How this ps unit works??

[samy555]

The circuit in the following figure
(from: **broken link removed**
circuits.com/circuits/power-supply/dc-power-supply/891/variable-dc-power-supply) represents a variable power supply
Transistor Q1 is npn type
**broken link removed**
This was corrected in the following figure
**broken link removed**
I have thought carefully about this circuit, but I could not get to the mechanism of how it work!
I think that when the transistor Q1 operates, it must turn off Q2.
I try to simulate it using multisim10, but it didn't work, that is when I change the value of any of P1 or P2 the output is fixed at near the retectified voltage.
Any help in clarifying the way this circuit work would be welcome, thanks

 

[ronsimpson]

The bottom circuit is correct.
You are correct that Q1 turns off Q2 and Q4.

This is not a great power supply. It is not temperature stable. It should work but like many internet circuits there are no values.

 

[samy555]

Sorry, there are valuse
•P1 = 500R
•P2 = 10K
•R1 = 2.2K-1/2w
•R2 = 2.2K-1/2w
•R3 = 330R
•R4 = 150R
•R5 = 1R-5W
•C1 = 35V-3300uF
•D1 = 1N5402
•D2 = 1N5402
•D3 = 5mm Red Led
•C2 = 63V-1uF
•Q1 = BC182
•Q2 = BD139
•Q3 = BC212
•Q4 = 2N3055
•SW1 = SPST Mains Switch
•T1 = 36VCT-Transformer

http://www.hobby-circuits.com/circuits/power-supply/dc-power-supply/891/variable-dc-power-supply

Can you explain to me how it works??

 

[ronsimpson]

R2 turns on Q2.
...Q2 turns on Q4.
.....Q4 pull up the output voltage.
P2 divides the output voltage. If P2 is set to divide by 10 then when the output voltage reaches 6 to 7 volts the base of Q1 is at 0.6 volts. This turns on Q1 and pulls the base of Q2 down, turning off Q2 and Q4.

Current limit: When the voltage across R5 is too large, Q3=on causing Q1=on and turning off Q2 and Q4.

 

[samy555]

R2 turns on Q2.
...Q2 turns on Q4.
.....Q4 pull up the output voltage.
P2 divides the output voltage. If P2 is set to divide by 10 then when the output voltage reaches 6 to 7 volts the base of Q1 is at 0.6 volts. This turns on Q1 and pulls the base of Q2 down, turning off Q2 and Q4.

Current limit: When the voltage across R5 is too large, Q3=on causing Q1=on and turning off Q2 and Q4.

Can I understand from your words that Q1 is only making Q2 ON or OFF???

 

[audioguru]

Q1 is not a switch, it is linear.
Q1 always conducts a little so that Q2 and Q4 also conduct a little.

When the output voltage is a little too high then Q1 turns on a little more which turns Q2 and Q4 off a little so that the output voltage is reduced down to normal.

When the output voltage is a little too low then Q1 turns off a little which allows Q2 and Q4 to turn on a little more so that the output voltage is increased up to normal.

It is called Negative Feedback.

 

[samy555]

Q1 is not a switch, it is linear.
Q1 always conducts a little so that Q2 and Q4 also conduct a little.

When the output voltage is a little too high then Q1 turns on a little more which turns Q2 and Q4 off a little so that the output voltage is reduced down to normal.

When the output voltage is a little too low then Q1 turns off a little which allows Q2 and Q4 to turn on a little more so that the output voltage is increased up to normal.

It is called Negative Feedback.

Thank you guru

But why can not that transistor be a switch?
I know that when a voltage of 0.7 volts is applied between the base and the emitter of Q1, the resistance between the emitter and the collector becomes very small
Therefore, the base of transistor Q2 connected ground, and thus becomes OFF??????

 

[ChrisP58]

The transistors can be saturated switches, or non-saturated linear elements. It all depends on how they are biased.

In this circuit, under normal operating conditions, they are biased to work in linear mode. This means that they are all hovering on the edge of being turned on. *

This is why the output voltage can be regulated. If the transistors were used as switches, the output voltage would be either zero, or a little less than the voltage at C1.


* Q3 is biased off unless the circuit is in current limit mode.

 

[unclejed613]

The bottom circuit is correct.
You are correct that Q1 turns off Q2 and Q4.

This is not a great power supply. It is not temperature stable. It should work but like many internet circuits there are no values.

it's not temperature stable because the B-E junction of Q1 is the voltage reference. this adds a -2.2mV/°C temperature coefficient to the reference voltage, which, multiplied by the divider ratio of P2, becomes a much larger voltage drift.

 

[samy555]

The transistors can be saturated switches, or non-saturated linear elements. It all depends on how they are biased.

In this circuit, under normal operating conditions, they are biased to work in linear mode. This means that they are all hovering on the edge of being turned on. *

This is why the output voltage can be regulated. If the transistors were used as switches, the output voltage would be either zero, or a little less than the voltage at C1.


* Q3 is biased off unless the circuit is in current limit mode.

That was a very good point
can you please tell me how to distinguish between them (switch and linear)
If you gave me examples I would really appreciate you
thank you

 

[unclejed613]

shown in the pic is linear and switch operation of a transistor. in linear operation the output is somewhere between the rail voltage and 0V. for a given change in input voltage, there is a corresponding proportional change in the output voltage. the transistor acts as a variable resistance. the second circuit shows switch operation. the input voltage is a pulse, and the transistor output is rail voltage or zero. the transistor is either completely on (output at 0V) or completely off (output at rail voltage). there's no "in between".

 

[ChrisP58]

To use a bipolar transistor as a switch, you drive the base with much more current than is needed for whatever the collector current you are controlling.

For linear use, you control the base current to be just enough to make the collector current what you want it to be. You generally use feedback to dynamically adjust the base current as necessary for whatever the circuit is supposed to do.

In this circuit, we are trying to control the output voltage. The base current for Q4, the output transistor, comes from Q2, whose base current is supplied by R2. Q1 is setup as the error amplifier. It's base current is set by a voltage divider formed by the two haves of P2. If the output voltage is too high, more current is feed into the base of Q1, and it's collector steals current away from the base of Q2, controlling the output of Q4.

 

[samy555]

Thank you very much for both ChrisP58 and unclejed613
Remains now to understand how transistor Q3 works as a current limitter??
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