Why use two capacitors in parallel?

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.

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.

On Nov 22, 2006, at 8:39 PM, Dan wrote: 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

I have no quarrel with that!

Bill

-- Fermez le Bush

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?