Impedance is one of the most confusing aspects of electronics - I will de-mystify impedance by taking an extremely casual approach. I have known electronic enthusiasts who still couldn't even mentally visualise the concept of impedance even after 25 years

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ESSENTIAL IMPEDANCE BASICS


 

Authored by Ian C. Purdie VK2TIP

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What is impedance?

First comes the simple answer. If the demand exists we will provide a complex answer to impedance later.

Now here comes one of the most confusing aspects of electronics - which I will de-mystify by taking an extremely casual approach, so what's new!. I have known electronic enthusiasts who still couldn't even mentally visualise the concept even after 25 years.

I'll keep it dead simple, very inelegant but dead simple and give all the purists heart palpitations. I bet you walk away with a better understanding though.

If you need to know the technical answer for impedance and you should, then consult one of the must read texts I will have suggested elsewhere.

A simple example of impedance

Assume you have available these 4 items on your bench:

(a) A series of eight fresh AA type 1.5 volt cells to create a total of 12 volts supply.
(b) A 12 volt heavy duty automotive battery - fully charged.
(c) a small 12v bulb (globe) of very, very low wattage. and;
(d) a very high wattage automotive high-beam headlight.

Now if we connect the extremely low wattage bulb to the series string of AA cells we would expect all to work well. Similarly if we connect the high wattage, high-beam headlight to the heavy duty automotive battery all will be well. Well for a time anyway. Both of these sets are "sort"of matched together. Light duty to light duty and heavy duty to heavy duty.

Now what do you think would happen if we connect the high beam headlightto the series AA cells and conversely the low wattage bulb to the automotive battery?.

In the first case we could imagine the high beam headlight would quickly trash our little tiny AA cells. In the second case our min-wattage bulb would glow quite happily at its rated wattage for quite a long time. Why?, therein lies my expanation of impedance. Consider it!

The heavy duty battery is capable of delivering relatively large amounts of power but the series string is capable of delivering only relatively minimal power. The first is a low impedance sourceand the other, in comparison is a relatively high impedance source.

On the other hand the high beam headlight is capable of consuming relatively large amounts of power but the minature bulb is capable of consuming only minimal amounts of power.

High Impedance loads and Low Impedance loads

Again the first is a low impedance load and the other is high impedance load. If you're keen to apply ohms law you will discover why, research it through the text books.

Meanwhile take a well deserved coffee or tea break now and think it over. Me?, I'll just have another beer while I'm waiting for you.

Good break?. If you were paying attention you would now be able to understand an analogy - a particularly rough but effective one;

Imagine a tiny caterpiller chewing on a large blade of grass - no problem plenty to eat there. Now on the other hand imagine a poor cow stuck in a desert with only one similar blade of grass available to eat. I hope you have some better understanding now.

General expression of Impedance

The term impedance is a general expression which can be applied to any electrical entity which impedes the flow of current. Thus this expression could be used to denote a resistance, a pure reactance, or as is most likely in the real world, a complex combination of both reactance and resistance.

Don't get overly concerned if you're a bit confused by that statement at the moment. However this does then lead us on to "Q".

Footnote on Impedance - 10th April, 2002

A reader wrote to me and suggested:

Hi Ian,

Thanks for the great site. I find it absolutely invaluable. It's rare to find find such high quality, free information.

I finally got the concept of impedance matching through my head. Here is a point that you might add to you discussion on impedance and the lightbulb/battery analogy:

When mismatched, neither system will deliver its designed light output. The high current bulb will deliver very little light energy when powered by the low current batteries. The low current bulb will only deliver a fraction of the light energy that the high current battery is capable of deliving. But, match the sources to the loads, and both systems run at 100% efficiency!

BOOK - A Practical Introduction to Impedance Matching by Robert L. Thomas

 

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RELATED TOPICS ON IMPEDANCE

1. capacitance

2. inductance

3. "Q"

4. reactance

5. resistance

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Updated 15th May, 2000

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