# question on tansistor as switch Volume III - Semiconductors

#### rst

Joined Jul 12, 2013
2

is the direction of current of the battery from negative to +ve, is it opposite as it indicated above. I am not clear, is the battery to supply current to transistor? is it the direction of current from battery to solar cell then to the base?

#### WBahn

Joined Mar 31, 2012
30,057
The arrows are showing the direction in which electrons flow. Since electrons are negatively charged, the current (which is the flow of charge per unit time, not the flow of charge carriers per unit time) is in the other direction.

This is the distinction between "conventional current" and "electron current". Engineering, because it is much more heavily focused on a consistent mathematical treatment of circuits and systems, almost universally uses conventional current.

Personally, I don't recommend using electron current because it sets you up for difficulties as circuits get more complicated because you have to remember when and where to throw in minus signs instead of just letting the math take care of itself naturally.

Having said that, the person that established the E-book on this site decreed that it would use electron current and not conventional current. They had every perogative to do so.

#### rst

Joined Jul 12, 2013
2
thanks for replying. Is the lamp lit by the battery or by the solar cell. I am don't understand reason of connecting two sources of energy in one circuit. Which source activate the transistor, the battery or the solar cell.

#### ScottWang

Joined Aug 23, 2012
7,400
The conception of NPN transistor and current, normally the C and B need to adding a resistor to limiting the current.

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#### WBahn

Joined Mar 31, 2012
30,057
The solar cell's current goes through the base-emitter pins of the transistor. The battery's current goes through the collector-emitter pins of the transistor. The presence of base-emitter current permits (and controls) the collector-emitter current. One way to think about it is that the base current "turns a valve" that controls how much collector current you can get.

In general, the reason you use a circuit like this is that the solar cell can't provide enough power to drive the lamp directly, but you want the lamp's brightness to be controlled by the amount of light shining on the solar cell.