Go Down

Topic: Question on a design by adafruit - level shifting (Read 169 times) previous topic - next topic

liudr

The adafruit esp8266 board:

https://www.adafruit.com/products/2471

Nice big picture shows two diodes marked as B5 serving as 3.3V to 5V level shifter (left side 5V, right side 3.3V). Their schematic say 1N4148 but I don't know if that was correct. They probably only wanted the SMD footprint to be correct.



I found the diode to be OnSemi B5 ESD protection diode, essentially a transient voltage suppressor for 5V devices:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0ahUKEwjey5nymMDOAhXHwiYKHaeTAdgQFggqMAI&url=http%3A%2F%2Fwww.onsemi.com%2Fpub%2FCollateral%2FESD5B5.0ST1-D.PDF&usg=AFQjCNEzt9Lj8TEtyfCFbNEXCOq72sn75w&sig2=mpZMbDBnTJbl-rQK01JyOw&bvm=bv.129422649,d.eWE&cad=rja

So my question is: do TVS get regularly used for voltage level shifting in industry? I'm not an electrical engineer so I don't know if this is a common practice. Thanks.
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

Graynomad

#1
Aug 14, 2016, 10:50 am Last Edit: Aug 14, 2016, 10:51 am by Graynomad
That will work if the right side has a pull up resistor to 3v3.

As for a TVS diode being used, maybe they have a smaller Vf so the low voltage level is better (IE lower) on the 3v3 side, but I don't know why if that's not the case.
Rob Gray aka the GRAYnomad www.robgray.com

liudr

No, the design only has a pullup to 3.3V on RESET. The RX is directly connected to ESP8266. Maybe ESP8266 has an internal pullup. But won't this design require some largish current to pass through the TVS from the 5V side? Say the 3.3V RX is connected to the diode/TVS, and the same junction is connected to a resistor then 3.3V power, then the junction would be somewhere above 3.3V when the 5V RX is HIGH, am I correct?
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

Go Up
 


Please enter a valid email to subscribe

Confirm your email address

We need to confirm your email address.
To complete the subscription, please click the link in the email we just sent you.

Thank you for subscribing!

Arduino
via Egeo 16
Torino, 10131
Italy