Switching transistor question

Thread Starter

bisctboy

Joined Nov 3, 2008
26
The attached picture shows how a switching transistor can use the 1v from a solar cell to turn on a light. Is it possible to reverse the transistor so that the switch turns on when no light is present? In other words, do transistors come in "normally closed" and "normally open" versions? I need to put both types in my circuit. If the sun is shining, I need current to flow in one part of my circuit while blocking the flow of current in another part of my circuit. Conversely, when the sun is not shining, I need the opposite to be true.

Is the above the difference between npn and pnp?

Thanks,
 

Attachments

R!f@@

Joined Apr 2, 2009
9,916
Regardless of whether you want the 'switch' on or off your proposal would simply destroy the transistor.
Well Said.


You always need a base resistor, a 1K ohm is agood value to start, and remember that the load current is less than the Ic of the transistor, and also heat dissipation is one thing to keep in mind.

Rifaa
 

hobbyist

Joined Aug 10, 2008
892
Here is a quick example of a inverter
When solar cell has no light, than all current flows through LED, turning it on.
When solar cell has light on it than transistor begins to conduct drawing current away from the LED, causing the LED to shut off.

Click on the top left hand corner to enlarge it.
 

Attachments

Thread Starter

bisctboy

Joined Nov 3, 2008
26
A small solar cell won't produce enough base current to damage a transistor.
That's what I thought as well since I am using a 2v (.5v 400mA X 4) solar cell. I am having a hard time "writing" what I want to do so I have attached a hand-sketched picture of what I'm trying to accomplish. Obviously, my sketch lacks the resisitors, capacitors, etc that go with the IC's. It's just a rough draft. How would I implement transistors or Sgt Wookies "Light Detectors" into my circuit? I know they have to be connected to my solar cell to detect it's voltage.

Thanks,
 

Attachments

steveb

Joined Jul 3, 2008
2,436
.... do transistors come in "normally closed" and "normally open" versions? .....
The closest thing to a "normally-closed transistor" is a depletion mode FET. JFETs are usually depletion mode and you can get MOSFETs in depletion mode, although they are more rare than enhancement mode. JFETs will typically have higher resistance than MOSFETs.

Ab enhancment mode MOSFETs or a bipolar junction transistor (like you've shown in your figure) would be like a "normally-open transistor".

I'm not necessarily suggesting you use FETs in this applications. I'm just answering your question.
 
Top