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Author Topic: Electrical Faux Pas  (Read 21257 times)

Offline z_p_e

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Re: Electrical Faux Pas
« Reply #15 on: November 13, 2007, 11:42:41 PM »
wattsup,

There are two main capacitor types: polar, and non-polar.

Non-polar caps are ceramic, film, mica, mono etc. These can be used for AC or DC with no concern about polarity.

Polar caps are electrolytics, tantalums, etc. They will have a + or - sign on their case. These can be used for AC or DC as well, but care must be exercised in both cases.

When used for DC, make sure you observe the polarity of the capacitor in relation to the power supply. Of course make sure the voltage rating is ok too.

When used for AC, the "safest" way to use them is by connecting two of them in series, and back-to-back. This is now a non-polar electrolytic, and these can actually be purchased, although they are not too common. Remember that when you do this, your final capacitance value will be one-half of that screened on the case. Back-to-back means connecting both +'s or both -'s together. It should not matter which one you choose.

You can "cheat" with an electrolytic when a high value capacitance is required and you don't want to use two in series, but only with "line-level" type signals....nothing of significant power or voltage swing. "Cheap" or poorly designed audio electronics often does this. It will work, but don't expect hi-fi results, the audio will actually become distorted.

The final way of using an electrolytic, or polar capacitor with AC is with a circuit that has a large DC offset. A perfect example would be to decouple the output of an audio amplifier that is using only a single DC supply. The amplifier will normally be biased half way between the supply voltage, and since you do not want this voltage constantly powering your speaker, you insert a capacitor in series between the amp output and the speaker to decouple this DC. In this case, you may use a single electrolytic cap, just make sure the +'ve lead is at the amp output, and the neg lead is connected to the speaker terminal. The other terminal of the speaker must be connected to ground in this case.

Cheers,
Darren

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Re: Electrical Faux Pas
« Reply #15 on: November 13, 2007, 11:42:41 PM »

Offline sparks

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Re: Electrical Faux Pas
« Reply #16 on: November 13, 2007, 11:42:53 PM »
    Sorry for messing up this thread
« Last Edit: November 14, 2007, 06:15:20 PM by sparks »

Offline wattsup

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Re: Electrical Faux Pas
« Reply #17 on: November 14, 2007, 12:52:26 AM »
@Sparks

You should post this in the TPU Discussion thread instead of this thread cause we are using this thread to discuss general electronics. Good post though.

@z_p_e

Again regarding the caps, and thanks for your previous post which I am starting to understand.

On DC circuits, I rarely see a capacitor that is in series (except on the large TPU that has those two black caps in series). They are usually in parallel to other components or between the + and - lines. I have tried putting caps in series but this does not work as I cannot get any voltage out of the other cap terminal.

Let's say you ran a small dc motor off of a battery. You can put the cap on the + and - before the motor, but you cannot put it only in series on the positive line. Even with transformers, I have not been able to. Are there special caps for this or is this simply impossible.

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Re: Electrical Faux Pas
« Reply #17 on: November 14, 2007, 12:52:26 AM »
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Offline z_p_e

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Re: Electrical Faux Pas
« Reply #18 on: November 14, 2007, 01:25:08 AM »
wattsup.

Capacitors are used as follows:

For DC, the capacitor is always in parallel to the load or battery. So the capacitor is "across" the battery or load, not in series with them. Capacitors do not "pass" DC voltage.

For AC, the capacitor is usually used in series for AC coupling (or decoupling of DC), except when used in filter circuits. In filter circuits (such as a simple RC), the capacitors can be in a "shunt" (parallel) configuration.

A couple other tidbits:

A transformer gives a similar situation to a series capacitor; unless the input is changing (i.e. AC), there will be no output. It's a little more complicated than that, but in general and to keep it simple, this is true.

A pure DC voltage is one that does not vary with time. But don't call a DC voltage that changes (drifts) from 10V down to 9.5V over a two day period AC either....it's just a drifting DC voltage ;)

So any time you are "switching" your battery on and off in relatively quick succession (either manually, or with an oscillator and MOSFET switch), this should be considered an AC source.

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Offline z_p_e

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Re: Electrical Faux Pas
« Reply #19 on: November 14, 2007, 05:31:48 AM »
Vortex,

I think in terms of ?transit time? and ?switching time?, SM may have been making a generalization and perhaps meant both in combination? If precision timing and instantaneous correction is required and involved, then I believe the bottom line is ?how fast can the device cleanly transfer its input to its output??

Inter-electrode capacitance in BJT's will cause group delay and slower switching for pulses, and I suspect this will have a far worse effect than the transit time through semiconductor material. In the end, it messes with the intended output, delaying higher frequencies more than lower ones. As a result our output pulse is "sloppy".

Tubes typically have much lower inter-electrode capacitance compared to BJT's and MOSFET's, but the transit time (strictly speaking) may not be faster. As you say, there is a physical distance the electrons must travel, and that takes time. In semiconductors, junction distance is very short compared to tubes, but the velocity of propagation through the pn material may be significantly lower than c, and as such the race might even out.

According to this web page: http://www.john-a-harper.com/tubes201/  tube transit time is about 1ns.

Let?s assume a Plate to Cathode distance of 1cm (0.01m). If the electrons traveled at a constant speed of light (they don?t), this transit time would be considerably less?.around 33ps. This is a factor of about 30 times less than c. Evidently, the average electron velocity of propagation in tubes is much less than c?unless of course the 1ns figure is incorrect.

Just for comparison, here is a recent comparator device from National:

?Comparators feature sub-nanosecond propagation delay.

May 22, 2007 - With 700 ps propagation delay, dual 21 mA Model LMH7322 features rise and fall times of 160 ps and dispersion of 5 ps at greater than 100 mV overdrive.?

In other areas, transistor switching times are getting down to a few pico-seconds:
http://www.cs.clemson.edu/~mark/464/transistors.html

So ultimately, I don?t know what to say of the assertions that tube transit times are much less than SS, other than in regards to the TPU, it may not matter because in the end both work.

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Re: Electrical Faux Pas
« Reply #19 on: November 14, 2007, 05:31:48 AM »
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Offline sparks

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Re: Electrical Faux Pas
« Reply #20 on: November 14, 2007, 07:31:53 AM »
 Can't find delete button sorry for confusing this thread :-[
     

Offline angryScientist

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Re: Electrical Faux Pas
« Reply #21 on: November 14, 2007, 11:44:30 PM »
@z_p_e

I find a discrepancy between your terminology and mine.
Quote
So any time you are "switching" your battery on and off in relatively quick succession (either manually, or with an oscillator and MOSFET switch), this should be considered an AC source.

Switching a direct current on and off is what I, and most everyone else, would call pulsed DC. It is still a direct current because the current would at no time change direction. It will still flow from negative to positive and the two will never switch positions.

That being said I would like everyone else to know that it exceedingly easy for a pulsed DC to turn into an AC current. For instance you could put it through a transformer to get AC out the secondary. Or you could put it through a simple coil and get a reversal of current flow during the off part of the cycle. In some cases you could consider a capacitor to reverse a direct current while it is discharging.

Sorry if I'm nitpicking. The rest of the post was great. I just have full knowledge of the value of having words mean the same thing to the sender as they do to the receiver. It's the only way to communicate.

Free Energy | searching for free energy and discussing free energy

Re: Electrical Faux Pas
« Reply #21 on: November 14, 2007, 11:44:30 PM »
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Offline z_p_e

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Re: Electrical Faux Pas
« Reply #22 on: November 15, 2007, 03:18:23 AM »
Hi AngryScientist,

You are correct. I would technically call it "pulsed DC" as well.

Sometimes I (and probably many folks) loosely call anything that is changing with time...."AC". Even the "Master" himself called it "AC with a DC component". Sorry for the lazy slip-up.
;)

Offline wattsup

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Re: Electrical Faux Pas
« Reply #23 on: November 20, 2007, 05:17:12 PM »
z_p_e

I removed my initial questions as you are right to keep this more generic.
« Last Edit: November 21, 2007, 01:43:08 AM by wattsup »

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Re: Electrical Faux Pas
« Reply #23 on: November 20, 2007, 05:17:12 PM »
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Offline z_p_e

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Re: Electrical Faux Pas
« Reply #24 on: November 21, 2007, 01:01:10 AM »
Wattsup,

Those are pretty specific questions really only pertaining to Otto and Roberto's work. I would encourage you to ask them, as they are more qualified to answer them.

We should try to keep the questions here as generic as possible I would think.

Cheers,
Darren

Offline wattsup

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Re: Electrical Faux Pas
« Reply #25 on: November 23, 2007, 12:32:53 AM »
@z_p_e

Well I'm back again and I will keep this one generic, hoping your help will be good for use elsewhere. This is about transistors.

I initially though transistors were miniature on/off devices (like relays) but it seems this is not the case at all. Am I correct to say that depending on the voltage applied to the gate as a percentage of the rated voltage of the transistor, the collector will conduct towards the emitter that percentage of the signal coming from the collector to the emitter. Would this be a correct assumption. Then why would you also call it an amplifier?

So if you wanted to use a transistor as a 100% on/off component, you would still need to pulse the gate 100% on/off, which would still require more control via a timer or oscillator to pulse the gate. Is this right.

My next question will obviously then be about oscillators.

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Re: Electrical Faux Pas
« Reply #25 on: November 23, 2007, 12:32:53 AM »
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Offline z_p_e

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Re: Electrical Faux Pas
« Reply #26 on: November 23, 2007, 02:52:30 AM »
Transistors come in a few forms:

BJT
JFET
MOSFET

There is a reason the British call triode tubes "valves", because in terms of electric current, that's exactly how they behave. I will try to be simplistic in the following description.

Think of the above 3-terminal devices as current valves. They all have a pin in which we supply with a voltage/current (the input end of our valve), a pin in which the voltage/current exits (the output end of our valve), and a pin that controls the amount of electron flow from the supply side to exit side (the control valve itself).

Think of the "gain" (amount of amplification) of one of these devices as the ability to control a potentially large amount of electron current (input end to output end) with a much smaller amount of current (or voltage) via the Base or Gate.

BJT's (bipolar junction transistors) are "current controlled current sources". In other words, a small change in current on the Base, will result in a much larger change of current through the device from end to end (Collector to Emitter). This is how they achieve gain or amplification.

JFETs and MOSFETs are "voltage controlled current sources". In other words, a small change in voltage on the Gate, will result in a much larger change in current through the device from end to end (Drain to Source). This too is a form of gain, even though the control and output units are different.

So you can see that all these 3-terminal devices are really current devices (even tubes). Everything is based on electron current flow from one end of the device to the other, and the amount is controlled by the "valve" terminal (Base or Gate), which can be a voltage or current control, depending on the device.

Voltage output is developed by circuit topology, and is generally due to current through a resistor.

In order for there to be control of the electron flow, the control input (Base current or Gate voltage) must be in reference to one of the device's other terminals, and that is via the Emitter (BJT) and Source (JFET/MOSFET).

Each device has a "turn-on" voltage, which means for a BJT, there must be about 0.7V between the Base and Emitter, and about 4V between the Gate and Source for MOSFETs (MOSFETS generally need more like 10V to fully turn them on, despite the 4V spec).

When the devices are fully "ON", they are considered to be in "saturation". What that means is the magnitude of current through the device has reached a maximum, and will not increase any further, even if the control input is increased.

By causing these devices to alternate between fully OFF and ON (cutoff and saturation), they are acting like a switch. If you placed a 20W light bulb in series with this "switch", you could then turn this 20W bulb ON and OFF by using only a few milliamps (BJT) of current to do so.

I hope that sheds some light on how BJT's and MOSFET's work.

Oscillators are a more complicated topic to describe, but post your question and we'll see what comes of it

Cheers.


Offline wattsup

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Re: Electrical Faux Pas
« Reply #27 on: November 26, 2007, 03:31:55 PM »
@z_p_e

Thanks for your last response as it has cleared up many questions. If ever you get tired of helping please feel comfortable to stop any time. I could go on and on, so again thanks for everything. Your one good explanation is worth hours of mulling through so much info and still getting lost.

1) Can an emitter of one transistor be fed to the base of another transistor?

2) Do zener diodes work like regular diodes but that they only let current pass when it has reached a certain voltage. If yes, is this like an on/off valve that opens when a minimal voltage is reached or do I have this backwards. Do they consume much power?

3) I have an oscillator with four pins on it. How do these work?

4) Toroidal ferrite cores come in a variety of sizes and values. Do the values indicate the amount of magnetic field they can emit and if not, please explain how we can understand ferrite specs.

z_p_e I know I have many question so please answer any you want and when you want. There is no rush.

Offline duff

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Re: Electrical Faux Pas
« Reply #28 on: November 26, 2007, 06:52:30 PM »
@wattsup

You may be interested in following series of Free books. They are well written from a technicians perspecitve and will probably answer most of your questions :

Lessons In Electric Circuits
A free series of textbooks on the subjects of electricity and electronics

http://www.ibiblio.org/obp/electricCircuits/


"Volume III - Semiconductors"  will answer most of your transistor related questions.


-Duff

Offline z_p_e

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Re: Electrical Faux Pas
« Reply #29 on: November 27, 2007, 02:40:16 AM »
@z_p_e

1) Can an emitter of one transistor be fed to the base of another transistor?

Generally speaking, yes. A good example utilizing this type of connection is called a "Darlington Pair". Was there something specific you were thinking of doing?

Quote
2) Do zener diodes work like regular diodes but that they only let current pass when it has reached a certain voltage. If yes, is this like an on/off valve that opens when a minimal voltage is reached or do I have this backwards. Do they consume much power?
All regular junction diodes are zener diodes. In other words, regular diodes also have a zener voltage, but zener diodes are manufactured to have a specific breakdown voltage. "Zenering" occurs when the diode is reversed-biased to a sufficient voltage to cause a breakdown of the p-n junction, when normally, no current would flow. Regular diodes aren't used for zeners normally, because their reverse breakdown voltage is usually in the thousands of volts. Once a diode is in a "zenering" condition, further increases to the applied reverse voltage does not appreciably increase the voltage across the diode, and this is why they are used in regulator circuits.The amount of power dissipated by the zener diode depends on the excess voltage applied that is over and above its zener voltage, times the current through the diode.

Quote
3) I have an oscillator with four pins on it. How do these work?
Not sure what you are referring to here. You'll have to be a lot more specific..i.e. what part number etc.

Quote
4) Toroidal ferrite cores come in a variety of sizes and values. Do the values indicate the amount of magnetic field they can emit and if not, please explain how we can understand ferrite specs.
Recommend you download and read this:
http://www.allegromicro.com/en/Products/Design/arnold/coretran.pdf
http://www.arnoldmagnetics.com/mtc/pdf/SoftMag.pdf (same as above)

There are folks here that are more knowledgeable than I on the subject of magnetics and cores, perhaps they can chime in and provide a hand.

In regards to your questions, no problem, keep them coming. I will help out when and where I can. Others, feel free to contribute as well. Thanks duff for the link.

Cheers,
Darren

 

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