What's the input impedance of battery?

[bittware]

What's the input impedance of battery? or what's the input impedance of an output port of an MCU? or what's the input impedance of a switching power supply's output port?

As everybody knows, the output of a voltage source ideally has infinitely small output impedance, but what's the magnitude of its input impedance looking back to its output terminals? How can I set up test to measure its input impedance?

Anyone could explain?

Thanks in advance.

 

[laktronics]

Hi,
The input impedance of a battery is its internal resistance, however small it may be. The input impedance of the out put pin of an MCU has two values, one each for high level out put and low level output. Normally for high level, it is its pull up resistance to Vcc. For low level, it is the onswitch resistance to ground. Similar resiatance/impedance you can find out in case of switching regulator also. In short, the input inpedance of a voltage source is its output impedance and is a measure of how much its output will drop when you draw a current from its output.

The input impedance is measured by measuring the open circuit output voltage and the output voltage when a current is sourced from or sunk by the output. The change in output voltage Δv÷Δi is the internal impedance of the source. In effect it is a measure of the load regulation of the voltage source.

Regards,
Laktronics

 

[~E3016~]

Here is the picture

**broken link removed**

 

[FvM]

I hear a strange try of defining a difference between input and ouput impedance. Reasonably the other contributors ignored this confusing point, but I wonder what may be meaned at all?

A battery impedance may be also measured as a frequency dependant property by impedance spectroscopy, determining the parameters for more complex equivalent circuits.

 

[bittware]

Hi,
The input impedance of a battery is its internal resistance, however small it may be. The input impedance of the out put pin of an MCU has two values, one each for high level out put and low level output. Normally for high level, it is its pull up resistance to Vcc. For low level, it is the onswitch resistance to ground. Similar resiatance/impedance you can find out in case of switching regulator also. In short, the input inpedance of a voltage source is its output impedance and is a measure of how much its output will drop when you draw a current from its output.

The input impedance is measured by measuring the open circuit output voltage and the output voltage when a current is sourced from or sunk by the output. The change in output voltage Δv÷Δi is the internal impedance of the source. In effect it is a measure of the load regulation of the voltage source.

Regards,
Laktronics

I think the load regulation is more like output impedance. What really bothered me is what's difference between input and output impedance when talking about voltage source alike output port.

 

[FvM]

Can you give any indication why there should be a difference between input- and output impedance. It seems to me as splitting hairs.

 

[IanP]

as far as a battery is concerned, there is no such a thing like input [or output] impedance ..
one can only talk about it’s internal resistance [impedance] ..
https://data.energizer.com/PDFs/BatteryIR.pdf

If you intend to measure the internal resistance of a battery follow this:

A very simple measurement can be done by loading the cell with a resistor R1 and measure the viltage U1. Then use another resistor R2 and measure U2. Chose the resistors so that the resulting current is about half the normal current in operation and about twice that current. That will give you a representative value.

Then solve the two equations U1 = E - Ri*U1/R1 and U2 = E - Ri*U2/R2, where E is cell EMF and Ri is cell internal resistance, for Ri.

If you are going to use the cells in a more dynamic situation, it may be better to measure the impedance, Zi. You do that by injecting an AC current (use a blocking capacitor for DC) and measure the resulting AC voltage and current. Calculate Zi from U and I. Select frequency to suit the application. Remember that a high HF impedance usually can be improved by putting a capacitor parallel to the cell.

rgds,
ianp

 

[bittware]

I hear a strange try of defining a difference between input and ouput impedance. Reasonably the other contributors ignored this confusing point, but I wonder what may be meaned at all?

A battery impedance may be also measured as a frequency dependant property by impedance spectroscopy, determining the parameters for more complex equivalent circuits.

I've found I asked a silly question. Yes, battery only has output (internal) impedance, because it is 1-port network.
My original intention is to ask how to figure out what the input impedance of a switching regulator is when looking into output port.
Can you give me some hints?

 

[FvM]

I see, that I misunderstood your intention. Regarding the terms input or output in connection with impedances: In my understanding it's only a simple way to distinguish which port of a two-port device is meant.

If I analyze a switching regulator as a two-port, then I would use the term input impedance for the input port (power input) and output impedance for output port. Both ports have an impedance depending on internal structure and parameters and (generally) the connected load/source at the other port.

Impedance is a small-signal parameter, valid for a given operating point only. It's generally a komplexe frequency dependant value. The above discussion of the battery example was mainly referring to DC impedance respectively internal resistance, which is the most important parameter of a battery.

For a switching regulator, the AC impedance is becoming more important. It's determined to an important part by the control loop, but also based on structural parameters as the output filter dimensioning. Analyzing a switching regulators output impedance is important to check, if stability with a certain load can be achieved. When designing a switching regulator and it's control loop, the intended output impedance is one of several important design parameters.

 

[bittware]

I see, that I misunderstood your intention. Regarding the terms input or output in connection with impedances: In my understanding it's only a simple way to distinguish which port of a two-port device is meant.

If I analyze a switching regulator as a two-port, then I would use the term input impedance for the input port (power input) and output impedance for output port. Both ports have an impedance depending on internal structure and parameters and (generally) the connected load/source at the other port.

Impedance is a small-signal parameter, valid for a given operating point only. It's generally a komplexe frequency dependant value. The above discussion of the battery example was mainly referring to DC impedance respectively internal resistance, which is the most important parameter of a battery.

For a switching regulator, the AC impedance is becoming more important. It's determined to an important part by the control loop, but also based on structural parameters as the output filter dimensioning. Analyzing a switching regulators output impedance is important to check, if stability with a certain load can be achieved. When designing a switching regulator and it's control loop, the intended output impedance is one of several important design parameters.

But if the switching power supply output is subject to external interference, say EMI, its output should be deemed as input instead cause noise may sneek into its internal loop through this port. In which case, I need to design an EMI filter to block external noise. But the proper EMI filter design highly depends on filter's source impedance (here is the noise source) and load impedance (here is the power supply's output port's input impedance which may sound a little weird). That's is what I really care about.
So could you show your idea on this?
Thanks a lot!

bittware

 

[FvM]

I don't see a particular problem from the EMI question. The terms input and output can be used to indicate a signal or energy flow, but they didn't change the character of impedances itself. The SMPS output impedance would be determined by the output C and it's ESR (and possible assembled high frequency bypass capacitors) in the EMI frequency region of interest. It's an important parameter in designing a filter. But the impedance value (komplexe and frequency dependant) is the same that is seen as output impedance by a load.

 

[bittware]

I don't see a particular problem from the EMI question. The terms input and output can be used to indicate a signal or energy flow, but they didn't change the character of impedances itself. The SMPS output impedance would be determined by the output C and it's ESR (and possible assembled high frequency bypass capacitors) in the EMI frequency region of interest. It's an important parameter in designing a filter. But the impedance value (komplexe and frequency dependant) is the same that is seen as output impedance by a load.

Say an RC filter in between power supply output and load, can you state that its impedance is same from both power side and load side?

 

[pjfs]

The input and output impedance for a battery are equal. The thevenin equivalent is an ideal voltage source in series with the internal Resistance denoted as Ri. This is for DC, for AC parallel capacitance and series inductance may be added to get a more accurate model.