Автор Тема: Импеданси и съгласуване на импедансите (impedance matching)  (Прочетена 8660 пъти)

Неактивен mzk

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Става въпрос за това: http://hyperphysics.phy-astr.gsu.edu/hbase/audio/imped.html

Съвпадане на импедансите на генератора и товара, при което се повишава ефективността.

Някой може ли да го обясни на чист български език, и ако може да даде пример с генератор (бобина) и товар (капацитивен, индуктивен) и как се правят сметките?

Благодаря предварително!
« Последна редакция: Март 03, 2009, 11:35:52 pm от altium »

Неактивен mzk

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Re: Съизмерване на импеданса (impedance matching) помощ
« Отговор #1 -: Март 03, 2009, 01:07:18 pm »
Импеданс, формулата е:

Където Z e импеданс в омове;
R е съпротивление в омове;
XL е индуктивното съпротивление в омове
XC е капацитивното съпротивление в омове.

XL=2*pi*f*L
XC=-1/(2*pi*f*C)

( http://en.wikipedia.org/wiki/Reactance_(electronics) )

pi - 22/7 (3,14)
f - честота
L - индуктивност
C - капацитет

Отношение на навивките на трансформатора:

N = брой навивки,
E = напрежение,
I = ток в ампери,
Z = импеданс в омове,
P = първична намотка,
S = вторична намотка.

Цитат
A transformer consists of two coils that are magnetically joined because they are close to one another and because they are wound around the same magnetic core. The input signal goes to the primary coil and the output signal is taken from the secondary coil. The transfer of voltage will be in proportion to the number of turns in the coil. If the secondary coil has fewer turns of wire than the primary coil, the ac voltage across the secondary coil will be lower than the voltage across the primary. A transformer can be used to step up ac voltage or to step down voltage. With appropriate impedance matching, all of the power going into the transformer is passed to the load. The output impedance of the transformer is stepped down or up along with the voltage. The transformer provides voltage amplification, not power amplification.

The term “impedance” refers to the total opposition to current flow in an ac circuit, and is measured in ohms. Using a transformer to match the impedances of a source and a load provides maximum power transfer between them. Examples of situations where this is significant include an audio amplifier and its speakers, a radio transmitter and its antenna, or a microphone and its console input. Microphones are generally classified as either “low impedance” (50 to 600 ohms) or “high impedance” (10,000 to 40,000 ohms). Professional microphones are in the low-impedance category.

Because of their relatively low voltage output level, microphones should be used in circuits that are compatible with their impedance. If this is not possible, an impedance-matching transformer should be used. Two examples are shown above. This helps to ensure that maximum power transfer will occur from the microphone into the preamplifier or amplifier to which it is connected. As can be read in the Shure commentary (above), low-impedance circuits are able to accommodate long microphone cable lengths without increasing noise or reduction in signal strength.

Източник: http://www.coutant.org/matching/5.html
http://www.coutant.org/matching/
« Последна редакция: Май 25, 2011, 03:10:34 pm от mzk »

mi68

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Re: Съизмерване на импеданса (impedance matching) помощ
« Отговор #2 -: Март 03, 2009, 01:49:26 pm »
Практическият смисъл на тази гимнастика е да се намали реактивната енергия и педпазване от изгаряне на генератора(веригата).Ф-лите са верни и елементарни.Някой път се практикува успешно резонанса тогава инпеданса се правръща в активно(омично)съпротивление.Реципрочното на инпеданс(проводимост)е имитанс ще го проверя как се казва.От ф-лата се вижда,че при голямо омично съпротивление се получава голямо преводно отношение на т-ра и голяма фазова разлика.
« Последна редакция: Март 03, 2009, 01:55:35 pm от mi68 »

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http://www.mitedu.freeserve.co.uk/Theory/inzoz.htm

Input Impedance
Meter Method
From the AC impedance triangle, the input or output impedance of a two terminal network can be determined by measuring the small signal AC currents and voltages. For the input, the voltage is measured across the input terminals and the current measured by inserting the meter in series with the signal generator. Use a fixed frequency say 1kHz and set the generator level to around 20 mV RMS. For example, if you read 20mV RMS and 10uA for current, then the impedance is 2k. With high impedance circuits, the current will become very small and difficult to measure, so an alternative method is called for.



An easy way to measure small input currents,is to use a fixed resistor, as in the diagram above. Measure the AC voltage at points V1 and V2, then the input current, Iin becomes: (V2 - V1) / R1. The input impedance of the circuit under test is then found from V1 / Iin.
Example:
If you use a 10k resistor for R1 and measure V2=10.1mv and V1=10mV then Iin=0.1mV/10k =10uA. The input impedance would then be 10mV / 10uA or 1K.



Output Impedance
Meter Method
Output impedance may also be determined using a similar technique. A fixed load resistor is used and the output voltage is measured first with full load, then without the load.



In the diagram above, Zo is the output impedance of the network to be measured. The term network is a general term, as the circuit could be an amplifier, filter oscillator, etc. The network is shown as a thevinin source. Firstly the load resistor, Rl is removed and output voltage measured (V). Next the load resistor,Rl is placed in the circuit and the voltage measured again (Vo). The voltage drop across the output impedance,Zo becomes V - Vo, the series current, Io becomes Vo / Rl, and the output impedance is (V-Vo) / Vo/Rl or rearranging Zo= Rl(V-Vo) / Vo.