# How does this circuit work?

Thread Starter

#### newguy193

Joined Jun 26, 2021
14

It's my final project. I need to provide information of its working briefly and its equations. I'm seeing similarity to astable multi vibrator but I can't say how it works. Your help is appreciated

#### RBR1317

Joined Nov 13, 2010
714
What if instead of SW1 & Q3 there was an SW3 switching the emitter of Q1 to GND? Could you analyze that?

#### ericgibbs

Joined Jan 29, 2010
18,835
hi 193,
Welcome to AAC.
Hint: What is the purpose the two 'isolating' diodes?
E

Thread Starter

#### newguy193

Joined Jun 26, 2021
14
hi 193,
Welcome to AAC.
Hint: What is the purpose the two 'isolating' diodes?
E
Hi
I got no idea. Maybe to increase Vce for 0.6 volts(Von) for each cycle? or maybe providing voltage gap? idk

Thread Starter

#### newguy193

Joined Jun 26, 2021
14
What if instead of SW1 & Q3 there was an SW3 switching the emitter of Q1 to GND? Could you analyze that?
No idea. I don't know why is that switch there. I have the proteus simulation file for it, want me to put it here?
Its got an error though, I get zero output signal of it and I don't know why.

#### Audioguru again

Joined Oct 21, 2019
6,688
It is a very simple multivibrator but its diodes are in the wrong places.
A real circuit has noise that starts it. A simulator has no noise so you must add a startup pulse to get it oscillating.

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#### ericgibbs

Joined Jan 29, 2010
18,835
It's my final project. I need to provide information of its working briefly and its equations. I'm seeing similarity to astable multi vibrator but I can't say how it works. Your help is appreciated
hi,
The TS's College assignment is to describe how his posted Astable circuit works, not how to redesign an alternative astable circuit.

hi 193,
Please post your best effort at explaining your posted circuit.
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#### Audioguru again

Joined Oct 21, 2019
6,688

The teacher will give me extra points for pointing out the circuit errors.
The student's simulation probably has both multivibrator transistors turning on at the same time instead of alternating.

#### ericgibbs

Joined Jan 29, 2010
18,835
More likely to fail you for not answering the question.
If I was the examiner I certainly would give you a 0/10 score.

#### Irving

Joined Jan 30, 2016
3,875
It is a very simple multivibrator but its diodes are in the wrong places.
Really? Where would you put them then?

#### Irving

Joined Jan 30, 2016
3,875
It's my final project. I need to provide information of its working briefly and its equations. I'm seeing similarity to astable multi vibrator but I can't say how it works. Your help is appreciated
Forget about simulating it for now... Describe how an astable multivibrator works and then consider how the extra components impact on that...

#### Irving

Joined Jan 30, 2016
3,875
But the different positions shown in article 1 fig 2 & 3 serve a very different purpose to the TS' example (also shown Fig 5 of article 1).

#### ericgibbs

Joined Jan 29, 2010
18,835
hi 193,
Would the circuit run if the Q3 transistor was removed from the circuit,?

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#### Audioguru again

Joined Oct 21, 2019
6,688
Of course the circuit will not work if the switch does not turn on Q3 or if Q3 is not replaced with a wire.
Why doesn't the switch simply turn on the power supply +12V to the circuit instead of using Q3?

I showed and the first article posted by Bertus showed correct positions for the diodes so that the transistors are not destroyed by too high emitter-base voltage. The datasheets say Absolute Maximum Veb= 5V or 6V.

#### ericgibbs

Joined Jan 29, 2010
18,835
Of course the circuit will not work if the switch does not turn on Q3 or if Q3 is not replaced with a wire.
Why doesn't the switch simply turn on the power supply +12V to the circuit instead of using Q3?
hi agu,
It is a circuit configuration that intended to make the student have to figure out what are the function of the individual components have in the circuit, it is not necessarily a practical design.

You are over thinking the assignment.

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Thread Starter

#### newguy193

Joined Jun 26, 2021
14
hi 193,
Would the circuit run if the Q3 transistor was removed from the circuit,?

E
View attachment 242275
As the other member have already mentioned it won't work because the current path won't be provided for Q1. Yet, why Q3 and not a short circuit to ground?

I understand that you are trying to make me figure this out by giving hints instead of giving answers but I'd appreciate providing answer as tomorrow is the last day to deadline.

#### ericgibbs

Joined Jan 29, 2010
18,835
hi,
Which type of switch is shown in the circuit.?
E

Update:
The type label should tell you, what type of switch is being used.

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