Two input-voltage protection - Page 1
Forum section: electronics / beginners

author: paeion
author_rank: Contributor
timestamp: January 02, 2022, 09:14:44 am
content: Hi all,I am making my own DAQ with a TI ADS1256 ADC and the ESP32 micro to send load cell data over bluetooth to the PC. Long story short, I am an electronics newbie and unsure how to best protect the input voltage sources:I want to have a 7V input jack to supply my analog and digital linear regulators (5V and 3.3V respectively) and also connect the USB 5V to the 3.3V regulator so that I can power the ESP32 during programming - without enabling the analog side.So far I got a zener for over-voltage protection on the 7V input and two diodes on each input (see attached schematic). I removed bypass caps and proper connections for simplicity (can attach full schematic as well if needed).Am I on the right track or is my setup fundamentally wrong?Any advice would be greatly appreciated!P.S. Should I be worried about the inefficient 7-3.3V drop on the digital regulator with the ESP32 zapping an approximate peak 500mA ?

author: m k
author_rank: Super Contributor
timestamp: January 08, 2022, 11:11:15 am
content: You are on the right track but fundamentality is a nasty thing, it has different sizes and small ones can evade.Your 5000mV USB is dropped by the diode and your 500mA zap requires 1150mV drop.

author: Manul
author_rank: Super Contributor
timestamp: January 08, 2022, 01:05:16 pm
content: Things to keep in mind:1. Zenner is generally not very good in absorbing serious surges, because it has quite high internal resistance. If you really need that overvoltage protection, better choice is TVS diode.2. Know your linear regulator's required input voltage headroom and ensure that there is enough + some small safety margin. Keep in mind that your 7V source is not an ideal source and could have some variations, define a worst case. For example, it could be 6.5V. Is the headroom for LDO still enough?3. Peak current (you mentioned 500mA) is somewhat meaningless in a sense that it could be very short peak or quite long and you haven't said that. For example, very short peaks will be supplied by bulk capacitor so peak (and average) current through the series diode could be way less. Generally, for power dissipation calculation you care about the average. For headroom calculation - peak.4. With some circuits there migh be an issue if you power rails separately (only the 3.3V in your example). For example, if you have logic outputs from MCU connected to unpowered parts of the circuit, there could be conduction through ESD diodes into that unpowered rail.5. As a rule of thumb, if you have an external power source input, put a small electrolytic capacitor there, don't use MLCC only. Electrolytic capacitor helps to damp the ringing (voltage spikes) during plugging/unplugging due to it's higher ESR.

author: paeion
author_rank: Contributor
timestamp: January 09, 2022, 12:38:56 am
content: Thanks for the replies, theres a lot there I had not considered/ didn't know about previously.Some extra info:My motivation for over-voltage protection is the wrong use of the wall-wart on the input jack (e.g. a 15V adapter connected to the 7V jack).The ESP32 MCU worst-case is with wifi enabled with regular short peaks up to 500-600mA (based on internet findings), however I will primarily use Bluetooth which is closer to short 150mA peaksThe digital 3.3V regulator (NCP1117) has a peak 1.2V dropout at 800mA. The analog 5V regulator (LP2985) has a very low dropout voltage (280mV at peak 150mA). The max. input voltage is 18 and 20V for the analog and digital regulators, respectively.Some follow up questions I have:Even with a Schottky diode on the USB 5V, the margin on the 3.3V regulator will be very tight - Is a logic-level MOSFET and comparator a better option for switching on/off the 5V USB supply?Given that a worse-case might be a 15V wall-wart connected on the input jack... Which is a better use-case here? A resettable fuse + TVS or Zener diode. I have read about crowbar protection and using MOSFETs to the same effect, but surely the Zener combination is sufficient here for over-voltage regulation.