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Why does adding a linear regulator to this circuit cause the input voltage (V_c) to drop?

Power circuit schematic

  • The 9V input coming in is regulated. The voltage measured at the branch between D5 and R22 is ~8.7 V, so it should not be voltage sag due to the load of the regulator.
  • If I remove REG1 from the circuit, V_c reads ~8.7 V.
  • With REG1 present, V_c reads ~6.7 V. The drop in voltage is across R22.

The only other place in the circuit where V_c is used directly is to power a 2N7000 MOSFET used as an input buffer.

input buffer circuit

My best guess is that REG1 is somehow creating a voltage divider with R22, but I did not think this could happen. I don't believe my PCB has any shorts.

Note: I'm actually using a 78L05 regulator. I've tried two different ones (this and this). The full circuit diagram can be found here (on page 4).

Thanks.

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  • \$\begingroup\$ Check TO92 pins 123 = Out-gnd-In \$\endgroup\$
    – Tony Stewart EE75
    Jan 5, 2020 at 3:52
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 REG1 (TO92) pinout is correct. V_c on pin 3, and 5 V out on pin 1. \$\endgroup\$
    – KLing
    Jan 5, 2020 at 4:16
  • \$\begingroup\$ Load/Source impedance ratio acts as a voltage divider. Find out where the fault lies. \$\endgroup\$
    – Tony Stewart EE75
    Jan 5, 2020 at 4:18
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    \$\begingroup\$ Hi, I am not entirely clear on the purpose of having R22 as simply dissipates power and makes all voltages after R22 load dependant. Secondly, Is Vb a voltage reference? if yes, it would be better to create a reference from after your linear regulator. Thirdly, if you see sudden voltage drop without any load, it could be a faulty regulator, if not incorrect pin usage. The use of 2N7000 is not clear aswell in terms of what is outcome you desire from use of 2n7000 circuit? \$\endgroup\$
    – Nouman Qaiser
    Jan 5, 2020 at 5:31
  • \$\begingroup\$ I agree but L05 rated for 100mA means 3.3V drop max If more its a faulty regulator and should be hot \$\endgroup\$
    – Tony Stewart EE75
    Jan 5, 2020 at 6:44

1 Answer 1

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The regulator needs to have a capacitor, around 0.33uF nearby between input and ground. Also a small capacitor 0.1uF between output and ground is good practice. The regulator could be unstable without it, and without a little bit of load on the output. That may explain the current draw, and resulting voltage drop across R22. Speaking of R22, normally it is a bad idea to have a high resistance on the power feed to a regulator. Also, it is worth noting that the regulator will only work if the voltage drop from input to output is at least 2 volts. So if you need 5 V out, the input pin on the regulator must be at least 7 V.

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