Are these normal? 5V pin got only 4.76V and 3.3V pin got only 3.15V

I am working on MQ-2 smoke detector module by supply voltage from 5V pin of Arduino UNO R3. Normally, the MQ-2 module use only 115 - 130 mA. My UNO R3 use only 40 mA while running loop.

USB cable only: No load -> The voltage coming out 5V pin is 4.86V. No load -> The voltage coming out 3.3V pin is 3.28V. MQ-2 load -> The voltage coming out 5V pin is 4.76V. MQ-2 load -> The voltage coming out 3.3V pin is 2.81V. The analog output from MQ-2 module is very high (~110).

External 9V power adaptor via DC Jack: No load -> The voltage coming out 5V pin is 4.98V. No load -> The voltage coming out 3.3V pin is 3.30V. MQ-2 load -> The voltage coming out 5V pin is 4.78V. MQ-2 load -> The voltage coming out 3.3V pin is 2.88V. The analog output from MQ-2 module is very high (~110).

Use my own 7805 regulator apply to MQ-2 module: No load -> 5.02V MQ-2 load -> 5.02V. The analog output from MQ-2 module is very low (~55).

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spycatcher2k: Hi - I try to power as little off the Arduino as I can, and often use 7805's to power both the Arduino and attached Sensors, Displays ETC. this way I know I have plenty of power for external components and a known voltage throughout the design.

If we use external 7805 only, can we supply directly to 5V pin of Arduino UNO? (Don't plug in DC JACK and USB cable)

The Vin pin is required 7-12V.

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The old “can I power an Arduino board via the 5V pin” question.
The answer is : no.
I do that with a few projects, but the answer is still : no. Really, it’s a : no. No. Just no.

There are a few cases that the voltage regulator on the Arduino board was blown because of the reverse voltage. I have added an extra diode from 5V to VIN to bypass the voltage regulator.

A voltage at the barrel jack or VIN turns off the USB 5V power. That does not happen when the Arduino board is powered via the 5V pin, and current might go into the computer via the usb cable.
This does indeed happen, my 5V power supply does sometimes push current into the computer. So far, so good, nothing damaged yet, but it might be bad for other computers.

If you have a 5V power supply with USB connector, and you connect either the 5V power or the computer to the usb connector, and you don’t mind a blown voltage regulator on the Arduino board, then yes, go for it.

Koepel: The old "can I power an Arduino board via the 5V pin" question. The answer is : no.

We had yet another discussion on this a few days ago. The standard regulator on an UNO has an inbuilt diode from Vout to Vin, which prevents damage to the regulator in the case of connecting 5V directly to the 5V rail of the board.

That diode can withstand a peak current of 15A (for 100us). From the Onsemi datasheet:-

Protection Diodes The NCP1117 family has two internal low impedance diode paths that normally do not require protection when used in the typical regulator applications. The first path connects between Vout and Vin, and it can withstand a peak surge current of about 15 A.

Powering with 5V via the 5V rail will only be a problem if a (very low-resistance) load is connected to the Vin pin. As long as the Vin pin is unconnected, powering via the 5V rail is fine. A great many of us do this all of the time, and never experience any issues whatsoever. I'd say that the cases of regulator failure that you mention might be due to connecting something to or shorting the Vin pin to ground while the board is powered via the 5V rail.

I doubt if the diode could withstand continuous current, so it's best to ensure that nothing is connected to the Vin pin when powering the board in this manner.

Updated

I tested with 3 power adapters 9-12V by connected via DC Jack or use USB cable. All gave the voltage drop when add load at 5V pin. 5V pin = 4.8X Volt 3.3V pin = 2.8X Volt

Then I tested input power 9-12V at Vin pin and very lucky. All 3 power adapters work perfect when add load at 5V pin. 5V pin = 5.00 Volt 3.3V pin = 3.33 Volt

What do you think? What's wrong with DC Jack and USB Jack?

Can you make a photo of it ? I don't know what to think of it. What is going on with your 3.3V ? If the 5V is only 4.8V, then it must be 3.3V.

It is possible that all three of your power adapters are bad. The 130mA for the sensor should not be such a big problem. An Arduino Uno powered via USB can easily do that.

There's something strange going on here. As Koepel points out, it doesn't make sense that the 3.3V rail would also measure a low voltage, if the 5V rail is at 4.8V, with everything working normally when power is applied to Vin.

The symptoms indicate an excessive load on the 3.3V rail, pulling down both the 3.3V rail and the 5V rail, but that would still happen when you supply power via the Vin pin, whereas you say everything is fine when you power the board that way.

I don't understand this either.

If the voltage is being measured other than at the relevant regulator terminals then IR losses across the ground wiring due to the 130mA current could account for some drop in voltage on both 5 and 3.3V rails. Copper is not a superconductor, and 35um thick copper PCB traces and vias can have significant resistance in this sort of scenario.

You say the load is an MQ2. That's got a 33 ohm coil, so you're pulling ~100 mA from the 3.3v rail with one MQ2. The 3.3v regulator on the official boards is crap, and the one on most clones is even crappier. If you need to use 3.3v on the MQ2's coil, use an external 3.3v regulator. Those gas sensors are real power hogs.

Still doesn't account for why there's no noticeable voltage drop when 9V is applied to the Vin pin with the same load.

Maybe something wrong with a diode.

Updated!

GND.png

It’s my fault. If the voltmeter use the difference GND pin with the sensor, it will show 5V.
If the voltmeter use the same GND pin with the sensor, it will show 4.8V.
Maybe my UNO is not good. It should be the same ground.

snap0946.png

Also the internal regulator (NCP1117ST50T3G) can supply only
I = ((150 - 25) / 160) / 5 = 156mA