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How to use a transistor as an on-off switch?
In my current configuration the transistor varies its collector-emitter resistance between 0.1ohm and 100M. At the resistance values in the middle the transistor gets very hot and dissipates a lot of power.
I need to know how to make the transistor tend to be on or off, spending very little time in between. Would higher gain help? Certain zener configuration maybe?
What are your thoughts? Thanks for your expertise!
1 Answer
- 1 decade agoFavorite Answer
It sounds like you do not have a resistor connected between the collector and the power supply. When you apply base current to the transistor, the collector current is the product of the base current and the current gain (beta) of the transistor. The heat that you noted is the product of this current and the supply voltage.
To make the transistor OFF you would have to arrange that the base current is stopped--that is reverse bias the emitter-base junction. To make the transistor fully ON, you would have to arrange that the base current is sufficient to cause required collector current to flow. A rule of "thumb" in these cases is to make the base current about 10% of the required collector current. this should work for for moat any transistor. To make the transistor switch ON/OFF fast, you would have to have a rapidly changing base current. I don't believe a zener diode would be of any value in a simple switching circuit.
Source(s): Have been an Electronic Engineer for about 55 yrs., and worked with many types of transistors in the development of electronic equipment during the 1960's and 70's.
