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7805 regulator capacitors

Discussion in 'Power Electronics' started by HellasTechn, Nov 9, 2019.

  1. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Exactly.

    Yes but in my case 9V is not an option. I use 12V as input in my project which is used for driving orhter circuits and then it is stepped down to 5V to drive the microcontroller.

    I am going to double check with my multimeter and get back to you on that. 300ma is peak current consumpion. The average current consumption is about 210ma i think.
    I didnt mention that in post 1 because my actual question is about Capacitors value and type selection :)
     
    Last edited: Nov 11, 2019
  2. bertus

    bertus

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    42
    Nov 8, 2019
    Hello,

    When you lower the resistor to 15 Ohms, you will not reach the critical values for regulation.

    Bertus
     
  3. hevans1944

    hevans1944 Hop - AC8NS

    4,258
    2,008
    Jun 21, 2012
    @HellasTechn, you seem to be ignoring the datasheet recommendations concerning the capacitor values you should use. The input capacitor is a high-frequency bypass capacitor, needed ONLY if there is substantial distance between the 12 V "smoothing" capacitor and the three-terminal "7805" linear regulator input. It assumes there is NO resistor between the linear regulator input and the raw, filtered, DC output power supply. The output capacitor is a deliberately small value selected (only if needed) to avoid changing the linear regulator characteristics when the circuit is operating normally under load.

    The raw DC should be derived from a full-wave bridge rectifier and "smoothed" or filtered with an electrolytic capacitor whose value depends on the current being supplied and the allowable ripple on the input. At the peaks of the input voltage, this capacitor will charge up and then linearly discharge into the voltage regulator input until the input voltage begins to increase again. The maximum and minimum values of this pseudo-sawtooth wave form must be within the allowable input voltage ranges of the the linear regulator.

    So, knowing the discharge current of the filter capacitor is linear, and knowing the constant current of this discharge (why constant? because the output voltage is constant across a presumably constant resistance load, therefore the load current and capacitor discharge current is constant), and knowing something about the period of the pseudo-sawtooth wave form (about 1/120 seconds for 60 Hz line supply), you can calculate a conservative value for the smoothing capacitor. It is only if this capacitor must be physically located some distance from the regulator that the 0.33 μF input bypass capacitor is required.

    The output bypass capacitor is only recommended for noise reduction. Because the output and its load is an integral part of the closed-loop negative feedback linear voltage control circuit, you should not add large values of "smoothing" capacitors to this output, lest you disturb the feedback characteristics so carefully designed into this integrated circuit. In particular, DO NOT add 100 μF aluminum or tantalum electrolytics between the output and ground.

    Gather together and READ as many datasheets as you can find, from different semiconductor manufacturers, on the 78xxx series of three-terminal voltage regulators. IIRC, at least one of these datasheets explains why you do not want to add a large capacitance to the output terminal because it can actually cause instability in regulator operation.
     
    HellasTechn likes this.
  4. HellasTechn

    HellasTechn

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    Apr 14, 2013
    do you mean i will reach the critical values for regulation ?

    For now i can say for sure that the system is working. Like i said i will check again with the dmm.
     
  5. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Okay that makes sense.

    In my case the 12V comes from an SMPS far from the regulators location. There are at least 5 meters of cable between.

    So i have chosen wrong capacitor values. I should go with what the datasheet recommends.

    I have to say though that i have seen 7805 regulators oscillating the output when no capacitors are used. I am not sure why that happens.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    In that case a switch mode regulator is the less power hungry alternative. You can get them as 7805 plug-in replacements.

    This is for LDOs but the theory is comparable to 7805's behavior.
     
    HellasTechn likes this.
  7. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Here are the actual readings.
    The actual resistor value is 26 Ohms and With 12V input under normal operation the circuit consumes 180ma, the voltge across the resistor is 2.4V and the input to the 7805 is 9.6V
    The measurements where taken without the diode so subtracting 0.5V (actual diodes voltage drop).
    Thathe actual readings indicate that im working within limits.

    I cant mathematically explain it but readings are readings yes ?
     
  8. bertus

    bertus

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    42
    Nov 8, 2019
    Hello,

    The 26 Ohms and 180 mA do not match with the mentioned 2.4 Volts.
    When I do the calculations the voltage accross the resistor would be 26 Ohms X 180 mA = 4.68 Volts.
    I do not have an explanation why the voltage accross the resistor is 2.4 Volts.

    Bertus
     
  9. HellasTechn

    HellasTechn

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    Apr 14, 2013
    I know. I really do not know what to say. It probably has something to do with the 7805.
     
  10. Hunter64

    Hunter64

    10
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    Nov 20, 2018
    You mentioned a maximum current of 300mA. With the 26 Ohm resistor you'll lose 7.8V.

    Good luck with reliabillity.
     
    davenn likes this.
  11. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Ive been thininking about the math... They didnt "add up" because i forgot that i took the current measurement at the input of the device, not at the 7805 output. So i was way off.
    I have measured the current going out the 7805 (actually consumed by the 5V circuits) and goes between 70 and 90ma depending the state of the i/o pins. That explains the 9.6Volts on the 7805's input
     
    Last edited: Nov 13, 2019
  12. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Thank you all four your advices. Special thanks to sir hevans1944 for post #23

    Though i have seen many circuits using 7805 with aluminum electrolytic capacitor between the output and gnd, i guess that the people who wrote the datasheets know better thatn i do so i used the capacitors recommended by them. 300nf ceramic at the input and 100nf ceramic at the output.
     
    Last edited: Nov 13, 2019
    davenn, Hunter64 and Minder like this.
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