Voltage Divider Resistor Values

Hello,

I bought one of these as I need to connect a 5V Micro Pro clone to a 3.3V flash memory module.

I notice that the hookup requires 5V for one side of the board and 3.3V for the other. I intend to use a Voltage divider to scale 5V down to 3.3V are the Micro Pro doesn’t have a 3.3V output.

I understand the physics of voltage dividers, but would like some guidance on the size of the resistances.
e.g. 1K and 2K would give me 1.666V 3.333V
but so would
10K and 20K.
Obviously the current would be different for these circuits.
Can someone please advise on what size of resistors I should use?

This is the Flash Memory (Needs 3.3 V logic)

2 Likes

Hi John,

Great question!
Unfortunately the answer goes a little deeper.

If the 3.3V is for the power of the logic converter we’ll (possibly) need a bit more juice.
We can take a look at the datasheet for the level shifter: https://core-electronics.com.au/attachments/localcontent/txb0104_12695e2eec4.pdf
The figure we are after is the current draw through Vccb:

50mA is a lot when it comes to a voltage divider (since we cant assume a high impedance load this affects how we calculate the resistors). To get 50mA across the whole divider we’d need a total resistance of 100 ohms. The power being dissipated would cause them to heat up a lot!

A recommended solution

If you’re just breadboarding a 3.3V regulator like this one: Makerverse Voltage Regulator - 3.3V | Core Electronics Australia
Will output a healthy 3.3V using the 5V output that you can then plug into the logic converter.

Another easier but more expensive solution
The pro micro does come in 3.3V variants, it might be worth just grabbing one of these for this project?

Liam

Thanks for your detailed response Liam. I am making a homemade version of the Arduboy with a bigger screen SSD1309 2.4inch. The 3.3V microPros run at 8MHz instead of 16MHz which would make many of the games too slow.

So far I have it working well in its simplest configuration:


And I have added a battery which is currently charging (Fingers crossed it all works.)

But Lastly I need to have a W25Q128, which is rated at 3.3V, hence the need to convert Logic from 3.3 to 5V and back.

I am happy to add more components such as you suggest…

However, I just thought, I could use the 3.7V from the battery to power the board… Doh.

1 Like

Hi John.
As Liam says a voltage divider is not your answer here.
The data sheet for your storage module chip is available to download here

It also needs a VCC of 3.3V
This has to be provided somewhere so use this source for the 3.3V at the “B” port of the level shifter.
If you haven’t got a 3.3V supply you will have to arrange one but a voltage divider will not do this job.
Cheers Bob

1 Like

Thanks Robert. Much appreciated.