Jymc220bi Schematic 2021 Guide

The 2021 revision dissipates ~1.5W at 24V input. Glue a 10x10mm aluminum heatsink to U4 for improved reliability in high-ambient temperatures.

Earlier revisions used a fixed 1.2V reference for the boost. The 2021 version introduces a resistor divider between VOUT and FB (pin not even listed in generic datasheets!).

Through trial and error, repair forums found that the 2021 output voltage follows: jymc220bi schematic 2021

[ V_OUT = 1.25V \times (1 + \fracR_topR_bottom) ]

Where ( R_top ) is between VOUT and FB, and ( R_bottom ) is FB to GND.
If you see 5.2V instead of 5.0V, replace the top resistor (typically 100k) with 91k. The 2021 revision dissipates ~1

This is the heart of the 2021 redesign.

If you are using the schematic for repair, pay special attention to these four areas, as they are the most frequent failure points in 2021-manufactured units: Earlier revisions used a fixed 1

| Component | Value | Function | |-----------|-------|----------| | U1, U2, U3 | PC817 | 4-pin optocoupler | | R1, R2, R3 | 330Ω | Input current limiting | | R4, R5, R6 | 10kΩ | Pull-up to VCC_IO | | C1 | 100nF | Input noise filter |

How it works:
The STEP, DIR, and ENA signals pass through a 330Ω resistor into an LED inside PC817. The phototransistor side pulls the internal 5V line low via a 10kΩ pull-up. In the 2021 revision, a 100nF capacitor was added between each optocoupler output and ground to debounce noisy step pulses.