Hi,

In a controller I found a HFE4050BT that used to implement a level translator for a 24 VDC input signal.
How does this work? The HFE4050BT is only specified for 15V max. Unfortunately I do not have a schematic, only a photo of the board.

 

How does this work? The HFE4050BT is only specified for 15V max.

Looks like the 24 volt input goes through a voltage divider comprised of a large 2.2K and 2K resistors

 

Like this?


R1: 2.2k, R2: 2.0k

What are R3 and R4 for? And why is R4 connected to both pins on the upper left of the IC

 

R3 and R4 are there to limit the current that would flow through the input protection diodes inside the HEF405 when an overvoltage happens.

 

Looks like R4 (2Meg?) is providing hysteresis by positive feedback from buffer output to input. This makes the buffer into a Schmitt trigger - good for handling signals that change slowly.