Voltage regulators problem

Other Hardware General Electronics
1

I have a 7805 5v fixed voltage regulator and a LM2940CT-12v fixed regulator. I have tried a 14.5v input to the 12v regulator and then connected its 12v output to the 5v regulator. Both regulator voltage outputs were wrong. The grounds were common to both regulators.
I have then tried the connections without the12v output connected to the 5v input. 14.5v input to both regulators. common ground. The 12v regulator output was 12v but the 5v was not regulated.
I need the 5v as an input to the beetle board and the 12v to power a small dc motor.
Is there a way to connect the voltage regulators in series as I first tried so that both regulators will regulate?
Both regulators work if they are independant of the other one.

2

Are you sure this regulator is ok?

3
  • Confirm you had appropriate decoupling capacitors placed on the regulators.
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4

Post an annotated schematic showing how you connected them.

5

I had no decoupling capacitors. Is there a circuit showing how to use decoupling capacitors. Not sure what you mean by decoupling capacitors. I want the regulators to be connected in series. I have only the one power source.

6

  • Place a 10uF capacitor on the regulator output to GND.
    Place a 0.1uF (100nF) ceramic capacitor on the input to GND, as close as possible to the regulator.

  • Today most would use Buck Converters.

$3.00 each

IMG_3336
IMG_33361671×1443 346 KB

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7

are these capacitors only necessary on the 12v regulator or on both the 12v and 5v? Have seen on an old Dick Smith catalog for a fixed 5v regulator, 7805, 2500uf on input and 0.22uf on output. The text says the same basic circuit can be used for different voltage regulators for example a 12v fixed regulator.
The capacitors you refer to are in the same catalog for an adjustable output voltage.
Is there some basic reason why the regulators have to have these capacitors to function properly?

8
  • When there is a length of wire between the power source such as a battery and the regulator, the inductance of the wiring can cause oscillation on the regulator output.
    The 100nF ceramic capacitor close to the input of the regulator stops these oscillations.
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9

I highly recommend downloading the free datasheets for each regulator to see the recommended connections. They also specify a minimum capacitance requirement, and without it, there's no guarantee they won't oscillate, which is likely the cause of your issues. As they say, RTFM (Read The Finished Manual).

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