I saw a simple power supply schematic and the person used a 470uF
(polorized) and a 1uF (non-polorized) in parallel on the output of a
regulator.
I sent the person an email and he informed me he's seen it done that way on
other supplies (which I have too), but didn't know the reason.
Can someone explain this?
Thanks in advance!

But depending on it's type of construction, it can exhibit
characteristics of both an inductor and a resistor. Large electrolytics
like the 470 ufd have a lot of inductance and hence poor response to
high frequencies. Therefore, you put a smaller C in // to take care of
the HF. Yes the input is only 120 hz[2 X 60 hz] but HF can be generated
down stream and need filtering out.
Chuck P.

On 11/22/06 6:16 PM, in article
221120061816090705% snipped-for-privacy@m> >
Sometimes that is not enough. Often, at least in the days of tubes, low
level stages were decoupled from the power supply with RC low pass filters.
That prevents feedback oscillations under some conditions. A long time ago,
I had lots of trouble with TTL circuitry because it was so fast. I had to
use capacitors close to the chips to keep current spikes from leaving the
vicinity.
Bill
-- Fermez le Bush

It may help, particularly the sizing calculations, to think of the
situation not so much a case of filtering but more one of meeting the
transient energy demands of the load.
If the load, say, is trying to generate a step output, large
electrolytics on their own would not meet the energy demand fast enough.
The power supply output would momentarily droop and may even drop to
close to zero - producing all sorts of effects. Including an output
waveform that was more slope than step - if you were lucky. Extra,
smaller capacitors, sited appropriately, reduce the transient effects on
the power rails of transient energy demands.

In my understanding, phasors are rotating vectors.
Take a simple voltage vector at an angle of zero. That is precisely
defined.
Now consider a an x-y coordinate system to the right of the voltage vector.
The y part of the coordinate system is the instantaneous voltage; the x part
is time.
So I rotate my voltage vector counter-clockwise. At 45 degrees, the voltage
is one-half the peak value. At 90 degrees, the voltage is at its positive
peak value. So, as I rotate my voltage vector, from zero degrees to 90
degrees, it traces out a nice sine wave.
So the need for phasors, in my opinion, is when you deal with other vectors
tied to the same system frequency. In other words, the current with respect
to the voltage could be described as a phasor that typically lags the
voltage. The phasor satisfies the need for "with respect to" in dealing
with various vector quantities.
So the vectors represent a snapshot of what the vectors do with respect to
each other. More importantly, phasors are important with three-phase
systems where you maintain a 120 degree displacement between the source
voltages.
So I tend to think of a vector as being a phasor at some instant in time.
Maybe some learned fellow like Don Kelly can shine some further light on
this matter.
If it works for you, use it. *****************************
The phasor is a representation of A*exp(j*w*t) where A is a complex number
representing amplitude and includes phase information. Usually, only the A
is drawn as the"vector." You call it a vector because when you add complex
numbers together the corresponding arrows add as vectors do. BUT ONLY IN TWO
DIMENSIONS. This is a very special case. It cannot be extended to find
accelerations for curvilinear motions in three dimensions.
Hamilton extended the concept of complex numbers to three dimensions so that
he could handle what we now call vectors. To do so, he had to abandon having
a single imaginary unit, what is usually called j in electrical engineering.
He had to use three, i, j, and k. Because of that quaternions do not
commute. That is necessary because cross multiplication of vectors does not
commute. Quaternion multiplication also leads to to scalar and vector
products. In the end quaternions have not been used much by electrical
engineers. Heavyside detested them and he also detested Tait who promulgated
their use. In the end, the vector analysis popularized by Gibbs and others
prevailed.
Bill

Hello, all and from the IEEE Standard Dictionary of Electrical and
Electronics Terms:
"A phasor is a complex number. Unless otherwise specified the term
'phasor' is assumed to be used only in connection with quantitites related
to the steady alternating state in a linear network or system. Notes: (1)
The term 'phasor' is used instead of 'vector' to avoid confusion with
space vectors. (2) In polar form any phasor can be written Ae^jk(theta) or
A(angle theta) in which A, real, is the modulus, absolute value, or
amplitude of the phasor and theta its phase angle (which may be
abbreviated phase when no ambiguity will arise."
Hope the above provides some clarification and please excuse my font's
lack of properly showing exponential and Steinmetz notations. Sincerely,
John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

Do all 1mF capacitors fire after the same time lapse?
If so I would think putting two in parallel would produce two pulses. One
from the larger, after given time and at given charge and similarly from the
second. Kind of a bright - dim flashing bulb, with set time between the
flashes. Bit more complex with the timing, but is that what it would do?

characteristics and placed them across a load, you could see the voltage
across the load rise to one peak, due to the "fastest" capacitor, start
to fall, and then peak again as the "slower" capacitor took effect.
Much the same effect happens at Southampton, UK with the tides. It gets
one high tide, with the water coming up the Channel from the West, the
tide starts to fall and then Southampton gets a second high tide, as the
tide water comes around the Isle of Wight and into Southampton from the
East..

Thanks Sue.
I find it interesting to note also how the line between D 7 C throws up the
greatest waters in the Channel, and also this is the roughest water could be
found in the Channel. Occurrent X hours before / after high tide. It's
amazing the waters creeping from the Atlantic North (North Sea) and South
(Celtic and English Channel) around Great Britain collide there and not
somewhere else. Isn't that unexpected Sue??
Like torque on a long screwdriver. Jeesus, that wouldn't have happened
without textbooks!!
('s amazing what a Nautical Almanac tells one who studies.)

Brighton - Le Havre, Dover - Calais (or the other way around if you're
French of course)
that's what I reckoned when I was designing barges and narrow boats to sail
on the French Canals and to take in one off Channel crossings.
What you think Sue? Anywhere near?

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.