I want to turn off a load that requires about 200mA. I am only familiar
with the common emitter setup for a NPN transistor, which cant be used
to turn off a positive voltage supply. I am unsure of the proper
technique for the other setup that allows for the control of a positive
voltage. Thanks.
2006-08-25 13:37:15 · 7 answers · asked by Anonymous in Science & Mathematics ➔ Engineering
I want to turn on-off a 556 timer, but even when I remove it's ground supply it still outputs normally.
2006-08-25 14:55:20 · update #1
I think you want a PNP transistor with the emitter connected to the supply and the collector connected to the high side of the load.
Perhaps a 2N4402 would work if you ensure that it is always completely ON or OFF, with quick transitions between ON & OFF (so you don't burn it up). You didn't spec. the voltages; the 2N4402 is rated to 40V I think (not sure).
Connect a 500 to 10K resistor to the base. Switch the other end of the resistor to connect either to the supply or ground. (I assume the load is grounded.) If the base is High, the PNP switch is OFF; if the base is Low, the switch is ON.
2006-08-25 14:02:29 · answer #1 · answered by Tom H 4 · 0⤊ 0⤋
Easy to fire up a relay with an NPN - not so easy to turn a positive supply on and off with the NPN alone.
Let's start with the collector - this would necessarily be the positive supply you want to control. The base would then be biased off and turned on for conduction when needed. The emitter could be at ground potential through a current limiting resister ( higher than the load resistance) and tapped through a forward biased diode for the voltage source you need to control. I suppose it can be done, but there are better ways to do it.
2006-08-25 13:56:14 · answer #2 · answered by LeAnne 7 · 0⤊ 0⤋
What kind of control signals do you have voltage range? What is the voltage on the positive power supply ?
I personally preferr using mosfet. either N- or P channel mosfet. They are great for switching and control.
If you insist on using a N-p-n: What is the type of load you got, can you put the n-p-n at the ground end ?
In another word, the load will connect to power supply end, but the grounds will be connect to the collector of the npn transistor.
The npn transistor will act as a connector to the ground of the load. This has a draw back, the load is not completely connect to grounds. So, it depends on what is your load. what is it for ?
2006-08-25 14:36:36 · answer #3 · answered by Anonymous · 0⤊ 0⤋
Use a SCR. That is the proper device to switch power.
The only time you'd need a transistor is if you want to modulate the amount of power going through. Essentially using it as an amplifier.
2006-08-25 13:43:19 · answer #4 · answered by Anonymous · 0⤊ 0⤋
Remove the NPN transistor--Simple.
2006-08-25 13:41:03 · answer #5 · answered by Jeff P 2 · 0⤊ 0⤋
You can use a SCR ( Silicon controlled rectifier) to turn off your NPN transisor....You can conect two resistors to the base and collector of the transistor and make it use a switch....
2006-08-25 13:48:06 · answer #6 · answered by Ravi S 1 · 0⤊ 0⤋
Search the web for information on the common collector and the common base configuratons. One of them might be suitable for what you want to do.
2006-08-25 14:03:12 · answer #7 · answered by cdf-rom 7 · 0⤊ 0⤋