Digital Level Shift Used With Current Sources

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rpu_bus
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Joined: Sun Sep 18, 2016 6:07 pm

Digital Level Shift Used With Current Sources

Postby rpu_bus » Mon Jun 19, 2017 12:39 pm

K38 is the marking for BSS138 which is used to level shift voltage for I2C communications. It also works for higher voltages. The control boards have current sources that operate up to nearly the battery voltage (less BJT saturation voltage). The K38 is then used to level shift that down to a voltage that is safe for the MCU, before going into the current protection resistor. The protection resistor is sized to allow the MCU to pull down or sink a single current source. This means the MCU is well protected while gaining the ability to sink a 20mA current source from a 12V battery and therefore light up a string of LED's or a string of optical coupler inputs like found in solid state relays to turn on three phases with one control loop that still has enough voltage drop for an LED, and can be manually open with a switch.

DigitalIO_WithCurrSource.png
DigitalIO_WithCurrSource.png (14.81 KiB) Viewed 15 times

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