'Borrowing' power from a 12v supply to power my 3.3v ESP8266 module - Terminology and where to start

Thread Starter

LokisKitten

Joined Oct 25, 2017
5
I have a simple bedside LED lamp with a 12V 1A supply as standard. I wanted to convert it into an IoT device with an ESP8266 module and DHT22 temp and humidity module as well as the ability to switch on/off the lamp.

I know I probably want a relay attached to one of the GPIO pins to control the lamps on/off state but my issue is even more basic.

Powering the ESP8266 from the 12V 1A DC supply! How do I do that? Is it even possible? Surely it must be. I don't mind a sacrificing lamp brightness for the increased functionality!

Unfortunately I don't even knowthe basic starting point of what to search for! I THINK, maybe, a "voltage divider" is what I want? But everything I read says "voltage divider" circuits are a terrible idea and you need X and before you know it I'm looking at schematics that make a modern PC motherboard look basic.

Also from my fading school knowledge of Ohms law if I reduce voltage (V) from 12 to 3.3 then surely I'm affecting the amperage.

Surely it can't be hard to 'borrow power' from the 12V supply in the way I want? Does anyone have at least a starting point to help me out?

Apologies for what must be a frustratingly ignorant question! I hope you can help. Thank you.
 

MaxHeadRoom

Joined Jul 18, 2013
28,692
You may be able to use a simple 3 pin regulator, but first you need to do a little reverse-engineering on the unit to establish a few criteria, is this power supply built in or in the form of a wall-wart?
Max.
 

dendad

Joined Feb 20, 2016
4,476
Check to see if the plug pack is a DC or AC out. It is most probably DC.
Then a switch mode regulator will be best as it takes a lot less power from the supply and will not get hot.
These are good..
https://www.ebay.com.au/itm/Voltage...885311?hash=item1ed219317f:g:RfMAAOSwke9Z6quW
Adjust it to 3.3V output.
There are quite a few buck converters available. I've used the above in a few projects.
Don't go with an linear regulator as it is way too inefficient and the lamp power supply could be close to the limit anyway.
Using a linear reg for 3.3V 200mA put will take about 2.5W from the plugpack, while a buck switchmode will be about 0.75W.
 

Thread Starter

LokisKitten

Joined Oct 25, 2017
5
You may be able to use a simple 3 pin regulator, but first you need to do a little reverse-engineering on the unit to establish a few criteria, is this power supply built in or in the form of a wall-wart?
Max.
It's a DC wall wart. I'll look up "3 pin regulator". Thanks for the head start!! Normally IU'm the first to jump on Google to find the answers I need but when you don't even know what to search for then its hard! Thanks for the help!

Check to see if the plug pack is a DC or AC out. It is most probably DC.
Then a switch mode regulator will be best as it takes a lot less power from the supply and will not get hot.
These are good..
https://www.ebay.com.au/itm/Voltage...885311?hash=item1ed219317f:g:RfMAAOSwke9Z6quW
Adjust it to 3.3V output.
There are quite a few buck converters available. I've used the above in a few projects.
Don't go with an linear regulator as it is way too inefficient and the lamp power supply could be close to the limit anyway.
Using a linear reg for 3.3V 200mA put will take about 2.5W from the plugpack, while a buck switchmode will be about 0.75W.
Fantastic - thank you. This looks exactly like what I'd need. I'm going to look up soime tutorials to learn the theory behind regulators then buy some to do my project. If I'm feeling I'll see about making my own!

I'm in the UK but do you think this 6 pack of LM2596 buck convertors would be just as good:https://www.amazon.co.uk/LM2596-Con...coding=UTF8&psc=1&refRID=XVRNX8QNHBXVKPQVSCZH

Funnily enough you mention not using a linear regulator and I see what you mean. Because I bought a linear regulator from one of these cheap Chinese sites and it has a huge heatsink on compared to the buck you've pointed out which doesn't so I'm guessing the inefficiency is via heat production. I'll leave the linear regulator and grab a buck using an LM2596. But more importantly research what on earth each one of those are!!

Thank you both - very much appreciated!
 

philba

Joined Aug 17, 2017
959
A linear could well work. The ESP8266 can be power hungry, it has a fairly large draw on startup (~300 mA) and when actually transmitting packets it gets close to 300 mA. In sleep mode, it consumes < 1 mA and idling it runs between 80 and 35 mA (depends on operating mode). For this application, if designed to mostly sleep, it think it would be just fine. If the average draw over a window of a few seconds could be kept to below 40 mA then the linear would have to dissipate about 1/3 Watt. A TO-220 package would see a temp rise of about 20C so it would not get that hot - probably well less than 60C. An SOIC8 with something like 6 cm^2 of board space would probably be ok too. I'm assuming any relay would be a 12V kind so the only real draw is the esp itself.
 
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