Pages: 1 2 [3]   Go Down
Author Topic: Transistor issue  (Read 3143 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
Edison Member
Karma: 116
Posts: 2205
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

with the 240 ohm resistor we are driving at about 4.3V/240 = ~18ma

That's not how that chart works.

You take your desired Ic, divide it by the 10x or 50x curve to get the Ib (100ma - 20ma). From that, you calculate your base resistor for 5v drive -> 43ohm - 200ohm.

Alternatively, your Ib with a 220ohm base resistor is 18ma, yielding a Ic of 18ma * 10x - 18ma * 50x, or 180ma to 800ma.

For switching applications, you should simply forget about hFE.

Offline Offline
God Member
Karma: 7
Posts: 602
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

For switching applications, you should simply forget about hFE.

Look, it's not that cut an dried.  They are only showing two lines, IC/IB  = 10 and 50.  There a an infinite number of possible lines you could draw in there, and they behave in a fully predictable manner.  Just because they only show 2 lines does not mean that other lines do not exist.  Or that if you wander off the space between those two lines by a tiny bit, the transistor will behave in a hugely different fashion.  A line for a ratio of 55 would be just a tiny bit higher than the line showing the ratio of 50.

My calculations are right.  The transistor, will be in saturation, VCE will be approximately 0.055V with IC @ 1amp, and it will be at less than 10 degrees above ambient temperature.

You are obviously set on this, so I will not try to convince you further.  This argument is just starting to get inane.  If you want to prove me wrong, get a ZTX851, drive the base from 5V through a 240 ohm resistor, allow 1 amp IC (5V through 5 ohms, perhaps) and take some measurements.  If it blows up, or even get's luke-warm, I'll play for the transistor and send you $5 for your trouble. Okay?

Other than that, let's agree to disagree.
« Last Edit: December 11, 2012, 03:57:49 pm by BillO » Logged

Facts just don't care if you ignore them.

Pages: 1 2 [3]   Go Up
Jump to: