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  1. #1
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    Regulated Power Supply

    I design a circuit of Modbus and GSM with two different regulators.
    Modbus need voltage >4.8 and GSM need 4 volt so i used 7805 (1Amp) with Max485 and atmega8 converter and LM2596 (3Amps) Adjustable regulator to power up GSM board . and input supply is 12V/2A adapter. when i use this circuit to read data from inverter and send data using http request.
    my 7805 goes heat up but circuit working properly . please suggest me what change should i make ??

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  2. #2
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    Re: Regulated Power Supply

    Hi,

    First step would be to calculate PD of 7805: (Vin - Vout) * I. If it's 12V in, then 7V * (Modbus + ATMega currents).

    Good idea to review datasheet graphs for 7805 about current draw at different Vin - Vout. I think about 100mA is a nice current for that size voltage drop without a heatsink or forced cooling.

    Possibly - depending on current draw - need to heatsink the 7805.

    If 7805 is the regulator you must use you could bypass it with a BJT, figure 13 and 14 of STMicro 78xx datasheet has the circuit diagrams and formula.

    Use a dropping resistor on input to 7805, but that just wastes the same amount of energy, at least it would be outside the regulator. You can do the same thing with a mosfet (using it as a dropping resistor), I believe.


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