NPN Transistor & welcom

[dropcheck]

Hello everyone. New here and I wanted to say Hi first of all.

2ND: Working with zener diode to bias an NPN transistor as a power supply. Believe the transistor is used to increase current to drive circuit.

At normal input voltage (30Vdc) everything works fine. At lowever voltages (20Vdc) the circuit does not work. I "believe" the reduced input is reducing my Ib, therefore current gain is less.

Does the current gain Hfe vary with input voltage and current's? I see the spec sheet rates gain at Ic =100mA. My Ic (Ie) is more around 7.5mA under good conditions and drops to 2.2mA when not working.

any ideas or explanation of how current gain is determined or varies under different inputs?

thanks,
JS

 

[IRstuff]

Yes, it varies. You should consult the manufacturer or put the part on a tester. An HP4145 would give you the data in a couple of minutes

TTFN

 

[dropcheck]

I've contacted to manufacture and waiting to hear back. another question: What techniques could I use to measure the currents (Ib, Ic, Ie) I've used my Fluke meter and measured Ib = ~7uA and Ic=Ie = 7mA. I contacted Fluke to ask about accurance because the signal is phone simulator (19Hz ringing)

Fluke said meter worked to 20Hz and I should measure another way.

any ideas?

Jon

 

[itsmoked]

dropcheck; you can put appropriately sized resistors in all those transistor leads (or use existing ones) and measure the voltage across them with a scope. From that you can do all the gain calcs you want.

By just messing about with your supply voltage and looking at the results on a scope you can answer all your questions and probably learn a lot quickly!

 

[IRstuff]

What class is this for?

TTFN

 

[dropcheck]

thanks for the tips. I've taken the scope and took some measurments, just need to run some more calcualations. Been using this project increase my knowledge of Pspice.

Looks like my "load" requires more current than power supply can produce at the lower input voltages.

not for a class. Sort of a nick /nack project for myself and to learn more about transistors also. Learning in real world is differnt than my classes at school. all the "given" specs and information didn't vary in school like they are now.

 

[zappedagain]

Dropcheck,

Welcome to the real world of electronics, where things are not always as they seem; the physics always wins! It looks like you are off to a good start so keep exploring!