ESP8266 Community Forum

- Fri Jan 15, 2016 3:50 pm #38892 Hi!
I'v made an ESP8266 with a small program in it that lights up 3 white leds when get some data from internet.
Powered with the Arduino Uno 3.3 Volt everything works fine.

But I want to use it with an AC/DC transformer taken from an old phone which give +5V and 350mmA.
I want to connect it to the ESP8266 and I'm trying to do this with a resistance divider.

Here is the question: is everything safe with my circuit? I've read the mathematical expression on internet and I'mot sure because with that formula I can use also resistors of 1Kohm and 2Kohm, but I didn't understand which is the difference.
Can someone help me?
Thank you.

Here is the sketch, how I did it (the button is for uploading code in the ESP8266):

- Fri Jan 15, 2016 9:59 pm #38922 Your V2 calculation is correct only when only the resistors are connected. At that time the DC drain will be I=V/R=5/(100+200)=17mA, all waisted warming the resistors. BTW I am sure you meant to write V2=V1*(R2/(R1+R2)).

Once the esp etc. is connected you have another equivalent resistance added (which is dynamic) and your V2 calculation is wrong.

If we assume (for simplicity) that a constant 70mA is running through the esp+LEDs from a proper 3.3v source (a reasonable value) then we can calculate it is equivalent to R=V/I=3.3/.070=47ohm. This is in parallel with the 200ohm so 38ohm combined (this is the effective R2) and now V2 will be V2=5*(47/(100+47))=1.6v. Not good.

Better to use an LDO, a DC/DC or at a pinch a suitable diode(s) to drop the voltage from 5v.

- Sat Jan 16, 2016 4:51 am #38936 Thank you for your reply, very clear and yes, I meant (R1+R2).
There are a lot of LDOs, can you please link me one that I can use in my circuit instead of R1 and R2? For example, this should work with my 5V 350mmA power?
http://www.aliexpress.com/item/Free-Shi ... 9.7.5djtAx
Thanks!

- Sat Jan 16, 2016 7:27 am #38953 I am not a hardware person, but in my experience the 1117 is a very common part. It has very different performance from different manufacturers, but can usually handle 800mA or above so will work for you.

This is not a "real" L(ow)DO, with a 1v (or higher) DropOut, and often has a high Iq (often a few mA) so is unsuitable for battery usage.

HTH