Overheating LM3875 - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

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
Reply
 
Thread Tools Search this Thread
Old 30th June 2012, 01:27 PM   #1
diyAudio Member
 
Join Date: Jun 2012
Default Overheating LM3875

I'm building an amplifier with the LM3875 chip. When I connected it to the powersupply and speakers for the first time it started to do pretty loud bangs with approx one second intervals without any input signal. The chip turned rather hot int just a couple of second before I disconnected it from the socket. There is the same problem to both of the amplifiers (stereo) which i think is wierd. Because i don't think i would do the same mistake to times, but who knows.

I also disconnected the speakers and had it connected to the socket for just a few seconds before it turnde very hot (very hot = barely can't touch it hot).

I connected a 20 ohm resistor on the speaker outlet to be able measure the DC voltage. It started at just a couple of mV but it was rising, had after a few seconds, when I had to disconnect the power supply due to the heat, the voltage had been rising to about 700 mV.

I have been folowing a circuit diagram similar to the one in the product sheet for the LM3875.

I don't think there is any problem with my power supply because it has a stable positive and negative output. I have checked for shorts but can't find any, but I might have to check one more time.
Could I have been connecting wrong polarity to the wrong power input to the amp (quite shure i haven't, but maybe) But I don't dear to try change the polarity if it is right as I have it now, if somthing might get damaged.

Any suggestions of what may be wrong?
  Reply With Quote
Old 30th June 2012, 02:15 PM   #2
Puffin is offline Puffin  United Kingdom
diyAudio Member
 
Join Date: Feb 2006
Location: U.K
What is the circuit? What is your power supply. Sounds like it is oscillating.
  Reply With Quote
Old 30th June 2012, 02:17 PM   #3
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
Are the chips on a heat sink ? And of course how have you wired the power supply ( pin numbers and polarity ).
__________________
AM
  Reply With Quote
Old 30th June 2012, 02:51 PM   #4
diyAudio Member
 
Join Date: Jun 2012
I have followed the two schematics ( power supply and amp) showed in the first post in this thread HiFiForum.nu - Sten-Klen: mer gainclone åt folket...(Kubmätt). But on the power supply I have to smaller capacitors parallel with the two 10000uF, not sure if that is totally correct , but from what I could see in the picture in the thread it shuld bee like that. The powerssupply circuit is connected to a 2 x 18V 4.44A torrodial transformer where the two middle cords are twisted together to make, at least what I think, the some kind of "zero" or earth. The output is positive and negative 25V when the power supply is not connected to the load. I suspect it might go down to ~18V when it is under load.


No it is not mounted to any heat sink but I don't think it should be that hot that quick. I suspect the oscillating noise could come from the chip's thermal protection that turns it on and off, but I'm not 100% sure it has thermal protection.

I can take some pictures if that would be any help.
  Reply With Quote
Old 30th June 2012, 03:11 PM   #5
diyAudio Member
 
Join Date: Jun 2012
Ok, I will try with a heat sink. But I think it is strange that it heat up that quick and make those loud bangs I explained in the first post. It takes just 2 or 3 seconds for it to heat up.
  Reply With Quote
Old 30th June 2012, 07:41 PM   #6
Puffin is offline Puffin  United Kingdom
diyAudio Member
 
Join Date: Feb 2006
Location: U.K
Have you made the amp exactly as the Datasheet schematic?

This "some kind of zero or earth" is fundamental to the correct operation of the amp. How have you connected it?

Also, you should always have the Op-Amp connected to a heatsink before testing.

Do you have a safety light-bulb tester in circuit when powering up? If not make one before you test it again. It will light up if there are any problems with your PS or amp circuit.

Test the power supply first before attaching it to the amp.

Yes. Take some pictures

Last edited by Puffin; 30th June 2012 at 07:44 PM.
  Reply With Quote
Old 30th June 2012, 09:34 PM   #7
tsiros is offline tsiros  Greece
diyAudio Member
 
tsiros's Avatar
 
Join Date: Jun 2012
Location: Patra, Greece
Send a message via MSN to tsiros Send a message via Skype™ to tsiros
to overheat that quickly there must be some sort of shortcircuit somewhere. the bangs might be the LM trying to protect itself? just a shot in the dark

what input does the amp have when you test it?

i feel bad that i do not know enough to be of help. i should be, dammit. i'm supposed to be a physicist
  Reply With Quote
Old 30th June 2012, 10:02 PM   #8
diyAudio Member
 
Join Date: Jun 2012
I haven't made it exactly as the one in the data sheet

Click the image to open in full size.
I have done exactly like this one.

Here is the data sheet with the schematic https://www.national.com/ds/LM/LM3875.pdf

The difference seems to be that there is no capacitors connected input and output in in the product sheet and there are some differences in the component values

As you can see in the pictures i have connected the gray and red cable from the transformer to ground according to this schematic. Click the image to open in full size.

I'm not sure what you mean with "safety light-bulb tester", but i have tested the PS with a voltmeter and it gives out a positive and negative 25V when not under load. I think this will go down to about 18V which the transformer is rated to. I have tested the PS with two 12V lightbulb in series between both positive to gnd and negative to gnd and they shine nice and steadily.

The rectifier bridge might look a bit strange but it is just one "double-diode" (which I hope works just like two separate diodes) and the four other is two pair of parallel connected diodes (to handle the current).

I tried to mark the positive, negative and ground in the pictures but i'm not sure if that will make it any clearer because it got a bit messy.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Last edited by guran604; 30th June 2012 at 10:26 PM.
  Reply With Quote
Old 30th June 2012, 10:17 PM   #9
diyAudio Member
 
Join Date: Jun 2012
Quote:
Originally Posted by tsiros View Post
to overheat that quickly there must be some sort of shortcircuit somewhere. the bangs might be the LM trying to protect itself? just a shot in the dark

what input does the amp have when you test it?

i feel bad that i do not know enough to be of help. i should be, dammit. i'm supposed to be a physicist
I tried both with and without input but it made no difference, just the banging sound.

I have looked for shorts but i couldn't find any. But since it is the same problem with both amps (two identical for stereo) it seems unlikely I have made the same mistake two times.

I also fugured it might be the amp tries to protecting itself from overheating, but I cant fugure out what is causing it

Maybe the pictures i posted in my last post will help you use your physicist skills to solve the problem
  Reply With Quote
Old 1st July 2012, 01:18 AM   #10
tsiros is offline tsiros  Greece
diyAudio Member
 
tsiros's Avatar
 
Join Date: Jun 2012
Location: Patra, Greece
Send a message via MSN to tsiros Send a message via Skype™ to tsiros
this is the PS test with the lightbulb that people are mentioning

Building a Gainclone chip amp power supply.
  Reply With Quote

Reply


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
overheating amp sndtch6 Solid State 1 21st April 2004 10:30 AM
Can overheating be the cause? Super Solid State 7 24th December 2001 12:58 PM


New To Site? Need Help?

All times are GMT. The time now is 11:33 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2