BC547 vs BC517

Ian0

Joined Aug 7, 2020
3,761
BC517 is a darlington: you should be able to replace two of the BC547 s with it.
The circuit isn't properly biassed, so I'm surprised that it works at all.
 

Ian0

Joined Aug 7, 2020
3,761
Without any proper bias setting, it's pot luck whether it works or it doesn't. It depends on its operation on leakage current, which is highly temperature dependent, and will vary wildly from one transistor to another.
Try cooling it down if it's on all the time, or warming it up if it's off all the time!
 

Thread Starter

LAOADAM

Joined Nov 21, 2018
658
Without any proper bias setting, it's pot luck whether it works or it doesn't. It depends on its operation on leakage current, which is highly temperature dependent, and will vary wildly from one transistor to another.
Try cooling it down if it's on all the time, or warming it up if it's off all the time!
Thanks.
Replaced the BC517 by PN2222a, SEEMS ok NOW.
BTW. what's the easy way to set bia of here?
 

Ian0

Joined Aug 7, 2020
3,761
I am very surprised to read that it can sense DC. If it only sees DC, what causes base current to flow?
It's easy to see what causes current to flow - a gain of about a million at a conservative estimate, probably 10 million, would only need 1nA leakage C to B in the first transistor to cause 10mA in the LED. But I don't see how it can sense the presence of DC in a nearby wire - probably it works like divining rods.
 

Thread Starter

LAOADAM

Joined Nov 21, 2018
658
It's easy to see what causes current to flow - a gain of about a million at a conservative estimate, probably 10 million, would only need 1nA leakage C to B in the first transistor to cause 10mA in the LED. But I don't see how it can sense the presence of DC in a nearby wire - probably it works like divining rods.
Thanks.
 
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