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First PCB order. Requesting Advice on Electrical Schematic.

Former Member

Other than the 10K Resistor that is not connected to anything, I am wondering if anyone can spot any issues with the schematic below?

 

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  • 0

    The 78L33 is not a suitable regulator for this application - you are taking power from a USB connector using only the power pins (itself rather bad practice) so you supply voltage may be as low as 4.5V according to specs but down to 4V in real life. The &*L33 (page 4 of ST's data sheet) has a dropout voltage (max) of 1.7V so it needs a supply of at least 5.03V to remain in regulation. Change the 78L33 for a modern low drop regulator.

    You have no small ceramic caps across the supply to the processor - add a 0.1uF ceramic cap to ground for each VCC pin on the processor. They must be placed close to the processor pins. You need similar caps for ISP1.

     

    The ESP8266 will need more decoupling - are you using  a ready made module or a bare chip. Don't use the bare chip unless you have access to, and know how to use RF test gear.

    I would expect  a WiFi chip to draw quite large peak currents - on the little module using this chip they use  a much beefier regulator than you have. Check out circuits on the web for these modules and ask yourself why yours is so different.

     

    MK

  • 0 in reply to michaelkellett

    MK,

     

    Thank you for the insight. I will work on sourcing a better voltage regulator (any suggestions?) as well as adding filters to my ATmega328P-MU, ESP8266, and the 6 AVR pins. Could you shed some insight on what you would do to not take power directly from the power pins of the USB connector as you indicate is bad practice? 

     

    For clarification, the battery terminal is attached to a 3.7V 500mAh lithium ion polymer battery. I am currently using the ready made ESP8266 version 3, but I also have version 7 and 12 that I am tinkering with. I put the voltage regulator in the circuit because the ESP8266 is only rated for 3.3V, but I see what you mean as the ESP8266 will probably draw 300mA at peak, and the 78L33 is only rated for 100mA.

     

    Because the battery nominal voltage is 3.7V, do you think I can get away with removing the voltage regulator altogether? (i.e. How much less time do you think the chip will last? I have seen the ESP8266 work at 5V.)   

     

    Thanks Again

     

    Frank Vigilante

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  • 0 in reply to michaelkellett

    MK,

     

    Thank you for the insight. I will work on sourcing a better voltage regulator (any suggestions?) as well as adding filters to my ATmega328P-MU, ESP8266, and the 6 AVR pins. Could you shed some insight on what you would do to not take power directly from the power pins of the USB connector as you indicate is bad practice? 

     

    For clarification, the battery terminal is attached to a 3.7V 500mAh lithium ion polymer battery. I am currently using the ready made ESP8266 version 3, but I also have version 7 and 12 that I am tinkering with. I put the voltage regulator in the circuit because the ESP8266 is only rated for 3.3V, but I see what you mean as the ESP8266 will probably draw 300mA at peak, and the 78L33 is only rated for 100mA.

     

    Because the battery nominal voltage is 3.7V, do you think I can get away with removing the voltage regulator altogether? (i.e. How much less time do you think the chip will last? I have seen the ESP8266 work at 5V.)   

     

    Thanks Again

     

    Frank Vigilante

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