How does a transistor works as a switch?

[Pavi98]

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.

 

[Nigel Goodwin]

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]

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

 

[danadak]

Could not find easily imported spice model for a LiO battery so this is a crude
sim, where panel (V1) is swept from 0 - 10 V.

Regards, Dana.

 

[Pavi98]

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]

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]

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.

 

[AnalogKid]

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?.

Looks like a garden light circuit. Super-duper-el-cheapo.

ak

 

[Ramussons]

How does the transistor get its base drive? Leakage through the Solar Panel?

 

[Nigel Goodwin]

How does the transistor get its base drive? Leakage through the Solar Panel?

Yes, the battery discharges through the solar panel (and in this case the 10K and BE junction) - which is why solar panels usually have either an internal or external rectifier to prevent it.

 

[tepalia02]

Can't understand where the Base current for BC547 will come from. Confused.....

 

[Papabravo]

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.

 

[upand_at_them]

Current flow: