Transistor turning on without base voltage?

Thread Starter

iinself

Joined Jan 18, 2013
98
Hi,
could someone please explain why the LED glows in the following circuit without any base voltage when I touch the base lead. I observe that when I touch the base the Vbe voltage reads as 3.7V, when not touched it is fluctuating at 300mV. The transistor is MJE3055T.

I thought the transistor can conduct only when the base-emitter is forward biased but without any direct voltage just by touching the base the transistor seems to conduct??

Thanks
Arvind
 

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MrChips

Joined Oct 2, 2009
30,706
The base-emitter junction of a bipolar junction transistor (BJT) responds to current, not voltage. The LED will light up with a collector current of less than 1mA.
If the typical current gain of a BJT is about 300, it only takes about 3μA from your finger to turn on the LED. Your body acts as an antenna and is picking up a lot of electrical signals, mainly 60Hz AC power from around your room.
 

Audioguru

Joined Dec 20, 2007
11,248
I doubt the transistor will turn on when the base is touched if the circuit is in the middle of a desert hundreds of miles away from civilization (no AC mains signals).
 

killivolt

Joined Jan 10, 2010
835
Do "EM's" affect the Transistors P/N junction?

Is it the same reason why on occasion the transformer lighting would set off my, capacitive Touch circuit.
 

Thread Starter

iinself

Joined Jan 18, 2013
98
The base-emitter junction of a bipolar junction transistor (BJT) responds to current, not voltage. The LED will light up with a collector current of less than 1mA.
If the typical current gain of a BJT is about 300, it only takes about 3μA from your finger to turn on the LED. Your body acts as an antenna and is picking up a lot of electrical signals, mainly 60Hz AC power from around your room.
Thanks, got that. Current amplification happens whether base-emitter is forward biased or not. So a BJT is a current amplification device and not a voltage amplifaction device. When the LED turns on can I say the BJT is in the ON state? One more question still remains as to why is my voltmeter showing 3.7V when my supply was only 3.1 and all of this 3.7V is on the base-emitter junction ??
 

#12

Joined Nov 30, 2010
18,224
An idea that is not normally told to noobies is that transistors have a conduction ratio that goes all the way down to microvolts Vbe. They don't, "break over" like zeners or LEDs. If you want to say your transistor is, "on", that is OK with me. Fact is, a bipolar transistor can be manipulated to "on" way down in the nanoamp range.
 

Audioguru

Joined Dec 20, 2007
11,248
A voltmeter can be set to show AC or it can be set to show DC. You did not say which.
3.7V is problably the level of the AC signal picked up by nearby mains electrical wiring.
 

Thread Starter

iinself

Joined Jan 18, 2013
98
A voltmeter can be set to show AC or it can be set to show DC. You did not say which.
3.7V is problably the level of the AC signal picked up by nearby mains electrical wiring.
I checked again, it is DC volts ! When I don't touch the base then Vbe is fluctuates a lot around 300mV and when I touch it it is 3.7V. Yes it seems the only explanation is that by touch I am supplying the additional .6V, interesting !
 

Audioguru

Joined Dec 20, 2007
11,248
DC can be positive or negative. The DC voltmeter shows which but you did not say which.
I suspect the AC signal picked up by you caused the base of the transistor to swing 0.7V positive and 4.4V negative. So it shows the difference which is -3.7V.
 

killivolt

Joined Jan 10, 2010
835
DC can be positive or negative. The DC voltmeter shows which but you did not say which.
I suspect the AC signal picked up by you caused the base of the transistor to swing 0.7V positive and 4.4V negative. So it shows the difference which is -3.7V.
Ok, then.....
 
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