Input Impedance Explained: Example 1-5 Analysis

[PainterGuy]

Hi,

Please have a see Example 1-5 on the given link:
http://img854.imageshack.us/img854/8376/inputimpedance.jpg

The example mentions input impedance. What is it in simple words? The book says: Because this input impedance is across the measured terminals , a small current flows flows through the multimeter...

What is book saying? Please help me with it. Thank you very much for this.

Cheers

 

[KingNothing]

It is saying that you have a resistance presented by the multimeter which acts like a resistor in parallel with R_L. This is called the input impedance of the multimeter, because it's the resistance across it's inputs.

If the input impedance of the multimeter is too low, say for example, it is the same as R_L, then the current will split equally between R_L and the multimeter, and it will significantly affect the circuit.

 

[Zryn]

When you connect a multimeter to a circuit, in theory the multimeter has zero effect on the circuit, but in reality a multimeter is just another circuit with some finite resistance.

Imagine if you have the circuit pictured, and you put another 6kR resistance across the first one, (i.e. you connect its terminals where you would connect the terminals of a multimeter to measure the voltage across the original 6kR resistance), the current is now split between the two resistances. What happens if you put a 12kR across the 6kR instead, or a 18kR? What happens when you increase the added resistance up to 6MR, or 600MR? In each case the effect of the added resistance on the circuit lessens, but it can never be zero, even if the effect is reduced to a negligible value which we can approximate as zero, as in the case of theory.

Input Impedance in this case is the term used to describe the value of the multimeter's resistance.

Impedance is a combination of Resistance (resistors) and Reactance (inductors and capacitors) and is a more accurate, broad and almost interchangeable term.

 

[vk6kro]

Impedance is a "catch-all" term that can be used when you don't know if there are inductors or capacitors present.

In the case of analog multimeters, this "impedance" is almost entirely resistive.

They are just saying that you should be aware that the "open circuit voltage" may be changed if the output actually has a multimeter across it.

If you had a real open circuit voltage of 9 volts and a Rth of 10 K then measuring with a multimeter with 200 K resistance will give a reading of 9 * ( 200 K / 210 K) or 8.57 volts.

Measuring with a 1 megohm digital multimeter would give a reading of 9 * ( 1000 K / 1010 K) or 8.9 volts, which is a lot better.

I imagine you know all this anyway.

 

[sophiecentaur]

For DC measurement, the input resistance is all that need bother you but, if you want to make AC measurements then the input Impedance (which will include the effect of the inevitable Capacitance) may be relevant.

 

[PainterGuy]

Many thanks, everyone. I undertand it now with your help.

Cheers

 

[Bob S]

My (older) meter is a Simpson model 260, with an input impedance of 20,000 ohms per volt. So the input resistance depends on the dc range setting (but not the voltage reading). So you would get different voltage readings depending on what voltage range setting you use.

Bob S

 

[Jiggy-Ninja]

My (older) meter is a Simpson model 260, with an input impedance of 20,000 ohms per volt. So the input resistance depends on the dc range setting (but not the voltage reading). So you would get different voltage readings depending on what voltage range setting you use.

Bob S

That's typical with analog meters. Digital meters will usually show 10-20Mohms no matter what the voltage is set at.

 

[sophiecentaur]

An analogue meter is, essentially, a sensitive microAmeter. If it has a Full Scale Deflection of 100uA then, to register 1V FSD, it will need 10kOhm total resistance in series. For higher current ranges you just put resistances in parallel.
Pre-solid state O level Physics, I love you!

 

[PainterGuy]

Hi Sophiecentaur,

Just asking this out of curiosity. You can skip it if you like. You lives the US and you mentioned O Level Physics which is very much UK. Are O Level books used there in the States? Thanks for the information.

Cheers

 

[sophiecentaur]

I am UK based and Schooled. O
Level died many years ago when Science had to become 'accessible' to all. In the UK , now, they tell students about Quarks before they even know what an Electron Volt is! To hell in a handcart, I'm afraid. As for O Level books - second hand bookshops or eBay would have to be where to look. Good luck.

 

[PainterGuy]

I am UK based and Schooled. O
Level died many years ago when Science had to become 'accessible' to all. In the UK , now, they tell students about Quarks before they even know what an Electron Volt is! To hell in a handcart, I'm afraid. As for O Level books - second hand bookshops or eBay would have to be where to look. Good luck.

Thank you for letting me know this. Though, O- and A-Level qualification is still very popular in many commonwealth countries such as Singapore.

Cheers

 

[sophiecentaur]

A level is alive and well in UK but about 6 months behind where it was.