MCU malfunctions powering (via a 24V to 3.3V LDO) and Mosfet - Page 1
Forum section: electronics / beginners

author: Royflashlight
author_rank: Newbie
timestamp: July 14, 2023, 12:47:37 pm
content: Hi there, first time posting hereMy MCU malfunctions when powering both it (via a 24V to 3.3V LDO) and a load using a 24V-driven MOSFET.Works fine when 24v does not power LDO and they have common GND.Any ideas why?I have attached a simple picture of the circuit.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 14, 2023, 12:51:47 pm
content: On real fundamental level, the schematic seems to be correct, but really many possibilities for things to go wrong, not going to enumerate each possibility that comes to mind, please post at least a photograph showing how parts are laid out (or full schematic + layout drawing), and actual part numbers used, and what the load is, and what is supplying 24V.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 14, 2023, 12:59:55 pm
content: Are you tying two power supplies together via your circuit board?

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 14, 2023, 01:14:37 pm
content: It's a lot of voltage drop, even 50mA will easily overheat most LDOs (1W) if not using heatsink.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 14, 2023, 01:19:25 pm
content: Yeah. Without knowing what the MCU is, we can't know if it's consuming 1mA (a small AVR/PIC on default clock/power settings) or 500mA (some flagship Cortex-M7 thing).

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 14, 2023, 01:49:27 pm
content: Also, being the voltage difference so high, the LDO might oscillate under certain conditions, like too low ESR output caps (Typical issue with ceramic caps).5 ->3.3V is easy, but 24->3.3V with wrong decoupling might be radiating several watts into the RF spectrum

author: Siwastaja
author_rank: Super Contributor
timestamp: July 14, 2023, 02:00:27 pm
content: And going from 24V to 3.3V does not require an LDO regulator. Standard drop-out one will do fine. If I have to guess, "LDO" = 1117, possibly a counterfeit one. If 1117, these are really sensitive to correct type of output capacitor. This is why I requested full schematic, or even just a photograph gives a lot of information with minimum effort for the OP.

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 14, 2023, 02:12:49 pm
content: I'd say LDO has become pretty much the new standard for meaning "Linear regulator".It was a thing when something not wasting 1.5V was rare (78xx, LM317, etc), so LDO was invented!But nowadays they're everywhere. I got some cheapPJSEMI PJ75AL33SALDOs, withstanding (up to) 36V 100mA while dropping only 25mV.

author: tooki
author_rank: Super Contributor
timestamp: July 14, 2023, 08:59:48 pm
content: Quote from: DavidAlfa on July 14, 2023, 02:12:49 pmYeah but I don’t think we should be condoning or encouraging the use of “LDO” as a synonym for linear regulators. I’ve seen people talk about the “7805 LDO” which is categorically wrong. There’s no reason to encourage this or even accept it. We should instead educate, as Siwastaja did.

author: tooki
author_rank: Super Contributor
timestamp: July 14, 2023, 09:02:09 pm
content: Quote from: Royflashlight on July 14, 2023, 12:47:37 pmIt would help if you actually describe what the malfunction is. “MCU malfunctions” could mean anything. Be specific. Tell us exactly what parts you’re using. And let us know any measurements you’ve done. Being secretive (a common thing with new members) doesn’t help you get answers!

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 14, 2023, 10:35:13 pm
content: Sure, I'm not encourating that, but I find such case very often, Isimply tolerate it, better to not start a war everytime I see LDO meaning linear reg.First step here would be to have a lok at the vdd rail with the scope.Then find out if it "malfunctions" instantly or Iof it takes a while for that, then it's probably the LDO going nuts.