Transistor Thoery Help. Thermal Runaway - diyAudio
 Transistor Thoery Help. Thermal Runaway
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 Pass Labs This forum is dedicated to Pass Labs discussion.

 Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
diyAudio Member

Join Date: Jan 2002
Location: Williamsport, PA
Transistor Thoery Help. Thermal Runaway

A digital dweeb is requesting the assistance of any knowlegable analog guru out there.

Consider the simple NPN current source shown in the attachment.

If V1 is an ideal 0 impedance voltage source (referenced to -V) we know that the voltage across R1 is about 5v. Using ohms law we can determine the current through R2 is 5v/0.1 = 50A. (I know these numbers are large, I just picked them for ease of calculation).

Here is my question. Since the current through r2 is largely independant of beta, could we conculde that this transistor circuit is very resisant to thermal runaway (or anything else that would cause beta to increase)?

Thanks in advance for the help.
Attached Images
 irsc.jpg (4.4 KB, 181 views)

diyAudio Member

Join Date: Apr 2003
Location: London
Re: Transistor Thoery Help. Thermal Runaway

Hi,

Quote:
 Originally posted by hugeli60 Using ohms law we can determine the current through R2 is 5v/0.1 = 50A. (I know these numbers are large, I just picked them for ease of calculation).
Unfortunately, your values are completely unrealistic - you'll need more than 800 V supply just to get this circuit into a linear region - and dissipation on R2 would be about 40,000 W . It is, I suppose, possible - but I wouldn't try it at home... .

For a bit more down-to-earth approach, let's take R1 = 5 Ohm. Than the current would be about 1 A and supply voltage over 24 V will work. And yes, this circuit will only slightly increase the collector current with the temperature rise - main influence is the Vbe voltage ("0.7 v") - it has a negative TC about -2mV/degree C. So 100 degrees increase in temperature will result in 200 mV increase in voltage on R1 and the collector current - about 4% .

Al

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post ricsmuts Car Audio 39 22nd May 2008 03:03 PM Dan2 Solid State 6 25th March 2008 07:11 PM Wagener Solid State 19 22nd May 2004 10:44 AM azira Solid State 46 31st January 2004 09:57 AM Circlotron Solid State 12 23rd September 2002 02:31 AM

 New To Site? Need Help?

All times are GMT. The time now is 06:21 AM.