Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Darlington pair conduct current when I touch the base pin with fingers

Status
Not open for further replies.
M

magnetra

Full Member level 5
Joined
Apr 21, 2005
Messages
263
Helped
10
Reputation
20
Reaction score
7
Trophy points
1,298
Location
27.45N, 85.20E KTM, NP
Activity points
3,375
I connected a BC547 pair into Dalington configuration with a LED in collector as load. I've connected the base of the pair to gnd via 1M resistor. Now when i touch the base pin with my fingers, the darlington pair seems to conduct enough current to light the LED. What is happening here?Are my fingers source of charge??
Regds
Magnetra
 

Re: Darlington

The Darlington pair has a gain which is the product of the gains of the individual transistors. Thus, if each of them has a gain of 100, then the total gain is 10000.
That is very high.

So the voltage induced by your body (due to line frequency pickup primarily), will create a very low current into the base of the Darlington. But given the very high gain, it is sufficient to turn on the LED.
If you were to connect a scope to the output, you would at some point see that the output voltage is not DC, but looks like clipped AC. That is because the induced voltage is AC, but the transistor BE junction rectifes it and amplifies only the positive half.
 

Re: Darlington

Yea thats good enough. Yea I've noticed that when ever i touch the oscpe's probe with my fingers, it shows a roughly 50Hz signal, very similar to the city supply. So that's the line pickup by the body. Hey doesn't it have any effect on our body???
 

Re: Darlington

So far no effect has been confirmed, although some people claim that living near a power station, where the field is much stronger actually increases the chances of developping cancer.
But, again, these are just some controversial findings, I am not aware of any serous studies that would link the two.

Anyway, I cannot live without electricity, so whatever the risk, I will take it.
 

Re: Darlington

hi
you can find sufficient information of Darlington pairs at Gray & Meyers book
 

Re: Darlington

because we are in high density 50HZ AC line field, when you touch the BC547's

base, the voltage that induced by AC field provide some current into

the BC547's base.


best regards




magnetra said:
I connected a BC547 pair into Dalington configuration with a LED in collector as load. I've connected the base of the pair to gnd via 1M resistor. Now when i touch the base pin with my fingers, the darlington pair seems to conduct enough current to light the LED. What is happening here?Are my fingers source of charge??
Regds
Magnetra
Click to expand...
 

Re: Darlington

I think ESD(electrostatic discharge) is a possible answer
you can learn about it in the art of electronics
 

Darlington

ESD discharge won't keep the LED lit for long enough to visably see. This effect is also very easily noticed in mosfets, so much so that physical contact isn't required to trigger it because it's voltage based. That's how voltage detector pens work. In the case of a mosfet though if it were and ESD discharge it would fry the gate.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top