Help me with circuit analysis

[Maverickmax]

Circuit Analysis

Hi

I have been given a circuit to connect my microcontroller without any explaination or how the circuit was devised like this way.

So I would be very grateful if you help me to analysis the circuit.

The PWM (2.5v) is generated to waveform at 50Hz but it is square waveform. I understand that two RC filters are included in this circuit to implement 500Hz low pass filter. The question is why two low pass filter is needed?

The output of circuit must be approximately 200mV and I understand that attenuation approach is used but I have not got a clue how to work it out.

How to calculate the voltage between two 33kohm

I look forward to hear from you asap

Maverick Max

 

[Miguel Gaspar]

Re: Circuit Analysis

This is a very complex analisys. There are two three ways, first cualitative, second cuantitative using laplace and tird simulation.

The first, during the transition low to high all capacitors present very low impedance, so i= 2.5/(33k+33k+10k) that will be decreasing acording time passes and capacitors will acquire charge.

Then the signal PWM goes to 0V and both capacitors gives energy to the source and the 10k Ohms resistor, so they are discharging.

The problem is t find the balance between charge and discharge. I recomend you use some circuit simulator to see how that will operate.

 

[Maverickmax]

Re: Circuit Analysis

F(Hz)=1/(2*pi*C*R) can be used to determine the frequency cut-off

Vout=(R2/(R2+R1))*Vin

That is all I know but I dunno what the best approach to calculate the component's value in order to get 200mV at the output

Maverick Max

 

[Miguel Gaspar]

Circuit Analysis

It is not so easy, becasue there are two capacitors and they are not in parallel so you can't get an equivalent
try laplace

 

[Maverickmax]

Re: Circuit Analysis

Well

The guy who designed the circuit in bloody 10 mins and gave the circuit. Then he left home without explaination! I would not able to see him for 4 weeks as he has gone for a long vacation!

I am trying to work it out but it is driving me bonker!!

Maverick Max

 

[Miguel Gaspar]

Circuit Analysis

please tell me how could I help you

 

[Kral]

Re: Circuit Analysis

Maverickmax,
The square wave consists of a funamental (50Hz), plus odd harmonics in the relative ratios of (2/Pi)*1, (2/Pi)*(1/3), (2/PI)*(1/5), etc. The purpose of the filter is to attenuate the harmonics without excessive attenuation of the fundamental. A single pole filter has a slope of 6dB/octave. An n pole filter has a slope of 6n dB/octave. So the more poles you have, the greater will be the harmonic attenuation compared to the fundamental attenuation. I don't have time to provide a complete analysis, but here is how you may procede to calculate the voltage at the junction of the two 33K Ohm resistors. Number the components R1, R2, R3, C1, C2 (left to right).Let
Z1 = R3||XC2.
Z2 = Z1 + R2
Z3 = Z2||XC1
Where
XC2 = capacitive reactance of C2 = 1/(2PI*f*22E-9), etc.
|| means "parallel combination of"
Then,
The voltage divider ratio from input to the junction of R1, R2 is given by
A = Z3/(Z3+R1)
You can now calculate the component of each harmonic of the input.
This is a lot of work. It would be a lot easier to use a circuit simulator such as SPICE.
Regards,
Kral

 

[Miguel Gaspar]

Re: Circuit Analysis

There is the wave form at output

 

[Maverickmax]

Re: Circuit Analysis

Hi

Can you please tell me what is //?

Here is another circuit which is known as ladder network.

R1, R2, R3, R4 & R5 (From left to right) represents 5ohm, 10ohm, 5ohm and 15ohm respectively.

Can you please tell me how to calculate the voltage point at red and green point?

Maverick Max

 

[Kral]

Re: Circuit Analysis

Maverickmax,
Regarding your 2nd circuit: Label the Horizontal Resistors R1, R2 (left to right). Label the "vertical" resistors R3, R4 (left to right).
.
Here's a trick that works well for ladder circuits:
.
Let the input voltage = Vin
Assume the output voltage is 1V.
Calculate the current (I4) thru R4 (assuming 1 Volt). This current is equal to the current thru R2. You can now calculate the voltage (V123) at the junction of R1, R2, R3. V123 =1 + (I4)(R2).
.
Calculate the current (I3) through R3 = V123/R3)
.
Calculate the current (I1) thru R1. I1 = I3 + I4.
.
Calculate the voltage at the input Vin' = V123 + (I)(1R1).
.
Scale the calculated voltages by the factor
(Vin)/(Vin').
.
For multi-stage ladder networks, you proceed right to left in the same iterative manner. For the 2-stage circuit shown, other standard methods, loop currents, etc would work as well. However, for multi-stage networks, the iterative method is much simpler.

Regards,
Kral

 

[Maverickmax]

Re: Circuit Analysis

Hi

Cheer for calculation. I have worked out the voltage between 33kohm and (Xc2//10kohm) for the 1st circuit above. According to my calculation, the voltage would be approximately 65mV.

Please correct me if Im mistaken


Thank you

Maverick Max

PS Assuming that Vin is 2.5V DC....

I almost forgot to mention the voltage is approximately 600mV between 1st 33kohm and 10kohm. Please check it out if there is any mistake

MM

 

[raybo]

Re: Circuit Analysis

we got impedances so now here is the skinny

ztau=z1z2/square root z1z2+z2square/4 =square rootz1z2/square root 1+z1/4z2 ohms you figure it out

 

[Miguel Gaspar]

Circuit Analysis

First form right to left 5+15 = 20

20 in parallel with 10 = 6.67

now you have a 2.5 source a 5 Ohms and a 6.67 Ohms

At red point you have V red = 2.5*6.67/(5+6.67) = 1.43

Now at point red you have a source and two resistors
V green = 1.43*15/(15+5) = 1.0725V

 

[Maverickmax]

Re: Circuit Analysis

Hi

I have been given a circuit to connect my microcontroller without any explaination or how the circuit was devised like this way.

So I would be very grateful if you help me to analysis the circuit.

The PWM (2.5v) is generated to waveform at 50Hz but it is square waveform. I understand that two RC filters are included in this circuit to implement 500Hz low pass filter. The question is why two low pass filter is needed?

The output of circuit must be approximately 200mV and I understand that attenuation approach is used but I have not got a clue how to work it out.

How to calculate the voltage between two 33kohm

I look forward to hear from you asap

Maverick Max

Hi again

As I mentioned the calculation for above the circuit:

I have worked out the voltage between 33kohm and (Xc2//10kohm) for the 1st circuit above. According to my calculation, the voltage would be approximately 65mV.

Please remember the input voltage would be 2.5VDC so can you please help to vertify that my calculation is right or wrong?

Is about 65mV at the output?

Maverick Max