## Looking for a simple voltage devider to make this work.

7 posts / 0 new
Author
Message

I'm wondering if this can be done without a mosfet?

I have the following set up and working, the 3v TTUL works because the z-doids  but the circuit can be broken at the blue line and that seem like a bad situation to me. That device is not going to take the two 5v pulls well. So I'd rather put some type of a voltage divider in here so that the two can be disconnected. Though I'm trying to achieve this without a mosfet mainly because I'd like to prescribe the fix to a "common" individual and not make it too complicated. If there is no other way, so be it.

This being my divider logic at the moment.

Last Edited: Thu. Jul 4, 2019 - 12:43 PM
Total votes: 0

You want a 5v tolerant input to your 3v logic?

Just put a series resistor, say 1k between the 5v signal and 3v input. The chip's protection diodes will keep the voltage on the pin withing range and the resistor will limit the current to a couple milliamps. 1K will be low enough you can still use the 50K or so internal pullup resistor. If you want less current, use a bigger series resistor, switch off the internal pullup and put a pullup (if needed) to the 5v on the input end of the resistor.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut.

Total votes: 0

No, 1k would work. The Device is a de10 and does have internal 50k's

https://www.digikey.com/en/datas...

I'm guessing it has protection diodes in there.

So keep it as simple as this for the regulation.

Total votes: 0

Torby wrote:
The chip's protection diodes will keep the voltage on the pin withing range and the resistor will limit the current to a couple milliamps.
A common method for amps and comparators that would be a subtle defect for some MCU (ESD/EOS/EFT/lightning suppression takes a very small time duration to effect; therefore, very brief over-voltage)

Some MCU are 5V tolerant.

[then a page about isolators though there are other methods]

PIC32MZ1025DAK176 - 32-bit Microcontrollers

"Dare to be naïve." - Buckminster Fuller

Total votes: 0

I'm wondering if this can be done without a mosfet

What?  This is the first sentence & forget to mention what you want to do.

That device is not going to take the two 5v pulls well.

Which device? U2?

Why not tie your 1K part to 3.3v?

Please state what you need, rather than how the circuit is not working.

I'd like to prescribe the fix to a "common" individual

Lead solder is doctor recommended.  Use as directed.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Total votes: 0

u2 is my client that is attempting to communicate to the DE10.  With the entire circuit together it works fine. The "concern" is when the break is made.

u2: This is a USB device that when attached to A) both USB lines are pulled high. This tells the u2 to enter in none USB mode. So the device detects the high condition on both lines and begins a simple Clock / Data communication. This works fine but if u2 is removed and A) is still connected that is going to pull high on the 3v ttl serial lines as the de10 is providing the 5v for the client. Reading the data sheet for the de10, this seem unwise. I'd like to find a solution to prevent this condition when its unplugged. Maybe all I need is a z-diode to prevent anything over 3.3v back in to the de10? Something like

Last Edited: Fri. Jul 5, 2019 - 02:54 AM
Total votes: 0

Zeners aren't a good choice - look at the actual voltage spread over temperature and current. Use proper level shifters. If you're going to the length of using a fpga, then some level shifters are a trivial solution.