Two circuits combination issues

Thread Starter

circuitnaiver

Joined Oct 1, 2021
22
Hi, All,

I am not very good at circuit design. I have the following circuit.

The green dashed area is for changing the duty cycle. The yellow part is for increase the voltage amplitude. And the red part is the output load.

I have tested the partial circuit in the green and yellow area seprately. They worked well as expected. But when I connect them together as shown in the figure, it stops working and errors are detected in the simulation. I do not really know why it does not work. Can anyone help me out with it?

Thank you!!

1633678475893.png
 

ericgibbs

Joined Jan 29, 2010
14,145
hi,
This is your circuit in LTSpice, which part does not work in your Sim and what errors are reported.?

E
Check my circuit for accuracy.

EG 883.pngEG 884.pngEG 884.png
 

AnalogKid

Joined Aug 1, 2013
9,480
Rather than dive into 20 Questions, just tell us: what is the intent of the circuit? What is it supposed to do? It appears to take a +/-12 V pulse and amplify it into a +20 V pulse. If that is the intent, then the schematic needs a little work.

There is nothing in the green circuit that will vary the 555 output duty cycle, because R3 and C3 are fixed components. If the idea is to reduce any input pulse width to a fixed, narrower pulse width, then the monostable circuit can do this.

Also, powering a 555 through a 2 K resistor will not work in the real world.

As eric indicated, a 741 cannot run on +/-30 V supplies. They exceed the part's maximum ratings.

What is the purpose of D2?

ak
 
Last edited:

crutschow

Joined Mar 14, 2008
28,170
Always look at the data sheet limits before you do a design.
Simulator models do not have those limits (voltage, current, power, etc.) and thus ignore them in the simulation.
 

Thread Starter

circuitnaiver

Joined Oct 1, 2021
22

Thread Starter

circuitnaiver

Joined Oct 1, 2021
22
Rather than dive into 20 Questions, just tell us: what is the intent of the circuit? What is it supposed to do? It appears to take a +/-12 V pulse and amplify it into a +20 V pulse. If that is the intent, then the schematic needs a little work.

There is nothing in the green circuit that will vary the 555 output duty cycle, because R3 and C3 are fixed components. If the idea is to reduce any input pulse width to a fixed, narrower pulse width, then the monostable circuit can do this.

Also, powering a 555 through a 2 K resistor will not work in the real world.

As eric indicated, a 741 cannot run on +/-30 V supplies. They exceed the part's maximum ratings.

What is the purpose of D2?

ak
Hi, this design is going to make the circuit with the +-12V square wave input can output 6V, -12V square wave after R2 (with the original duty cycle), and 6V , -12V (or 0V if the negative output cannot be made) at the output of the op amp.

"There is nothing in the green circuit that will vary the 555 output duty cycle, because R3 and C3 are fixed components. If the idea is to reduce any input pulse width to a fixed, narrower pulse width, then the monostable circuit can do this.'
---- YES, it is for narrowing pulse width.

"Also, powering a 555 through a 2 K resistor will not work in the real world."
---- I was trying to directly output a 6V, 0V square wave at the output of the 555 timer. But it seems like not working as expected. The output amplitude is too low in the physical circuit testing.

"As eric indicated, a 741 cannot run on +/-30 V supplies. They exceed the part's maximum ratings."
---- Yes, I will fix it.

" What is the purpose of D2? "
---- D2 is used for making the lower level on the square wave become 0V.

Thank you.
 
Last edited:

Thread Starter

circuitnaiver

Joined Oct 1, 2021
22
This is the design purpose. It is going to make the circuit with the +-12V square wave input can output 6V, -12V square wave after R2 (with the original duty cycle), and 6V , -12V (or 0V if the negative output cannot be made) at the output of the op amp.
1633714857430.png
 

ericgibbs

Joined Jan 29, 2010
14,145
hi c,
Looking at my posted circuit, try this small change.
Remove D2 diode, replace with a link wire.
Add a 47k from pin #3 of the 555 to 0vCommon.
Reduces the +/-30Vdc to +/-15Vdc.

Run the sim , report the result.
I will post the LTSpice asc file, later.

E
 

Thread Starter

circuitnaiver

Joined Oct 1, 2021
22
hi c,
Looking at my posted circuit, try this small change.
Remove D2 diode, replace with a link wire.
Add a 47k from pin #3 of the 555 to 0vCommon.
Reduces the +/-30Vdc to +/-15Vdc.

Run the sim , report the result.
I will post the LTSpice asc file, later.

E
Hi, I tried as you said. My simulation starts working. It seems like you make the op amp circuit in parallel with the 47k ohm Resistor. What is the purpose of doing this?

Besides, my simulation result is different from the simulation you did. Is it because 555 timer I used is different? Is NE555 and LM555 different?
 

Thread Starter

circuitnaiver

Joined Oct 1, 2021
22
hj
The 47k provides a DC path for the NI input of the OPA.
E
Hi, eric, I want to make sure I did want you said. Can you check the figure? I connect output pin of the 555 timer to a 47k resistor and then to the ground.

" The 47k provides a DC path for the NI input of the OPA. "
---- I am still not very clear about this sentence. Could you please explain a little bit more? Thank you.

1633719284900.png
 

AnalogKid

Joined Aug 1, 2013
9,480
" The 47k provides a DC path for the NI input of the OPA. "
---- I am still not very clear about this sentence. Could you please explain a little bit more?
In a standard, non-inverting opamp linear amplifier circuit, there must be a DC reference potential at the non-inverting (+) input. This, along with other circuit parameters, established the output DC operating point. Negative feedback drives the inverting (-) input such that it equals the non-inverting input.

ak
 

ericgibbs

Joined Jan 29, 2010
14,145
hi circuit.
When using LTSpice with your original sim circuit, I got convergence errors and LTS stopped, 'Sometimes .???'

I suspect in a practical build of the circuit the 555 and 741 would work OK.

It could be a model problem in the 555 output design or the 741 input design.

Adding the 47k DC path solved the simulation problem and has no effect on the actual circuit operation.

"The 47k provides a DC path for the NI input of the OPA. " Post #13, covers this point.

E
 
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