# Simulate a circuit diagram of a step-down chopper using Multisim/MATLAB

• Engineering

Attached below.

## Relevant Equations:

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Using Multisim or any Electrical Engineering software:

I tried to simulate using Multisim, I got the output voltage.
How to find the waveform for the current ?

Could someone confirm my answer please? Any help is really appreciated .

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berkeman
Mentor
Sorry, how is "Figure 3" the circuit diagram of a step-down chopper? Did that come from some textbook? I'm not seeing any relationship to Figure 2 yet...

Sorry, how is "Figure 3" the circuit diagram of a step-down chopper? Did that come from some textbook? I'm not seeing any relationship to Figure 2 yet...
It's given by the instructor.

berkeman
Mentor
It's given by the instructor.
Then could you please first explain how the circuit in Figure 3 is an inplementation of the block diagram in Figure 2? What circuit components in Figure 3 are in each block in Figure 2?

Then could you please first explain how the circuit in Figure 3 is an inplementation of the block diagram in Figure 2? What circuit components in Figure 3 are in each block in Figure 2?
I don't know what is the relation between the two figures. So, I only concentrate on figure(3) to simulate.

Then could you please first explain how the circuit in Figure 3 is an inplementation of the block diagram in Figure 2? What circuit components in Figure 3 are in each block in Figure 2?
I think there is no relation between them, because figure(3) is also given in another experiment without figure(2). And the same question is repeated ( simulate figure(3) ).

berkeman
Mentor
So, I only concentrate on figure(3) to simulate.
Okay, so let's disregard the thread title (I can fix it up later), and ignore Figure 2. Then tell me what you think the circuit in Figure 3 is doing... What is the LM741 opamp being used as (hint -- it's not being used as an opamp in that circuit). And what is the output of this circuit (hint -- it's not where you are probing...).

What is the LM741 opamp being used as
It should amplify the weak signal .

And what is the output of this circuit (hint -- it's not where you are probing...)
I tried to connect the oscilloscope to the 1k resistor, on the right , but nothing appears.

berkeman
Mentor
Your screenshot shows the 'scope probe on the output of your 100V power supply that is connected to the collector of the transistor through a resistor. The "output" should be the collector of the transistor...

Can you show all of the node voltages in the circuit? What are you doing with the voltage source on the left? I think the original circuit showed a sawtooth waveform into that pin of the LM741.

BTW -- Did you say what that LM741 is being used as?

Your screenshot shows the 'scope probe on the output of your 100V power supply that is connected to the collector of the transistor through a resistor. The "output" should be the collector of the transistor...

Can you show all of the node voltages in the circuit? What are you doing with the voltage source on the left? I think the original circuit showed a sawtooth waveform into that pin of the LM741.

BTW -- Did you say what that LM741 is being used as?
I connected a sawtooth waveform to pin2 of LM741 .
LM741 is used to amplify the signal .
I connected the scope to the collector of the transistor directly , but the result is still the same.

DaveE
Gold Member
Here's the thing about computer simulations: They simulate exactly what you program them to do. You can draw your circuit and just run the simulator, but, in my experience, you will have about 10-20% chance of it working. You know; garbage in, garbage out.

People make mistakes. A key engineering skill, in any discipline, is to check your work. So once you have drawn the schematic, you should spend some time checking carefully to make sure it is what you intended. This practice will save you lots of time and aggravation. For example, look at every pin on your IC and verify that it is connected to what you want it to be connected to.

BvU and berkeman
berkeman
Mentor
I connected a sawtooth waveform to pin2 of LM741 .
LM741 is used to amplify the signal .
I connected the scope to the collector of the transistor directly , but the result is still the same.
Please learn to plot multiple waveforms on the same display at the same time, and to plot a length of time that shows multiple cycles of interest in your switching waveform. Thank you.

Show on one plot of multiple cycles:
• The + and - inputs to the opamp (which is being used as a _______ [not an "amplifier"])
• The output of the opamp which is mostly the same as the base of the transistor
• The drain of the transistor

Here's the thing about computer simulations: They simulate exactly what you program them to do. You can draw your circuit and just run the simulator, but, in my experience, you will have about 10-20% chance of it working. You know; garbage in, garbage out.

People make mistakes. A key engineering skill, in any discipline, is to check your work. So once you have drawn the schematic, you should spend some time checking carefully to make sure it is what you intended. This practice will save you lots of time and aggravation. For example, look at every pin on your IC and verify that it is connected to what you want it to be connected to.
I tried to modified the circuit and here's what I got:
Output voltage:

Output current :

DaveE
Gold Member
No. I don't know what your new circuit is (I suppose the corrected version shown before), and your plots have no information about the waveform. 1V, 100V, 1mV? However, I will say this: it appears that your circuit is making pulses. The circuit you showed before (with some minor corrections) will also make voltage pulses.

Anyway, I'm glad your making progress, but don't you think you can check it yourself? What is this circuit supposed to do? Suppose the transistor was off what would the displayed voltage be? What about when it's on?When should it be on? When should it be off? Frequency? Duty cycle?

I tried to modified the circuit and here's what I got:
Output voltage:
View attachment 262156
Output current :
View attachment 262157

I tried to simulate the circuit again: