Separate out Grounds fom eachother

Projects General Guidance
1

My question is how to separate the 2 grounds. I want to separate the ground for the high-power LDO from the ground for the low-power LDO. I need to use an optocoupler at 24V, and I want to isolate it from my ESP32. One issue I encountered was that the grounds for the optocoupler and the ESP32 were common, which breaks the isolation I need. Therefore, I require separate grounds for each.

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One method that I have in mind is using a diode, but I want to know the common and professional methods for industrial purposes.

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2

You cannot.
If you need isolated grounds then you need to use two separate isolated power supplies
Why di you need separate grounds?

3

Jim, this is related to my other thread (24V with transistor to power the relay). This relay will carry 220V AC. Relay switching could cause 24V spikes/transients & I want to protect my MCU 3.3V circuit at an industrial grade reliability.

You had suggested using two isolated power sources in the other thread, which is complicates my design. Currently I am using 24V main supply and using LDO to get my 3.3V. I am curious if I could separate my 3.3V GND circuit & use a diode at GND common point (LDO) to protect reverse current/spikes from 24V circuit.

I will not be isolated, but will that be good enough protection for a reliable system? My other option will be to use a step down transformer from 24V to 3.3 V, which will give me a separate ground , but definitely increase cost & I want to avoid it.

4

A step down converter may or may not have separate grounds...
Use a cheap adapter... (not too cheap, for those may be dangerous).

5

You also said cost was a concern, so I'm hesitant to make suggestions just to have you tell me again it's too expensive.

What is you budget for all these parts and in what quantities?

6

As I already said no you cannot.

7

Thank you for valuable input. Aim is to minimize cost, no set budget. Looking for cheapest option without compromising reliability.

8

You now have two topics on the same problem and it's against forum rules.
If you want help, please delete one or the other.

9

It not clear what you mean by "reliability". Why do you think that have two grounds is more reliable than one?

10

By reliability, I am referring to longevity of MCU - not being damaged by surges/spike & other common events in industrial automation systems. Which is why I opted to use photocouplers for my MCU IOs, but didn't' know common GND still could cause an issue.

My 3.3V circuit is powered through an LDO which is fed from main 24V supply. I am looking for ways to protect my 3.3V GND at minimum cost, without external 3.3V power source.

11

I would like to add, the main 24V supply has reverse polarity protection with a diode on GND. The only risk I can understand is the internal 24V event passing through GND system & damaging the MCU. I can separate the 3.3V GND traces/pads from rest of the system GND, but it would still have a common point at the 3.3V LDO. So I am wondering, if I can just protect this common point?

I have looked in isolation transformers, but they cannot handle the 1A power requirement for my 3.3V circuit.

Another option is 1:1 DC transformer. that should physically separate the GND for 3.3V circuit. But haven't found a suitable option on LCSC.

12

There is no point in using optocouplers if the grounds are common. You must also have an isolated power supply.

Please post your entire schematic so we can see what you are trying to do.

13

Is my MCU protected?

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14

For the third time no diode in GND!
It serves no purpose and will just create ground problems.

15

Protected from what? What do you think will happen?

16

damage due to Surges/Transient/Spikes

17

First you need to identify the source of these surges/transients/spikes and then try to eliminate them.

So where do these surges/transients/spikes come from?

18

Only source of surges/Spikes is from Relay coil circuit to transistor . Can surges goes to esp32 from Collector/Emitter to base of transistor?

19

No but for extra protection you can put a TVS diode across the transistor from collector to emitter and pick a transistor with a high Vce rating

20

Great idea! I removed the optocoupler and am now using a transistor with a VCE rating of 40V. I also added a TVS diode with a clamping voltage of 40V across the collector and emitter to protect the NPN transistor from any voltage spikes caused by the coil. Thank you so much for giving your valuable time ! @jim-p