Differences between simulation and reality - LM723 design

Thread Starter

mike_the_begginer

Joined Dec 7, 2019
101
Hello. I made the attached schematic, and in the simulation, it is oscillating, but when testing the practical circuit, with the oscilloscope, it doesn't look to oscillate (the scope is a Owon SDS 1022).
The schematic: 723_schematic.jpg
Please find attached the simulation in LTSpice and the libraries.

I would like to know what should I do in order to find where is the problem, and why there is a difference between the simulation and reality ?

Edit: I closed the LTspice and opened it again after a few minutes, and now the simulation works correctly, with no oscillation.
I still have a question: how many W does D3 need to dissipate, in worst case scenario ? It is enough to use a 0.5W zener diode for D3 ?
 

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crutschow

Joined Mar 14, 2008
27,243
Your circuit may be marginal.

The difference between simulation and the real circuit may be the added small stray node inductances and capacitances of the real circuit.

What's the purpose of all the added circuitry at the output?
 
Last edited:

Thread Starter

mike_the_begginer

Joined Dec 7, 2019
101
How did you implement V2, an ideal voltage source that controls S1, in the real circuit?
V2 is a 0-25V, 0-3A bench power supply which is set at 12V and it is powering a switch load.
This is the schematic of the switch load:
switch load.png



Your circuit may be marginal.

The difference between simulation and the real circuit may be the added small stray node inductances and capacitances of the real circuit.

What's the purpose of all the added circuitry at the output?
I tested the circuit without C7, and C10 was 1nF instead of 10nF as shown in the schematic. Also, I used an TIP35C instead of TIP3055. I used TIP3055 in my simulation because I didn't found TIP35C in my library.
I also tested the schematic as shown in the original message, but using TIP35C instead of TIP3055.
In both cases, I tested the circuit with the switch load, in short circuit and by using different values of power resistors (5W each) in the switch load (1R, 10R, 47R, 100R). When I tested with power resistors, I used the following schematic for the switch load:

switch load resistors.png


I verified again in the simulation, but now the circuit seems to be stable, no more oscillations:

output of the circuit_vmax.jpg

output of the circuit_vmin.jpg
 

Papabravo

Joined Feb 24, 2006
16,188
V2 is a 0-25V, 0-3A bench power supply which is set at 12V and it is powering a switch load.
This is the schematic of the switch load:
View attachment 240376





I tested the circuit without C7, and C10 was 1nF instead of 10nF as shown in the schematic. Also, I used an TIP35C instead of TIP3055. I used TIP3055 in my simulation because I didn't found TIP35C in my library.
I also tested the schematic as shown in the original message, but using TIP35C instead of TIP3055.
In both cases, I tested the circuit with the switch load, in short circuit and by using different values of power resistors (5W each) in the switch load (1R, 10R, 47R, 100R). When I tested with power resistors, I used the following schematic for the switch load:

View attachment 240378


I verified again in the simulation, but now the circuit seems to be stable, no more oscillations:

View attachment 240379

View attachment 240380
I could not make out any details in the picture posted, but I thought there was a pulse statement near the voltage source. We sometimes substitute those for things which are supposed to oscillate. You can hardly say it is "set at 12V" with the pulse statement sitting there.
 

Thread Starter

mike_the_begginer

Joined Dec 7, 2019
101
The power supply is set at 12V, and the pulse statement is represented by the switch load. Probably I am not explaining enough, so please ask if you have any difficulty in understanding what I am trying to say.
 
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