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Topic: [SOLVED] Understanding Wemos Current Consumption (Read 17030 times) previous topic - next topic

cbrum11

Mar 17, 2017, 12:46 am Last Edit: Mar 18, 2017, 09:33 am by cbrum11
Hello,

I currently have 2 Atlas Scientific sensors running on a Wemos D1 pro and transferring data via MQTT.  All is well so far but after investigating the components in more detail I have a concern.

The "datasheet" for the Wemos can be found HERE

Each of the datasheets for the sensors can be found HERE and HERE

I am running each sensor via pins D1 and D2 using I2C.

I am powering the Wemos via a power supply to the Wemos 5V pin.  I have experimented with using both 3.3V power and 5V power to this pin and both seem to work well.

Two main questions.

1.  I'm very confused on what the 5V pin actually is?  Is this where I am supposed to apply power to the Wemos via an external power supply?  Or is this a 5V OUT pin that supplies power to OTHER devices when the Wemos is powered via the USB?  If this is the case, is the only way the Wemos is actually supposed to be powered is through the mini usb jack?

2.  I'm concerned about amps.  My power supply shows about 100mA used on startup (I'm guessing this is when the Wemos is working to make an internet connection) and then between 45mA and 55mA after settling and the sensors taking their readings.

The datasheet for the EC sensor states a max operating amp draw of 35mA and the PH sensor datasheet states a max amp draw of 18.3mA.

The Wemos GPIO pins are rated for a max amp draw of 12mA???  However, I'm really confused.  To my best understanding, I'm only using the GPIO pins for the I2C communication.  My guess is that these pins are working in the acceptable range (less than 12mA) because things appear to be running smoothly?  Thus, I'm sort of back to my first question.  Does the 12mA max draw also apply to the 5V Wemos pin?  If it does not, what is the max rating for this pin?

I'm really a novice here, and although things are working now, I'd like to understand if I'm putting the components in a less than desirable position (operating them at too high an amp draw).  Thanks for taking the time to read this.  Any and all help is GREATLY appreciated.

Respectfully,
Chase


pert

1. You could use it to power the D1 pro or you could use it for a 5V supply if the D1 pro is being powered from the USB connector. For more information see the schematic:
https://www.wemos.cc/sites/default/files/2016-09/mini_pro.pdf
Does the 12mA max draw also apply to the 5V Wemos pin?
No
If it does not, what is the max rating for this pin?
Whatever your power supply is rated for or the rating of the traces on the circuit board (which is much higher than any sane use of that pin). Keep in mind that the ESP8266 uses a lot of current while doing WiFi communication. I've heard reports up to 800 mA, though the datasheet says it's significantly less than that. If your power supply can't provide enough current for the ESP8266 and the other components then the board will not work reliably.

cbrum11

@pert

Thank you so much for your concise and clear answer.

I had looked at the schematic, but being an electrical novice, I wasn't sure how to completely put 2 and 2 together.  In an effort to become less of a novice, maybe you can help me with that as well?

At the top left of the schematic I found the +5V labeled as "9".

Right below it I see "+5V" labeled as going to the "RT9013" voltage regulator.  I looked up the datasheet and found it HERE.

On Page 3 of that datasheet it states a Current Limit MAX as .85 amps (850 mA).  Is this technically the highest current that should be pushed or provided through this pin?

I also see another pin labeled "9" towards the bottom center of the wemos datasheet.  This pin says it is connected to "CP2104" USB to UART bridge through a "4K7" resistor and goes to 3.3V.

Does this pin 9 have anything at all to do with the previously mentioned "9" that's associated with the 5.V pin?

Also, I'm assuming 4K7 is a mistake on that resistor label and it should be 47K?

Thanks in advance for your answer.  I think others will hopefully find a description of how you derived your answer really helpful.

Respectfully,
Chase



pert

At the top left of the schematic I found the +5V labeled as "9".
That's the pinout of the full board. Explanation of the "9" below.

Right below it I see "+5V" labeled as going to the "RT9013" voltage regulator.  I looked up the datasheet and found it HERE.

On Page 3 of that datasheet it states a Current Limit MAX as .85 amps (850 mA).  Is this technically the highest current that should be pushed or provided through this pin?
That's the 3.3 V regulator. The ESP8266 runs at 3.3 V. The current limit is the maximum available to the ESP8266 and the 3.3V pin. It has no relevance to the 5V pin other than being supplied by that pin or the USB jack.

I also see another pin labeled "9" towards the bottom center of the wemos datasheet.  This pin says it is connected to "CP2104" USB to UART bridge through a "4K7" resistor and goes to 3.3V.
That's the reset pin of the CP2104. It's tied to 3.3 V through the resistor to keep that pin high so the CP2104 never resets. The CP2104 creates a virtual serial port on your computer over USB, which allows your computer to upload sketches to the ESP8266 and communicate (e.g. Serial Monitor). This is necessary because the ESP8266 doesn't have native USB support and modern computers don't have serial ports so a "bridge" is necessary to allow communication. That's why you may need to install a CP2104 driver on your computer before you can use the D1 Pro.

Does this pin 9 have anything at all to do with the previously mentioned "9" that's associated with the 5.V pin?
Those 9s are just the physical pin number on the chip (or breakout board in the case of the previously mentioned 9). It's just the count of pins going counterclockwise from the top left corner of the chip (usually denoted by an indentation on the package).

Also, I'm assuming 4K7 is a mistake on that resistor label and it should be 47K?
No, that's the correct notation for resistors, not a typo. It means 4.7 kOhm. It's written that way to be more  clear than 4.7k, which could be misread as 47k.

cbrum11

That's the 3.3 V regulator. The ESP8266 runs at 3.3 V. The current limit is the maximum available to the ESP8266 and the 3.3V pin. It has no relevance to the 5V pin other than being supplied by that pin or the USB jack.
This makes so much more sense than what I was saying.  I guess, to rephrase, the voltage regulator doesn't supply current... and I'm not "driving currrent through it" like I made it sound. The voltage regulator provides a proper voltage and, in doing so, can output a rated, max current.

Quote
No, that's the correct notation for resistors, not a typo. It means 4.7 kOhm. It's written that way to be more  clear than 4.7k, which could be misread as 47k.
This answer by itself is worth a textbook.

Thanks again Pert.  The detail in your answer was exceptionally helpful in fully understanding things.  Experience is an efficient teacher, so I'm very thankful for the knowledge you provided...

-Chase

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