http://www.allaboutcircuits.com/vol_6/chpt_5/9.html
I'm having trouble with this experiment-- can't get the LED to turn on/off as experiment describes.
First off, I've tried two different N-channel JFET and get the same results: NTE312 and MPF102 (radioshack).
I understand that if you reverse bias the gate, by touching one hand on the 100K resistor and the other on the (-) side of battery, it should pinch off the N channel and no current should flow, turning the LED off.
However, here's what I noticed:
1. With the 100k resistor floating, the LED is randomly on or off. Draws about 8 mA.
2. When I touch the floating resistor with just one hand the LED dims. Draws about 2 mA. Maybe my body has a (-) charge?
3. When I touch the resistor with one hand and touch the (+) supply with the other the LED brightens with a current of 12 mA. This was expected.
4. When I touch the resistor with one hand and touch the (-) supply with the other the LED brightens with a current of 7 mA. Not expected. I would have thought the LED would turn off. But the current did decrease.
5. Connecting the resistor directly to the (-) supply brightens the LED with a current of 9 mA! Not expected!
Any suggestions??
I'm having trouble with this experiment-- can't get the LED to turn on/off as experiment describes.
First off, I've tried two different N-channel JFET and get the same results: NTE312 and MPF102 (radioshack).
I understand that if you reverse bias the gate, by touching one hand on the 100K resistor and the other on the (-) side of battery, it should pinch off the N channel and no current should flow, turning the LED off.
However, here's what I noticed:
1. With the 100k resistor floating, the LED is randomly on or off. Draws about 8 mA.
2. When I touch the floating resistor with just one hand the LED dims. Draws about 2 mA. Maybe my body has a (-) charge?
3. When I touch the resistor with one hand and touch the (+) supply with the other the LED brightens with a current of 12 mA. This was expected.
4. When I touch the resistor with one hand and touch the (-) supply with the other the LED brightens with a current of 7 mA. Not expected. I would have thought the LED would turn off. But the current did decrease.
5. Connecting the resistor directly to the (-) supply brightens the LED with a current of 9 mA! Not expected!
Any suggestions??