"robb" wrote in message ...

"robb" wrote in message
...
yes i googled and found lots of different mods to the basic
5v
*7805* regulator power supply
that is {AC in, bridge, 7805, 5v out}

(images on a.b.s.e)

i built and experimenting with the 5v DC psu
i came across a strange (to me) oscope waveform.

using most basic psu {18vac in, bridge, 7805, 5v out}
with a (470 Ohm + standard LED) power indicator on the 5v side

1st
i oscope the (bridge +) and (7805 out) and i see typical
expected waves... that is bridge + positive 16 v sine peaks
squashed together and 7805 out is 5v DC but with small dips at
intervals cooresponding to positive sine peak dips

You do not give us any indication about the settings for the oscope
for each trace. This complicates interpretation of the displays.

The bridge + signal does not match what I would expect to see.
The difference may be indicative of what is your problem. Normally
there is a large capacitor connected between the + and - outputs
of the bridge. This capacitor stores energy during the peaks of the
input sine wave and then provides this energy to the 7805 during
the minimums of the input sine wave.

Without the storage capacitor, the + output would look like a simple
full wave rectified sine wave (which is what I think that you have in your
display pictures). Without the storage capacitor, there is nothing to
provide power to the regulator while the input AC is near zero. This
is probably the reason that your regulator voltage is dropping out.

With the storage capacitor, one should see a DC voltage with some
ripple voltage. The ripple voltage should peak at about the input sine
wave peak minus two diode drops in the bridge (about 1.4 volts).
After the input voltage peaks, the ripple will show the capacitor
discharging as the supply current is being drawn from the cap.

The storage capacitor needs to be large enough to provide between
the input peaks. You can estimate the required capacitor size using:

I = C * dv / dt

Whe

I = maximum regulator current (in amps)

C = storage capacitor size (in farads)

dv = expected change in the voltage as the capacitor discharges.
This needs to be small enough so that the capacitor voltage does not
drop below the drop out voltage for the regulator. I.e. the peak
capacitor voltage minus dv should be greater that the regulator's
drop out voltage.

dt = time between the input sine peaks (i.e. 0.00833 sec for 60 hz)

At 60 hz, with dv = 4 volts, and 1 amp of maximum current, this gives:

1 = C * 4 / 0.00833

-- C = 0.002083 F or just over 2000 uF