Quote:
Originally Posted by Mike Copioli
Attached is a new schematic for the IR board. I will be posting the layout within the next few days. I am posting the schematic to get feedback from teams before I do a layout. Once the layout is done, I will post the Eagle and gerber files for any team that wishes to build there own board to replace the board provided in the KOP. I have added a diode for reverse polarity protection and an electrolytic cap to help with any ripple. I have also increased the value of the current limiting resistors for the LEDS from 240 to 330. Any other suggestions let me know. Also, the finish board will have a ground plane covering the bottom layer.
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A second and less obvious diode is common across the Vin and Vout pins of the regulator. This is a common design practice (but interesting left off the app note for the LM79L05) This diode protects from transients during power down. Imagine the device is powered up and then Vpwr is suddenly (0V) removed. This second protection diode is required to discharge the filter caps on the output by safely shunting the current around the regulator. Without this diode, the Vreg will be stressed with reversed polarity. VReg are hyper sensitive to any reverse voltage. As engineers we need to think beyond the "run" mode and also consider the power up and power down modes.
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Mode Vin Vout Diode
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Run 12V 5V Reverse Biased
Pwr Up 0->12V 0->5V Reverse Biased
Pwr Down 0V 5->0V Forward Biased
Hope this was helpful. Sorry the feedback is "late"