24VDC level translator

Thread Starter

Justus22

Joined Dec 8, 2018
12
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.

1.png
 

Thread Starter

Justus22

Joined Dec 8, 2018
12
Like this?
1.jpg

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
 

Sensacell

Joined Jun 19, 2012
2,821
R3 and R4 are there to limit the current that would flow through the input protection diodes inside the HEF405 when an overvoltage happens.
 

GeBJT

Joined Nov 3, 2020
19
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.
 
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