Tie voltage rails

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Tie voltage rails

This topic has 8 replies, 5 voices, and was last updated 15 August 2022 at 10:47 by Speedy Builder5.

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14 August 2022 at 18:28 #609581
Speedy Builder5
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@speedybuilder5
Dabling in electronics, Guidance needed (Again).

I am playing around with data transfer over wireless , using Arduino UNO and two 5volt to 3volt logic shifters and a 3 volt LoRa RFM98W radio card.

Spec Logic Shifter

In practice, the Arduino could have a USB cable attached to a computer and a 9 volt supply. The Arduino then regulates the supply to 5v and provides its own 3 volt line.

1/ Should I tie the Arduino 3volt output to both the logic shifters and the radio card 3 volt pins.

2/ same for the 5volt rail (excluding the radio card)

3/ same for the ground rail.

Thanks in advance

Bob
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14 August 2022 at 18:28 #32278
Speedy Builder5
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@speedybuilder5
14 August 2022 at 19:07 #609586
John Rudd
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@johnrudd16576
I’m not as edjucated as other members. Nor does my noledge equal or better others….

so here goes….

https://core-electronics.com.au/guides/how-to-use-logic-level-shifters-converters/

other members may have a better offering

Edited By John Rudd on 14/08/2022 19:08:08
14 August 2022 at 19:09 #609587
SillyOldDuffer
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@sillyoldduffer
Posted by Speedy Builder5 on 14/08/2022 18:28:56:

Dabling in electronics, Guidance needed (Again).

I am playing around with data transfer over wireless , using Arduino UNO and two 5volt to 3volt logic shifters and a 3 volt LoRa RFM98W radio card.

Spec Logic Shifter

In practice, the Arduino could have a USB cable attached to a computer and a 9 volt supply. The Arduino then regulates the supply to 5v and provides its own 3 volt line.

1/ Should I tie the Arduino 3volt output to both the logic shifters and the radio card 3 volt pins.

2/ same for the 5volt rail (excluding the radio card)

3/ same for the ground rail.

Thanks in advance

Bob

Yes.
- The Arduino provides 5V and 3V power for other devices.
- The level shifter needs 5V and 3V power to convert logic signals up and down.
- The radio card just needs a 3V supply but it needs to be a bit beefy because the transmitter claims 100mW out. I'd budget for the module drawing 150mW from the Arduino's 3.3V regulator, about 45mA. Should be OK but I couldn't find a Uno spec giving the capacity of the 3V regulator.
Dave
14 August 2022 at 20:39 #609595
An Other
Participant
@another21905
Found this online:

"You can use the 3.3V pin to power sensors and modules that need 3.3V power.

It can supply about 100 to 150mA of current. The 3.3V regulator is connected to the output of the 5V regulator. Drawing current from the 3.3V regulator will dissipate heat in both the 3.3V regulator and the 5V regulator. This means that if you connect a 3.3V device to the 3.3V pin, then it also limits the maximum current you can use for the 5V modules connected to the 5V pin."

Playing about with an nRF24 radio module, I found that the 3.3V supply should be treated with caution – probably depends on where the card comes from. Nowadays I much prefer a separate supply rather than relying on the Arduino itself – I've wasted too much time trying to debug problems eventually traced to the power supply.

Consider using an ESP32 module – it can be programmed using the Arduino IDE++, and is native 3.3V, so you don't need level shifters.

Hope this is some help.
15 August 2022 at 06:51 #609623
Speedy Builder5
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@speedybuilder5
Thanks for the advice gents. It looks like I should common the ground for all devices, take the 3.3volt from the Arduino for the level shifters and provide a separate 3.3volt for the radio card so as not to fry the Arduino 3.3v regulator. In which case, not tie the two 3.3volt supplies ?

Of course, there are several circuits for the radio card on the net not using level shifters with other comments saying it may fry the radio card and or Arduino. Its a greedy little thing !

Bob
15 August 2022 at 07:40 #609629
John Doe 2
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@johndoe2
I only have a partial understanding of the specific application you are asking about, but it is usually never a good idea to tie "voltage rails" together. Odd things or bad things could happen.

By all means tie earths together and, depending on the circuit; zero voltage lines together, but don't tie voltage lines together. The only time you might do so is to parallel batteries, but even that must be done with caution.

Something like a transmitter ideally needs its own dedicated supply, to prevent it pulling a voltage rail down and causing problems elsewhere in the circuit.

Edited By John Doe 2 on 15/08/2022 07:40:56
15 August 2022 at 10:33 #609656
Speedy Builder5
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@speedybuilder5
John, attached is my poor circuit diagram. (Ignore the comments on the diagram)

Bob
15 August 2022 at 10:47 #609661
Speedy Builder5
Participant
@speedybuilder5
John R – thanks for that link. It does explain in simple terms how to link the logic level shifters. In my diagram, I think that the logic level devices and the radio device should have their own dedicated 3.3v supply, and the 5volt supply from the Arduino. Tie the 5v supply and Grnd to the Arduino and the 3.3v and Grnd to both the logic shifters and radio card.

Bob
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