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How does a transistor works as a switch?

Pavi98

Member
Hello all
Could you please explain me about the working principle of the following diagram? I could not understand how the transistor works as a switch in this diagram specially how the voltage is divided to turn on the transistor

Thank you.

IMG_20220818_180600.jpg
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I suspect it probably doesn't work?, as the panel starts to charge the battery the voltage drop across D1 will increase to around 0.7V, driving the transistor base negative, turning the transistor off even more.
 

AnalogKid

Well-Known Member
Most Helpful Member
It does work.

as the panel starts to charge the battery the voltage drop across D1 will increase to around 0.7V, driving the transistor base negative, turning the transistor off even more.

Correct. During charging (daytime), the transistor and LED are off. D1 conducts to complete the charging circuit. At night, the solar panel is a low DC impedance. This elevates the voltage at the left side of R1, turning it on. D1 prevents the panel from discharging the battery.

Gold star for the reference designators.

ak
 

Pavi98

Member
It does work.



Correct. During charging (daytime), the transistor and LED are off. D1 conducts to complete the charging circuit. At night, the solar panel is a low DC impedance. This elevates the voltage at the left side of R1, turning it on. D1 prevents the panel from discharging the battery.

Gold star for the reference designators.

ak
How the transistor is turned off during daytime? Did you mean the impedance of the solar panel by the "dc impedance"?
Thank you
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It does work.



Correct. During charging (daytime), the transistor and LED are off. D1 conducts to complete the charging circuit. At night, the solar panel is a low DC impedance. This elevates the voltage at the left side of R1, turning it on. D1 prevents the panel from discharging the battery.

Gold star for the reference designators.

ak
I was under the impression that solar panels usually included an internal series rectifier to prevent discharging the batteries at night, presumably this panel must not have one?.
 

upand_at_them

Active Member
How the transistor is turned off during daytime? Did you mean the impedance of the solar panel by the "dc impedance"?
Thank you

During the daytime the solar panel is producing 5V. So conventional current flows from the solar panel + to the battery +, this is around the outer loop of the circuit. At night the solar panel isn't outputting anything so current is reversed because the battery is providing the current. Except that current won't flow backwards through D1, so the transistor base turns on.
 

Papabravo

Well-Known Member
Can't understand where the Base current from BC547 will come from. Confused.....
That is because you are not reading carefully. It comes from the positive terminal of the rechargeable battery, through the low impedance of the non-illuminated solar panel, through the resistor to the base of the transistor.
 

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