Thermal runaway question - diyAudio
Go Back   Home > Forums > General Interest > Car Audio
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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
Thread Tools Search this Thread
Old 17th May 2008, 10:12 PM   #1
diyAudio Member
Join Date: Nov 2006
Default Thermal runaway question

I built this amp X2 (see attachment) as a two channel car amp

It works great, I've biased it to about 20ma

If I push the amp I keep having thermal runaway. Obvoiusly I've done something stupid somwhere...... and it's cost me a small fortune in output transistors. I've tried almost no bias as well (5ma)with the same result.

Is TR10 supposed to be attached to the heatsink?

Any ideas?
Attached Images
File Type: jpg 400w.jpg (77.3 KB, 250 views)
  Reply With Quote
Old 17th May 2008, 10:29 PM   #2
diyAudio Member
Join Date: Mar 2002
I would surmise D1,D2, and D3 should be on the heatsink
  Reply With Quote
Old 17th May 2008, 10:35 PM   #3
diyAudio Member
Join Date: Nov 2003
Location: Louisiana
The bias components (transistor and diodes) need to be attached to the heatsink. You may be able to mount only the transistor on the sink for proper compensation but having all of them should provide better compensation.

Insert 5 amp fuses in the positive and negative rails between the supply and the amplifier. This will allow you to run the amp hard enough to make it heat up but should protect the power supply and the outputs. Drive it as hard as you can without blowing the fuses. Every couple of minutes, remove the input signal and re-check the bias current. Do this until it gets as hot as you intend to run it. If you have thermal protection, continue to check the bias current until it reaches thermal shutdown.
  Reply With Quote
Old 18th May 2008, 10:59 AM   #4
diyAudio Member
Join Date: Nov 2006
Thanks Perry

Will try that.

I've realised again just how destructive thermal runaway can be. Fuses are essential.
  Reply With Quote
Old 18th May 2008, 11:12 AM   #5
luka is offline luka  Slovenia
diyAudio Member
luka's Avatar
Join Date: Sep 2005
Location: in Slovenia :)
Send a message via Skype™ to luka

I would even try without fuses, + since they are so cheap and effective
home page @ @ 24/7 all year long
  Reply With Quote
Old 18th May 2008, 04:29 PM   #6
diyAudio Member
Join Date: Nov 2006
Default Is this circuit correct?

Is this circuit correct?

I have moved the transistors and diodes now, and it's a bit better.

What seems to be happening is both rails of output transistors turn on at high volume or high power transients, just below this level I can let the amp get very hot and it has no problem.

Only occurs with peaks
  Reply With Quote
Old 18th May 2008, 04:47 PM   #7
diyAudio Member
Join Date: Nov 2003
Location: Louisiana
If the bias current holds at all normal operating temperatures but it spikes for a few seconds just after a period of very high power output, the bias components are not close enough to the output transistors. The temperature of the output transistors is increasing but the temperature of the heatsink where the bias components are mounted is significantly cooler than the output transistors. Move the bias components as close to the outputs as possible.

This can also happen when the insulator material between the transistor and the sink is inefficient. The transistor sees a spike in temperature and it takes longer than it should for it to pass the heat to the sink. As the transistor returns to the temperature of the heatsink, the bias current returns to normal.
  Reply With Quote
Old 18th May 2008, 06:18 PM   #8
diyAudio Member
Join Date: Nov 2006
Maybe I should those thermaltrak output transistors I have lying around.

It's awkward to move that whole circuit to the heatsink, what would you reccomend is the minimum components?

Thanks for the advice so far.
  Reply With Quote
Old 18th May 2008, 06:42 PM   #9
diyAudio Member
Join Date: Nov 2003
Location: Louisiana
You should try heating the bias circuit components individually to see which one causes the greatest change in bias current. Heat the diodes as a group. If heating the transistor alone creates a significant change in bias current, try relocating only the transistor. If both the diodes and the transistor make a big difference in bias current and moving the transistor alone doesn't provide enough compensation, you'll have to move all of them.

If you mount them on a small piece of proto-board, they will be easier to relocate as a group.
  Reply With Quote
Old 18th May 2008, 06:53 PM   #10
diyAudio Member
Join Date: Nov 2006
I deserved that nice logical answer!

I could relocate most of it with two wires and proto board.

Once I get that right, I'd like to increase the VAS (not sure the drivers are being driven hard enough)

Once it's stable I will post the PCB I have done, which was designed to fit into a standard type car amp heatsink
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

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 Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
thermal runaway problem Dan2 Solid State 6 25th March 2008 07:11 PM
A question regarding thermal adhesives neil_kaye Parts 10 3rd March 2007 04:59 PM
thermal runaway diodes Wagener Solid State 19 22nd May 2004 10:44 AM
Will a true class B have thermal runaway? azira Solid State 46 31st January 2004 09:57 AM
Transistor Thoery Help. Thermal Runaway hugeli60 Pass Labs 1 1st June 2003 09:29 AM

New To Site? Need Help?

All times are GMT. The time now is 04:42 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2