Problems with regulator decoupling
On two
separate occasions recently equipment was not performing quite
correctly, both faults were later identified as the regulator decoupling
capacitors being open
circuit on 78 and 79 series devices.
On another
occasion I had needed to introduce a video distribution amplifier to provide
several feeds from one video source. The
unit I used was ex equipment and previously untested by me. I was pleased that it did what I needed to do but there was
slight patterning on the picture when viewed after the DA
I tried several things like multiple earth loops but could not remove the
slight interference to the picture. The
DA had previously been in use commercially so I guessed it was capable of
perfect pictures.
I first
tested the rail voltages for +/- 5v from 7805 and 7905 regulators which checked
out ok. I then tried adding more
smoothing to the main unregulated rails but no improvement. I then noticed 10 microfarad caps on each regulated output.
I connected another capacitor at the output of the negative regulator and
completely cleared the fault. Double
checking with a scope, although the rail was a perfect -5v it had a huge self
oscillation modulated on the rail. Replacing
the decoupling cap it was then completely smooth.
Don’t
forget these caps are often very close to regulators that get hot and over time may go open circuit causing strange effects. Always try a scope first
on supply rails to check for self oscillation
Denis [email protected]
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