# Plz i need explanation for this circuit

#### dragon_yugi

May 25, 2012
4
Plz i need explanation for this circuit

i plz i need answers for those questions

Q1: What is the type of connection of the current sense transistor ? (CE,CB,or CC )

Q2: What Mode of operation of Current Sense Transistor ? In case of protection and in case of normal operation ? "cut off, active mode or saturation"

Q3: Is the current limit circuit a feedback connection? Explain ! "but positive feedback or negative feedback? What is the controlled variable?...etc."

Q4: What is the function of the current sense resistor ?

Q5: How to calculate the current value ? Depending on the Resistor Value Rsense

Apr 7, 2012
3,635
Homework?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Perhaps you could start by showing us what answers you have to these already and we could comment on what you've got wrong or misunderstood.

#### dragon_yugi

May 25, 2012
4
not homework
it's an exercise

Plz i need to understand this circuit

#### dragon_yugi

May 25, 2012
4
Q1: What is the type of connection of the current sense transistor ? (CE,CB,or CC )

A1: CC
________________________________________

Q2: What Mode of operation of Current Sense Transistor ? In case of protection and in case of normal operation ?

A2: N.O:When the power supply is supplying current below the maximum level, current flows through the sense resistor and a small potential difference develops across it.
P.O: As the voltage across the current sense resistor reaches 0.6 volts, so the current sense transistor starts to turn on. When it does this, the voltage it the base of the main power supply pass transistor is pulled down, thereby preventing any increase in the output current of the power supply.
________________________________________

Q3: Is the current limit circuit a feedback connection? Explain !

A3: Yes, output voltage at Q1 emitter act as a voltage follower, means that the voltage will follow its base voltage. Because the R sense value is chosen to be a low resistance, the voltage will be appear at load as a full voltage delivered from voltage source. Actually there is a little voltage drop caused by Q1 Vbe (base-emitter voltage) and the resistor R sense, but this voltage drop can be neglected. If the load now draw more current, at some level, the voltage drop across R sense will reach the level at a point where the transistor Q2 begin to conduct, and the current will flow from its collector to its emitter, decreasing the base voltage of Q2. Because now the Q1 base voltage decrease, the voltage output of the Q2 emitter will also decrease as it works as a voltage follower circuit. When this output voltage decrease, the current to the load will also decrease. After this point of allowed maximum current, the more the load try to draw more current (by lowering its internal resistance equivalence), the lower the output will be delivered to maintain a constant current.
The output voltage can be adjusted using the output variable resistor.
________________________________________

Q4: What is the function of the current sense resistor ?

A4: The function of the current sense resistor is to control the value of the current, the max current will be given be
(0.6 or 0.7 (the volt between the junction of the transistor))/(Max current) [Ω]

________________________________________
Q5: How to calculate the current value ? Depending on the Resistor Value Rsense

A5: (0.6 or 0.7 (the volt between the junction of the transistor))/Rsense [A]

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Q1: What is the type of connection of the current sense transistor ? (CE,CB,or CC )

A1: CC

Consider the resistor as the signal source. Where and how is this signal applied?

Q2: What Mode of operation of Current Sense Transistor ? In case of protection and in case of normal operation ?

A2: N.O:When the power supply is supplying current below the maximum level, current flows through the sense resistor and a small potential difference develops across it.

Define "small"

What is the collector current?

P.O: As the voltage across the current sense resistor reaches 0.6 volts, so the current sense transistor starts to turn on. When it does this, the voltage it the base of the main power supply pass transistor is pulled down, thereby preventing any increase in the output current of the power supply.

How turned on is the transistor?

Q4: What is the function of the current sense resistor ?

A4: The function of the current sense resistor is to control the value of the current, the max current will be given be
(0.6 or 0.7 (the volt between the junction of the transistor))/(Max current) [Ω]

I would prefer to call the resistor a current sense resistor and the transistor something other than "the current sense transistor". Which is sensing current?

All up, good answers I think.

Re-read the questions. I think I have suggested what you need to do to address them completely.

#### dragon_yugi

May 25, 2012
4
Plz i want the right answers for the first 3 questions

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
I'm sure you do.

If you wrote those answers yourself you should have no trouble.

We don't give answers here. Just assistance.

#### CocaCola

Apr 7, 2012
3,635

If you wrote those answers yourself you should have no trouble.

My glass ball says trouble...

http://www.vidisonic.com/2008/07/10/current-limiting-circuit/
How Current Limiter Works
Look at the Figure 1, output voltage at Q1 emitter act as a voltage follower, means that the voltage will follow its base voltage. Because the R sense value is chosen to be a low resistance, the voltage will be appear at load as a full voltage delivered from voltage source. Actually there is a little voltage drop caused by Q1 Vbe (base-emitter voltage) and the resistor R sense, but this voltage drop can be neglected. If the load now draw more current, at some level, the voltage drop across R sense will reach the level at a point where the transistor Q2 begin to conduct, and the current will flow from its collector to its emitter, decreasing the base voltage of Q2. Because now the Q1 base voltage decrease, the voltage output of the Q2 emitter will also decrease as it works as a voltage follower circuit. When this output voltage decrease, the current to the load will also decrease. After this point of allowed maximum current, the more the load try to draw more current (by lowering its internal resistance equivalence), the lower the output will be delivered to maintain a constant current.

The operation of the current limiter is very straightforward. When the power supply is supplying current below the maximum level, current flows through the sense resistor and a small potential difference develops across it. The value of the resistor is chosen so that at when the maximum allowable current flows from the power supply, a voltage equal to the turn on voltage of the current sense transistor is developed across it. This is typically 0.6 volts, assuming that a silicon transistor is used.
As the voltage across the current sense resistor reaches 0.6 volts, so the current sense transistor starts to turn on. When it does this, the voltage it the base of the main power supply pass transistor is pulled down, thereby preventing any increase in the output current of the power supply. In this way it is very easy to calculate the value for the sense resistor using Ohms Law. It is simply 0.6 / maximum current. The current sense transistor should have a sufficiently large current capacity to be able to take the current the base of the main series pass transistor.

It's like a deja vu moment

Q1: What is the type of connection of the current sense transistor ? (CE,CB,or CC )

A1: CC
________________________________________

Q2: What Mode of operation of Current Sense Transistor ? In case of protection and in case of normal operation ?

A2: N.O:When the power supply is supplying current below the maximum level, current flows through the sense resistor and a small potential difference develops across it.
P.O: As the voltage across the current sense resistor reaches 0.6 volts, so the current sense transistor starts to turn on. When it does this, the voltage it the base of the main power supply pass transistor is pulled down, thereby preventing any increase in the output current of the power supply.
________________________________________

Q3: Is the current limit circuit a feedback connection? Explain !

A3: Yes, output voltage at Q1 emitter act as a voltage follower, means that the voltage will follow its base voltage. Because the R sense value is chosen to be a low resistance, the voltage will be appear at load as a full voltage delivered from voltage source. Actually there is a little voltage drop caused by Q1 Vbe (base-emitter voltage) and the resistor R sense, but this voltage drop can be neglected. If the load now draw more current, at some level, the voltage drop across R sense will reach the level at a point where the transistor Q2 begin to conduct, and the current will flow from its collector to its emitter, decreasing the base voltage of Q2. Because now the Q1 base voltage decrease, the voltage output of the Q2 emitter will also decrease as it works as a voltage follower circuit. When this output voltage decrease, the current to the load will also decrease. After this point of allowed maximum current, the more the load try to draw more current (by lowering its internal resistance equivalence), the lower the output will be delivered to maintain a constant current.
The output voltage can be adjusted using the output variable resistor.

________________________________________

Q4: What is the function of the current sense resistor ?

A4: The function of the current sense resistor is to control the value of the current, the max current will be given be
(0.6 or 0.7 (the volt between the junction of the transistor))/(Max current) [Ω]

________________________________________
Q5: How to calculate the current value ? Depending on the Resistor Value Rsense

A5: (0.6 or 0.7 (the volt between the junction of the transistor))/Rsense [A]

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