, 2018, 05:30:56 am
content: Quote from: praveen_khm on April 01, 2018, 03:33:44 amyou might have a look at the tl431 datasheet and modify the "voltage monitor" circuit to suit your needs

author: paulca
author_rank: Super Contributor
timestamp: April 01, 2018, 07:08:41 am
content: If you put an LM393 comparator in the loop with a trimmable voltage reference, maybe a dedicated IC reference, then calibrate it with your meter, it might work, but you need to power the comparator, presumably off the battery.Also, 2 lithium batteries are not fully charged at 7.4V, they are fully charged at 8.4V or 8.2V in older specs.This might give you some ideas on detecting fully-charged-ness. Though it ends up doing something slightly different.

author: Simon
author_rank: Unknown
timestamp: April 01, 2018, 08:39:58 am
content: So your putting the LED in series with the load ? The led can only take 20mA, that will be one smoking LED....... briefly, then a chared mess. You want your voltage test circuit in parallel with the load don't make the load part of the test circuit.

author: Zero999
author_rank: Super Contributor
timestamp: April 01, 2018, 08:49:18 am
content: Quote from: not1xor1 on April 01, 2018, 05:30:56 amhttp://www.reuk.co.uk/wordpress/storage/tl431-battery-voltage-monitor/For 8.4V:R1 = 12kR2 = 5k1R4 = 1kR3 = 680RI used the calculator, linked below, with V1 = 8.4V, the trip voltage and V2 = 2.495V, the TL431's reference voltage.http://sim.okawa-denshi.jp/en/rercal.php

author: Audioguru
author_rank: Super Contributor
timestamp: April 01, 2018, 11:07:16 pm
content: Go towww.batteryuniversityand learn about lithium ion batteries. Theymight not be fully chargedwhen the voltage of 2 series cells are 8.4V because when their charging and charged voltage reaches 8.4V then they might be only 70% fully charged. The charging is supposed to be stopped when the charging current drops to a low current and that is a full charge.

author: praveen_khm
author_rank: Contributor
timestamp: April 02, 2018, 09:40:41 am
content: Quote from: Hero999 on April 01, 2018, 08:49:18 amThanks for the circuit. I will look into this as it seems a feasible solution.