Equivalent Resistance Calculation

Thread Starter

Peter523

Joined Aug 15, 2021
91
Hello,
I constructed in Pspice this boost converter :
spice_boost_resis.png
Could anyone advise me how to calculate the equivalnt resistance of this circuit?
 

Papabravo

Joined Feb 24, 2006
16,938
Ohm's Law establishes a relationship between three quantities. If we know the values for two of them, the third one is determined.
 

crutschow

Joined Mar 14, 2008
27,891
Could anyone advise me how to calculate the equivalnt resistance of this circuit?
You change the value of the load slightly and observe the change in voltage.
The equivalent output resistance is then the change in voltage divided by the change in output current.
 

Audioguru again

Joined Oct 21, 2019
3,632
My cheap solar garden lights use that circuit to boost the 0.8V to 1.4V from a single Ni-MH battery cell to 3.5V to light a white or colors changing LED.
 

Thread Starter

Peter523

Joined Aug 15, 2021
91
Ohm's Law establishes a relationship between three quantities. If we know the values for two of them, the third one is determined.
I tried this. According to Ohms Law R=V/I. Firsty, i put on input 8.4V (on 75u inductor) and the input current was 6.3A meaning that resistance is 1.33 Ohms. When i put as input 6V the current was 9A meaning that resistance is 0.66. So i think that something is wrong.
 

Thread Starter

Peter523

Joined Aug 15, 2021
91
You change the value of the load slightly and observe the change in voltage.
The equivalent output resistance is then the change in voltage divided by the change in output current.
I would like to calculate the resistance of the whole circuit. According to this method and changing load from 4 to 3 Ohms resistance was calculated to 0.12 Ohms that seems strange.
 

DickCappels

Joined Aug 21, 2008
7,836
Your circuit is not a resistor. For example, with 0.1 volts in no current is drawn so the resistance appears to be infinite. You need to choose part of the operating range over which you perform crutschow’s calculation in post #7 to find the circuit’s dynamic resistance.
 

Thread Starter

Peter523

Joined Aug 15, 2021
91
Your circuit is not a resistor. For example, with 0.1 volts in no current is drawn so the resistance appears to be infinite. You need to choose part of the operating range over which you perform crutschow’s calculation in post #7 to find the circuit’s dynamic resistance.
Yes, but in both cases that i considered to perform crutschow's calculation, my circuit transforms the input voltage to standard 12V output. So i mean the circuit is on same operating range.
 

Papabravo

Joined Feb 24, 2006
16,938
I tried this. According to Ohms Law R=V/I. Firsty, i put on input 8.4V (on 75u inductor) and the input current was 6.3A meaning that resistance is 1.33 Ohms. When i put as input 6V the current was 9A meaning that resistance is 0.66. So i think that something is wrong.
You're going about this ALL wrong.
The input voltage fron the voltage source, V2, is a constant 7.2V
The RMS current, I(V2), delivered by V2( a 7.2V source) is 5.3232 Amperes (RMS), as shown by the measurement box in @Danko 's simulation. Those are the numbers you should be considering.

One more thing: RMS stands for Root Mean Square, and it is how to compute the average of an AC waveform for the purpose of computing power and other things. You might be well advised to brush up on this concept.
 

Polymorph

Joined Feb 6, 2009
6
Resistance only has meaning for a linear load. Or over a very narrow, specified range.
If you want to know what resistor would load the power source as much as this circuit, It is as simple as knowing the efficiency and the output power. That tells you the total power drawn. Then with the power supply voltage, calculate the current draw. Resistance is the ratio of voltage to current.
 

dcbingaman

Joined Jun 30, 2021
442
Resistance is defined as V/I. If this value is not constant as V changes then the circuit is non-linear and to talk about it's 'resistance' is meaningless. Unless you are talking about dynamic resistance which is taking dV/dI at a given operating point. That would be the limit of the ratio approached as delta V approaches zero.
 

Papabravo

Joined Feb 24, 2006
16,938
Resistance only has meaning for a linear load. Or over a very narrow, specified range.
If you want to know what resistor would load the power source as much as this circuit, It is as simple as knowing the efficiency and the output power. That tells you the total power drawn. Then with the power supply voltage, calculate the current draw. Resistance is the ratio of voltage to current.
The circuit has effectively been linearized about an operating point, and the TS asked for an "equivalent" resistance, which seems like a reasonable request. He is just having some difficulty with the volume of data being thrown at him.
 

Thread Starter

Peter523

Joined Aug 15, 2021
91
So, according to what @Papabravo and @Danko said the equivalent resistance is approximately 7.2/5.33=1.35 Ohms, right? But why when i give a different input voltage the resistance calculated is different?
 
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