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.

calculating saturated NPN drives me nuts

Status
Not open for further replies.

rompelstilchen

Full Member level 2
Joined
May 22, 2010
Messages
122
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,296
Location
fdsf
Activity points
2,308
Hello

I try to invert a 5V ttl with a bc818...I need to figure Rb when Rc is 1K (guess I dont need high current since there is no load)

in the datasheet, the BC818 has a gain between 250 and 600 (no idea what value to choose) for 100mA Ic

I tried many values or Rb, but it barely switches

I use this calculator : https://www.daycounter.com/Calculators/Transistor-Switch-Saturation-Calculator.phtml

please help

regards
 

Ic=betaForced*Ib for saturated BJTs.
Choose betaForced=10 or 20.

Now you know everything to select "Rb".
 

I tried 20k , does not work either...I dont get it , I tried 1K, 230k, 500k, now 20k, I should get closer somehow...that bloody p of s does not switch at all
from 5v, it barely goes to 4.5v

tried 10k also, it goes down to 3.8V...I already tried smaller values
 
Last edited:

Maybe you mixed up collector and emitter? That will reduce the gain a lot.
 

double checked everything
I tried a 50k pot for Rb, it should switch
I tried with another 818 that has a different id on its back, even worst, no matter what, the output is always 5v
I'll go to the shop tomorow and try to find 2222 bjt
everytime I try this simple setup, it takes me forever to make it work
I can also go with inverters I have but they are hex inverters, what a waste
a bjt would have been more elegant....
thanks anyway
 

Maybe you should try Rc=47ohm and Rb=100ohm.
 

With a 5V supply and a 1k load then the collector current is almost 5mA when the transistor is saturated. The "calculator" you used is completely wrong. The datasheet for your transistor and all little transistors shows a base current that is 1/10th the collector current for proper saturation.

You mentioned TTL. It has an output high of only about 3.5V when it has no load. The input of a TTL gate or inverter has almost no input current when it is high. Try an Rb that is (3.5V - 0.7V)/0.5mA= 5.6k ohms.
 

You don't need 100mA, not sure whether the transistor
you piked is good for 100mA anyhow but you need only
5mA or so. Pick a 1K pullup (Rc) and a 4.7K, 4.7K base
divider network (so threshold is ~ TTL 1.4V) and there
is your forced beta of 10-20.
 

Hi,

guess I dont need high current since there is no load)
There need to be a load: the pullup current caused by the pullup resistor.
Without pullup the circuit can not work.

Klaus
 

A TTL gate (if truly TTL, not CMOS) will have a weak pullup
and could function (with timing being rather sloppy and
skewed).
 

"Maybe you should try Rc=47ohm and Rb=100ohm. "

dude it works
so I have to have Ic=100mA ??? I cant choose a smaller current ?
jeee...I'll try with a 2222

- - - Updated - - -

found this and it's working

2222_inverter.png

maybe bc818 are meant to be for bigger loads or something
 

found this and it's working
Of course it works, they have betaForced~2.5

If you go to betaForced=10 as Audioguru is using, you will get less power loss.
 

"Maybe you should try Rc=47ohm and Rb=100ohm. "

dude it works
so I have to have Ic=100mA ??? I cant choose a smaller current ?
jeee...I'll try with a 2222

- - - Updated - - -

found this and it's working

View attachment 150153

maybe bc818 are meant to be for bigger loads or something

You can choose a small current. You'll just have to increase Rc to 150ohm and Rb to say 330ohm.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top