Clipping with Zener Diode - Page 1
Forum section: electronics / beginners

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 04, 2020, 06:10:15 pm
content: Hi everyone,I designed the circuit below. The purpose of it, is to keep the EN pin of the TPS62740 buck converter LOW while the wireless QI receiver is placed above a QI transmitter. Briefly, while the receiver is placed above the transmitter the RECT spikes with a voltage range around 3-12VDC. Detecting the spikes and adding an RC delay (is the period between thge spikes) I am able to keep EN pin LOW.I have test the circuit on a breadboard and it works as expected. After searching around a realized that in clipping circuits using zener diodes, is placed an in series current limited resistor.This is the XOR gatehttp://www.ti.com/lit/ds/symlink/sn74lvc1g86.pdf?HQS=TI-null-null-mousermode-df-pf-null-wwe&ts=1591292806522The input voltage of the XOR is 0 to 5.5V so a Zener with Vz = 5V it could be fine.. But there are parameters that I do not know how to take into consideration such as the Zener Impedance Zz and power rating. Should I case of those parameters in my applicatrion? Also I do not if it is required an in series resistor and how to calculate it for my case..Any advise?Thanks in advanceNick

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 04, 2020, 11:03:45 pm
content: It appears to me--looking at the datasheet--that RECT max is the battery voltage.My inclination is that the zener is not required.Why an XOR? Appears that you just need an inverter.Avoid drawing 4-way connections on schematics.

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 05, 2020, 05:31:47 am
content: Thank you for your answer WimberleytechQuote from: Wimberleytech on June 04, 2020, 11:03:45 pmRECT max is the battery voltage while Wireless EPT (Energy Power Transfer) is happening. When the battery is full, then, only spikes (digital pings) are appeared at RECT pin ranging between 3-12VDC (this is due to the QI protocol. Tx, digital ping the Rx in order to check if more energy is required). So, the zener diode is required to protect XOR's input..Quote from: Wimberleytech on June 04, 2020, 11:03:45 pmThis is to ensure that the load remains OFF (keeping EN pin LOW) while the Rx is placed above the Tx. When the Rx is been removed from the charger, the RECT pin goes LOW and after the RC delay, EN pin goes HIGH.

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 07, 2020, 08:57:58 am
content: Anyone?

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 07, 2020, 12:28:20 pm
content: Quote from: Nikos A. on June 07, 2020, 08:57:58 amHere is an article with calculations.https://www.electronics-tutorials.ws/diode/diode_7.htmlPut a resistor between the RECT pin and the zener.I looked at the datasheet and could not find info on the output resistance (or max current) associated with the RECT pin. If it is a relatively high output resistance, then a resistor may not be required.

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 13, 2020, 09:29:57 am
content: After doing some calculations, I found that I need a 130 Ohm resistor. I ignored the load current as XOR's input current is just 5uAThe input voltage is the RECT pin (varies from 17V to 10V DC). I calculate the max dissipated power (430mW) to be half of the rated (800mW) for the max input voltage (17V)I hope my approach to be correctThanks for you helpNick