How to drop 36V down to 5-16V range? - Page 1
Forum section: electronics / beginners

author: agehall
author_rank: Frequent Contributor
timestamp: May 16, 2021, 06:39:47 pm
content: I'm trying to replace a fan controller board with something a bit more custom based off an ESP32 module. The board has 36V input from the rest of the system and I'm trying to figure out the best way to get this down to the 5-16V range that my devel board can handle and I'm stuck.To get something quick and dirty up and running, I attempted a 1k+500R resistive divider and tapped the middle (approx 12V) to my board. But that does not work - it drops the voltage over the 500R to 5V and the board doesn't power on. I'm not sure I understand why this wouldn't work. I also tried tossing in an LM7805 between the 12V point and my devel board but that didn't solve anything.I've reverse-engineered the old board and the power supply for the 5V rail looks like this:On the board, D1-D3 looks like this:I assume that D1 is a reverse polarity protection diode (the text on it says "GM" and "45") and that D2 & D3 are zener diodes to drop the incoming 36V to something that the LS78L05 can handle safely. Does that sound like a correct assumption? D5 is an ST part marked "ER" and "Z406" but I've been unable to find datasheet for it.Can anyone give me some pointers on what I should do here? Do I need to replicate the D1-D3 to lower the voltage before my voltage regulator or should I look at some other solution?

author: PeteH
author_rank: Unknown
timestamp: May 16, 2021, 07:09:59 pm
content: Watch out.... Most people use the ESP 32/8266 modules and assume the "external" VIN is the full range of the LDO present (up to 16v) but in fact the range is limited to 5V due to the CP2102 or CH340.... Can't tell you how many people have destroyed the board by applying more than 5.5V/6V and wonder why the board no longer works.The ESP32 uses a lot of current at 3.3V depending on what you're doing with it...Get one of those higher voltage 3 pin "LDO replacement" step down converters. It needs to be fairly efficient to drop from 36V to 5V... And LDO would burn too much power.

author: ledtester
author_rank: Super Contributor
timestamp: May 16, 2021, 07:58:20 pm
content: QuoteA resistive divider has too much output impedance:https://electronics.stackexchange.com/questions/106718/when-would-i-use-a-voltage-regulator-vs-voltage-divider

author: agehall
author_rank: Frequent Contributor
timestamp: May 16, 2021, 08:42:41 pm
content: Quote from: PeteH on May 16, 2021, 07:09:59 pmIn this case, it is an ESP32MX-E board which has a VIN that is specified for 5-16V, so I should be good there. But I appreciate the concern - I can see how this would be overlooked by many.Quote from: ledtester on May 16, 2021, 07:58:20 pmThat makes sense. I knew there was something silly I was missing.Since posting my question, I've been looking thru my drawers and I finally found a DC/DC step-down module that accepts up to 40V input and can give me 16V output. With that module, my little monstrosity now works. I guess what I'll have to do on the PCB I'm designing, is to include a DC/DC converter that can deal with 36V (and some margin) and give me the output I need.

author: Zero999
author_rank: Super Contributor
timestamp: May 16, 2021, 09:04:08 pm
content: https://uk.rs-online.com/web/p/switching-regulators/1934004/https://www.tme.eu/gb/details/span02b-05/dc-dc-converters/mean-well/

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 16, 2021, 11:46:10 pm
content: You use one of those postage stamp sized/priced SMPS modules, but a quick and dirty solution is to just add a series resistor of a few hundred ohms in front of your fan. (Or two if dissipation is too high).For an estimate, run your fan from 12V, and measure it's current consumption.

author: varunkoganti
author_rank: Contributor
timestamp: May 17, 2021, 03:06:32 am
content: Please correct me if I am wrong.Objective of the project - Convert 36V DC to 5V-16V DC.The solution seems simple.You can use a step down converter.LM 2576 (Texas Instruments -https://www.ti.com/product/LM2576)LM 2576 (OnSemi -https://www.onsemi.com/pdf/datasheet/lm2576-d.pdf)The additional components you will be needing is schottky diode, inductor, capacitor & 2x resistors.The datasheets clearly mention how a typical application would loom like.Note - use the datasheet of the manufacturer for the respective device. ie Do not use an TI device with On Semi datasheet.

author: agehall
author_rank: Frequent Contributor
timestamp: May 17, 2021, 04:59:06 pm
content: Quote from: Doctorandus_P on May 16, 2021, 11:46:10 pmNo, that won’t work. The fan in question is a custom EBM Papst RG128/1300 that requires PWM. Just dropping the voltage isn’t really an option.The real circuit I’m designing includes a proper DC/DC switching regulator to drive the ESP32 board. I just wanted a quick and dirty prototype with stuff I have at hand to test things. Now that I managed to get a working prototype I think my real design will work just fine.I have to say though, this forum is wonderful - lot’s of good explanations and pointers in 24h! Thank you all! While I’ve been doing this as a hobby for years, I still learn tons from here every day.