using diodes to drop voltage? - Page 1
Forum section: electronics / projects

author: LuisHDR
author_rank: Newbie
timestamp: August 09, 2025, 05:42:30 pm
content: (L: H no defined, SMD 0805)The transformer’s output voltage is not clearly specified.I’m making a first attempt at reverse engineering with an LTE PCB I obtained along with its assembly diagram. I’ve reviewed the Quectel EG800K module, and according to its recommendations, I should use an LDO with a 470 µF capacitor before and after regulation, with a nominal voltage of 3.8 V and a current of at least 1.5 A. I found a 2 A LDO that had a certain cost, but later, after examining the PCB I got and probing here and there, I came up with this diagram. It seems they might be using diodes to reduce the voltage, but it doesn’t match the manufacturer’s recommendations. Comparing their diagram with the one I had in mind, it appears they reduced the cost by at least 70% in the power supply section (my cost is around $1), but it seems to be less reliable. As far as I know, the PCB is intended for production of at least 1,000 units.Here is the diagram recommended by Quectel.There might be another component in parallel that I’m missing, but this diagram is viable?This is what I had planned to use (much more in line with the manufacturer’s recommendations).

author: iMo
author_rank: Super Contributor
timestamp: August 09, 2025, 07:18:11 pm
content: The original in the ref design is perhaps the MIC29302 from Microchip or Micrel (LDO 3A, adjustable)..

author: Bud
author_rank: Super Contributor
timestamp: August 09, 2025, 07:33:53 pm
content: Your nominal voltage is smaller than Vmin ?

author: LuisHDR
author_rank: Newbie
timestamp: August 09, 2025, 07:54:25 pm
content: Quote from: iMo on August 09, 2025, 07:18:11 pmIt’s fine that I opted for a more “economical” LDO, but one that still falls within the parameters.

author: LuisHDR
author_rank: Newbie
timestamp: August 09, 2025, 07:55:09 pm
content: Quote from: Bud on August 09, 2025, 07:33:53 pmI suspect that the power supply is actually 6 V; I’m waiting for confirmation.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 09, 2025, 09:52:42 pm
content: Surely you mean Schottky diodes, Shockley diodes are quite another matter.

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2025, 10:34:45 pm
content: There is nothing wrong with using diodes to drop voltage. Just understand that their voltage drop does vary with current and temperature.

author: PGPG
author_rank: Frequent Contributor
timestamp: August 09, 2025, 11:40:13 pm
content: Quote from: LuisHDR on August 09, 2025, 07:55:09 pmWhat this 6V have to having Vmin=3.8 and Vnom=3.4?

author: microvia
author_rank: Regular Contributor
timestamp: August 10, 2025, 08:41:47 am
content: How about putting the inductor between the two electrolytics for further smoothing ?

author: Terry Bites
author_rank: Super Contributor
timestamp: August 10, 2025, 02:06:24 pm
content: Yes, and to be respectful we should all write them with an uppercase SAll eponymous items likewise. Zener, Darlington etc, etc.I cant say I've ever seen a Shockley outside of a text book. Has anyone?

author: Seekonk
author_rank: Super Contributor
timestamp: August 10, 2025, 10:59:24 pm
content: Hey, he stole my idea. I was going to patent that.This is my off grid garage where 200W 48V inverter runs directly off 60V solar panel, no battery. Needed just a little drop and in a rush used four bridge rectifiers in series. It is only a couple amps and I will probably never change it for something better.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 11, 2025, 11:00:03 am
content: A 1H coil?

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2025, 01:21:48 pm
content: Quote from: Seekonk on August 10, 2025, 10:59:24 pmI have done that before but used a big power shunt regulator to limit the voltage when no load was present, so under load, maximum power was available.

author: Seekonk
author_rank: Super Contributor
timestamp: August 12, 2025, 09:21:35 pm
content: I actually have a PWM shunt regulator called a water heater. Array voltage never gets much more than 1.5V above array power point voltage. Due to using standardized heater elements when demands of the house are low the array can overpower the heating element by another volt. This is the garage water heater just for the washer. This tank has the lowest priority diversion and gets excess last. All cycles are done with hot water. This front loader has no mold buildup because of hot water. You could eat out of the soap dispenser. Same machine at home has mold.

author: MathWizard
author_rank: Super Contributor
timestamp: August 13, 2025, 06:11:22 pm
content: I'll have to look up a hot-water boiler schematic, but yeah I wonder what would happen if you apply 12V from a high current computer PSU, I guess the current would be pretty low, and not impressive as a load tester.

author: tooki
author_rank: Super Contributor
timestamp: August 13, 2025, 06:44:41 pm
content: Quote from: LuisHDR on August 09, 2025, 05:42:30 pmThe 330 ohm resistor in series makes ZERO sense, to the point it's likely an error: with 5V in, it would limit the current to an absolute maximum of 15mA. To pull 1.5A through a 330 ohm resistor, you'd need to drop 495V across it, while dissipating 742 watts of power.