Vcc and powering sensors [SOLVED]

Greetings from locked down Colombia,

I've been developing this project, which has a bunch of active pressure sensors for a while. Their limited spec sheets claim that a 5V supply provides a 0.5-4.5V output. So far so good.

First, I realized that the SunFounder Mega2560s I had were stuffed with 3.3v regs, so my Vcc was delivering 3.3V instead of 5V. After getting 5.0 regs that was fixed, and Amazon refunded me for the defective Arduinos...

Then I realized (and read), that the USB supply doesn't provide a high enough voltage for the on board regs to reach 5V...

Then I added a Relay board and noticed with each activated relay the TFT screen grew dimmer!

So, I got a 12V external supply, delivering 42V!!!!

Now, I'm with a real 12V supply, but the Vcc pins deliver 6.7V, instead of 5V on the long double header of my Mega 2560 R3.

As I'm getting ready to put this whole thing in a box, how do I get to a reliable 5V supply on the sensors, so I can get reliable pressure readings?

Potential solutions: use the 5v regs I have with a good electrolytic capacitor, or some 5V DROKs I have in stock or can I tap onto a proper 5V regulated supply on the Mega 2560? It needs to power FIVE pressure sensors. Sorry, no info on power consumption...

I'm looking for simple solutions. With Covid, it takes more than a month to get any parts to Colombia...

Cheers

USB 5v does not go thru a voltage regulator.

The 5 volt pin on the Arduino is only meant for low power peripherals.

Do not power relays or motors with the Arduino 5v pin !

Always show us a good schematic of your wiring.

Use a DVM to measure the current going out of the 5v Arduino pin.


“ Now, I’m with a real 12V supply, but the Vcc pins deliver 6.7V, instead of 5V on the long double header of my Mega 2560 R3.”

Then your Arduino is ready for the bin.

I believe that type of sensor is ratiometric, they output 10% to 90% of their supply which should be close to 5V. That means your Arduino analog reference voltage (AREF) should be the same as the sensor supply voltage for good accuracy. So you really need a solid, stable 5V supply to Arduino's 5V pin, whether it comes from the onboard regulator or an external supply.
Also, you cannot power 75 mA relay coils from the 5V pin, too much load on the regulator, only sensors and a few LEDs. If the regulator has to drop 7V from a 12V supply, it's only capable of about 150mA without overheating.

Hi,
Power the senors directly from an external 5V supply, such as a mains powered USB supply, make sure the gnd of the 5V supply is connected to the gnd of the sensors AND the Arduino controller.

What are the sensors?
Link to where you purchased them would be helpful.

We need to see a circuit diagram of your project to give you more accurate information.

Thanks.. Tom... :slight_smile:

Wow, lots of answers, THANKS!

Link to one of the Sensors:

No information on power consumption there. But a similar, much more expensive unit from Atlas Scientific consumes 6mA. 5x = 30mA, maybe 50mA.

I stuck my Ammeter on it with 5 of them in parallel and it shows 11mA total. When I observe VCC while disconnecting the 5 units, there’s a difference of 35mV only. Don’t seem to be overburdening the regulator by much.

" Now, I’m with a real 12V supply, but the Vcc pins deliver 6.7V, instead of 5V on the long double header of my Mega 2560 R3." Then your Arduino is ready for the bin.

I measured again, with a 12V external Supply. Vcc is 4.98V, within spec! sorry for the confusion.

"Also, you cannot power 75 mA relay coils from the 5V pin, too much load on the regulator, only sensors and a few LEDs. "

Agreed

“If the regulator has to drop 7V from a 12V supply, it’s only capable of about 150mA without overheating.”

should I be using more or less than 12V to increase the available Current? I have a choice with a variable Buck Converter.

I’m adding the block schematic below:

I have a 24V/10A DC supply, which is needed to drive some ionization electrodes (about 2A).
The Arduino Mega won’t take 24V, so I’ll use a separate Buck converter 24-6V(or so) to supply the Arduino Stack and the sensors.
And I’ll use a separate Buck converter to supply the 8 Relay Board, to make sure I don’t get variations as miscellaneous relays turn on and off.

The LM2596 Buck converters are 2A continuous rated.

Cheers

Greetings;

I continued with programming and at one point was ready to unplug the system from the computer and let it live its own life.

So I plugged in the power as in the schematic (can’t call it sketch in this community ;D ) of the previous post. Within 4 minutes, the screen went blank, without a single Relay activated

Further investigation revealed a piping hot 5V reg on the Arduino, caused by the excessive current of the TFT screen (likely backlight).

So, I rewired as in schematic attached to this post:

24V supplies two buck converters, set at 9V and 5.1V.

5.1V supplies TFT screen and Relay Coils

9V supplies ARduino Mega

Arduino Vcc supplies Sensors, RTC, ethernet Shield and the Relay board electronics behind the opto-Coupler.

I then ran test, flipping all relays on/off, observing the various supply voltages: Fluctuations of less than 0.002V.

In operation for several days now, stand alone, measuring, communicating, displaying!

Case closed!

Thanks for your help.

Why are you proposing to persist in attempting to power the Arduino via "Vin"?

It requires 5 V. You put the 5 V in through the "5V" pin using a 5 V "switchmode" converter so that all the internals get the full 5 V and you connect the same supply to whatever sensors and other parts use 5 V.

That is how you power it and you will have no trouble with how many things the output pins are trying to supply other than the limits specified for the microcontroller chip itself.

There is some concern that powering it at 5 V might cause trouble under some circumstances, to a PC connected to the USB port. So when you program it, either disconnect the "5V" pin when you do so or use a modified USB cable with the 5 V wire disconnected.


Given that it has the opto-isolators it is a good idea to power the relay board from a separate "buck" converter and ensure the ground of the relay bard does not connect back to the Arduino, only to the "buck" converter.

Thanks Paul,

That might work too.

I do have an external USB connector on order anyway. I'll just connect the two data pins and solve the potential conflict.

I'll drop the Voltage from 9 to 5V and switch from Vin to Vcc...

Why did you specifically mention switching supply vs buck converter for the supplying the Arduino and sensors?

Cheers

trilife:
Why did you specifically mention switching supply vs buck converter for the supplying the Arduino and sensors?

I was distinguishing between providing power in a single step from the mains versus using a power supply providing 12 V and then using a switchmode "buck" converter to drop that to the necessary 5 V.

Complicating it by taking two step-downs makes little sense unless the intermediate voltage is required for some other purpose which you do not seem to have indicated.

I also stress the use of switchmode converters which are efficient at each stage, as against series linear regulators which produce much more heat which you then have to manage.

Got it.

Yes, I do use a switching supply for 20-24V DC to drive some ionization electrodes, controlled by the relays. A buck converter then drops the voltage again to 5V...

Cheers.

Now, if only someone could answer my questions about multiple email attachments in programming...