"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

2nd (**problem here **)
i add .1uF cap between 7805 out and ground (high frequency
decoupling ) and the bridge + output changes to some high
frequency fuzz on the tops of positive sine peaks.

When i zoom in on the ?fuzz? it is a series of about 1 uSec wide
spikes that are space about 10 uSec apart and the magnitude
follows shape of the sine wave ? (see pic in a.b.s.e) thats the
best this amateur can dexcribe

3rd ( ? problem fix ? )
all of these things by themselves will make fuzz go away....
- add a .1uF cap between the -/+ bridge output
- remove the LED
- add smoothing cap to either side

any ideas to help understand what this fuzz is ?

thanks for any help ,
robb

7805 needs about 2 volts of clearance to maintain regulation. Data sheet for
the 3 manufacturers I looked up shows it needing at least 0.3uf on the input
to prevent oscillation (which is what is causing the noise on your input
peaks), however you should use a much larger cap on the input to smooth out
the ripple from the bridge as well.

Linear regulators can be picky about their needs for input/output caps. It
varies even more with LDO types. Even the type of cap matters with some of
them...

Chris


"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

2nd (**problem here **)
i add .1uF cap between 7805 out and ground (high frequency
decoupling ) and the bridge + output changes to some high
frequency fuzz on the tops of positive sine peaks.

When i zoom in on the ?fuzz? it is a series of about 1 uSec wide
spikes that are space about 10 uSec apart and the magnitude
follows shape of the sine wave ? (see pic in a.b.s.e) thats the
best this amateur can dexcribe

3rd ( ? problem fix ? )
all of these things by themselves will make fuzz go away....
- add a .1uF cap between the -/+ bridge output
- remove the LED
- add smoothing cap to either side

any ideas to help understand what this fuzz is ?

thanks for any help ,
robb

"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

Thanks for reply Dan, More responses inline.

"Dan Coby" wrote in message
...
"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.


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


yea, it was getting to be long winded just telling the what part
and i forgot the exact settings, i thought the relative trace
differences would be enough.

e.g. trace without mod, then trace with a mod.


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.


well i was just experimenting with the most fundamental circuit
to try and learn and understand why those typically *Normal*
additions occur, such as ripple smoothincaps, decoupling caps,
etc

the simple method being start with atypical basic circuit measure
input/outputs, make a "typical" change and then compare the new
input/outputs note differences


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.


yes the sine wave hop hop hop, was the simple bridge (+) output
with no typical circuit additions (caps) they were 16v DC
measured but that must be an average as the divisions were set to
10 V vert/div so then ~ 22 volts at peaks

and as you pointed out at abot 5-6 volt the regulator cuts out
and causes the regulated output dips.


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.


interestingly, though not shown, i could only get the ripple with
fairly smallish cap (470 uF 35v) and high current load like a 12v
*auto brakes* light bulb and the ripple looked like a saw wave


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


thanks again for the usefull info and reply Don,
robb

Thanks for help Chris,
robb

"Christopher Ott" spamtrap at ottelectronics dot com wrote in
message ...
7805 needs about 2 volts of clearance to maintain regulation.
Data sheet for
the 3 manufacturers I looked up shows it needing at least 0.3uf
on the input
to prevent oscillation (which is what is causing the noise on
your input
peaks), however you should use a much larger cap on the input
to smooth out
the ripple from the bridge as well.

Linear regulators can be picky about their needs for
input/output caps. It
varies even more with LDO types. Even the type of cap matters
with some of
them...

"robb" wrote in message ...

....snip...

yes the sine wave hop hop hop, was the simple bridge (+) output
with no typical circuit additions (caps) they were 16v DC
measured but that must be an average as the divisions were set to
10 V vert/div so then ~ 22 volts at peaks

Okay, this gives us a little more information about what you have.

Yes, a typical DC measurement for a full wave rectified sine wave
does not match the peak voltage. Ideally the meter would measure
the 'rms' voltage. 'rms' stands for 'root mean squared'.
See http://en.wikipedia.org/wiki/Root_mean_square for more info
about calculating RMS voltages. rms voltages are used instead
of peak voltages since they track the power dissipation in a resistor.
I.e. a 1 volt rms across a 1 ohm resistor will dissipate 1 watt of heat.
This is true for both DC and AC signals. For a DC signal, the rms
voltage is equal to the DC voltage. For an AC signal, the peak voltage
in 1.414 (i.e. sqrt(2)) times the rms voltage. Thus a 16 volt rms AC
signal would have a peak of 22.6 volts.


and as you pointed out at abot 5-6 volt the regulator cuts out
and causes the regulated output dips.

Yes. The regulator will stop regulating properly when the input
voltage goes too low. A 7805 regulator does not have any form
of internal energy storage. Thus its output drops out when its
input voltage gets too low.

Do you get a reasonable operation of the regulator when you
add a storage capacitor? Please note that most regulators also
require some small capacitors for stability. These small capacitors
need to placed right next to the leads of the regulator.


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.


interestingly, though not shown, i could only get the ripple with
fairly smallish cap (470 uF 35v) and high current load like a 12v
*auto brakes* light bulb and the ripple looked like a saw wave

Yes. The ripple voltage will rise when the peaks of the input sine
waves occur. The diodes in the bridge will be forward biased
and then current will flow through the diodes to charge the storage
capacitor. After the peak passes, the diodes will be reversed
biased. The capacitor voltage will then drop as the load current
pulls charge off of the capacitor. This will look somewhat like
a saw wave.

The magnitude of the ripple is approximately given by:
I = C * dv / dt

Thus to see a 1 volt ripple voltage with a 470 uF cap at 60 hz,
you need to draw 56 mA.


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

There was a typo in my sentence. it should have read: "The storage
capacitor needs to be large enough to provide energy storage between
the input peaks."

Some of the 3 lead regulators like to sing.If this the case the only
way to fix it is to replace the regulator. I have found many of these
over the years.




On Mon, 26 Nov 2007 15:44:43 -0500, "robb" wrote:


"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

2nd (**problem here **)
i add .1uF cap between 7805 out and ground (high frequency
decoupling ) and the bridge + output changes to some high
frequency fuzz on the tops of positive sine peaks.

When i zoom in on the ?fuzz? it is a series of about 1 uSec wide
spikes that are space about 10 uSec apart and the magnitude
follows shape of the sine wave ? (see pic in a.b.s.e) thats the
best this amateur can dexcribe

3rd ( ? problem fix ? )
all of these things by themselves will make fuzz go away....
- add a .1uF cap between the -/+ bridge output
- remove the LED
- add smoothing cap to either side

any ideas to help understand what this fuzz is ?

thanks for any help ,
robb