Transistor Not Working as Expected

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shodai

Joined Apr 4, 2016
3
So I have been experimenting with transistors, trying to learn. However, I have run into a problem that I just do not get. All of the schematics that I look at have really high resistance for the base of the transistor (usually 10k) but when I use that it seems like nothing passes through. I change it to 1k and everything appears to work properly, with one pretty significant issue. It seems like the circuit work even when nothing is connected to the collector. So I am confused on both of these issues and was wondering if anyone has any ideas as to what I might be doing wrong?
 

dl324

Joined Mar 30, 2015
11,861
Sorry. Let me give the most simple example that I have tried.
One rule of thumb is to assume a beta of 10 when driving a transistor in saturation mode. To get 10mA collector current, you need 1mA of base current. With 100K, you're only getting 0.05mA. Try using 5K; 10K will also likely work. It depends on the beta of the transistor being used.
 

wayneh

Joined Sep 9, 2010
16,400
And then in many cases, beta is greater than ten and if minimizing the base current is important, you experiment. You use the largest ohms resistor that still works well. If you don't care about current and just want to be 100% sure it works, use the rule of thumb.
 

DGElder

Joined Apr 3, 2016
351
"It seems like the circuit work even when nothing is connected to the collector. So I am confused on both of these issues and was wondering if anyone has any ideas as to what I might be doing wrong?"

Define "works" precisely when you say "seems like the circuit work even when nothing is connected to the collector." What are your measured values?
 

crutschow

Joined Mar 14, 2008
25,645
A transistor "works" by the base current controlling the collector current.
If you have nothing connected to the collector than the transistor can't be working.
 
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