# Resisitor at the Base terminal

#### Pavi98

##### Member
Hello all,

It is recommended to use a 1/4 W 10K resisitor for the base terminal of a transistor (for driving an LED) and can I use 1W or higher wattage 10K resisitor with a instead of the 1/4 W resisitor? Will there be any difference?

Thank you

#### atferrari

##### Well-Known Member
The cost and the size.

#### Pommie

##### Well-Known Member
Do you have a schematic or link?

Mike.

#### Pavi98

##### Member
Do you have a schematic or link?

Mike.
Hello Mike,

The following simple diagram of a automatic LED On/off circuit.

Thank You.

#### Pommie

##### Well-Known Member
The mA into the base of that transistor is 0.7/10000 = 0.07mA. A typical BC547 has a gain of ~500 so the collector (and LED) current will be ~35mA. It needs a current limiting resistor (~600Ω) and the 10k changing to a lower value (say 2k). That circuit also wastes (0.7/5)% of the power generated = 14%. Also, if you are charging a Lipo battery (which it looks like) then that is very dangerous. Google Lipo charging danger for more info. The solar panel has to generate 3.7+0.7 = 4.4V before any charging takes place and the LED lights.

Mike.

#### Pavi98

##### Member
The mA into the base of that transistor is 0.7/10000 = 0.07mA. A typical BC547 has a gain of ~500 so the collector (and LED) current will be ~35mA. It needs a current limiting resistor (~600Ω) and the 10k changing to a lower value (say 2k). That circuit also wastes (0.7/5)% of the power generated = 14%. Also, if you are charging a Lipo battery (which it looks like) then that is very dangerous. Google Lipo charging danger for more info. The solar panel has to generate 3.7+0.7 = 4.4V before any charging takes place and the LED lights.

Mike.
Thank You Mike. It helped me a lot. I could find a 4V solar panel and I used a 3.7V rechargeble battery. Circuit works as intended. Whenever I put a flash light onto the Solar Panel, LED goes off and When I cover the solar panel, LED turns On. But the problem that I found is the LED glows faintly (5mm Clear, White). What can be the reason behind this?

Thank you.

#### Pommie

##### Well-Known Member
A few questions.
Is it a Lithium rechargeable battery such as a 18650?
How big is the solar panel? How many watts/Amps?

Mike.

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