# Transistor Switching, Minimum current for base?

Discussion in 'General Electronics Chat' started by Ishiyuu, May 25, 2016.

1. ### Ishiyuu Thread Starter New Member

May 25, 2016
3
0
Hello all,

I have a very general question that seems to not yet have been answered. In a circuit I am designing, What I want is a 2n2222 transistor to only fire above a certain current. In a simulator, I am applying a 361.1uA current to the base, and am getting a 36.1 mA current. In reality, is it possible to completely limit the emitter current to 0 without the base being 0?

2. ### dl324 Distinguished Member

Mar 30, 2015
3,378
651
Welcome to AAC!

How close to 0 do you require? Even with a large base resistor, there will be some leakage current.

For anyone to be able to comment on the circuit you're simulating, you need to post a schematic.

3. ### Ishiyuu Thread Starter New Member

May 25, 2016
3
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My apologies. My primary objective is to create a circuit whereby after 1 second of conducting, the circuit becomes "opened" for a period of time, where the base and collector connected cannot conduct current. I want to create this circuit in the most simple way as possible, without the use of gates or IC's.

4. ### dl324 Distinguished Member

Mar 30, 2015
3,378
651
If you want the transistor to be saturated for a certain period after being triggered, you can make a discrete one shot to drive the switch. That will take 3 transistors, a capacitor, and some resistors.

From wikipedia: https://en.wikipedia.org/wiki/Multivibrator

You could try using an RC network on the base of a transistor, but you won't get a sharp on-off transition.

5. ### crutschow Expert

Mar 14, 2008
13,501
3,375
How long is this "period of time"?

6. ### AnalogKid Distinguished Member

Aug 1, 2013
4,696
1,297
Yes. First, there is a minimum base-emitter voltage below which no base current flows. Once Vbe is large enough and base current does start to flow, there is a minimum current necessary to start collector-emitter conduction. These values are on the datasheet.

ak

7. ### wayneh Expert

Sep 9, 2010
12,388
3,244
No. If there is any base current whatsoever, it appears at the emitter. It cannot accumulate in the transistor!

Now, you could have a base-collector current if the collector voltage falls below the base voltage. But I don't think that's what you are asking.

8. ### Ishiyuu Thread Starter New Member

May 25, 2016
3
0
Thanks for the suggestions. I have found another solution however; I will use a relay instead