Current in heatsinks - diyAudio
Go Back   Home > Forums > General Interest > Everything Else

Everything Else Anything related to audio / video / electronics etc) BUT remember- we have many new forums where your thread may now fit! .... Parts, Equipment & Tools, Construction Tips, Software Tools......

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 11th August 2004, 08:14 PM   #1
Vivek is offline Vivek  India
diyAudio Member
 
Join Date: Nov 2000
Location: Bangalore, India
Default Current in heatsinks

Hi all,
I built an amp sometime ago and it has MJ2955/2N3055 at the output. The amp works normally (no hum etc.) but when working the output transistor heatsinks seem to be conducting some current. I can feel it when I touch the heatsinks. Is this a simple case of the collectors not being insulated from the heatsinks properly?
Thanks in advance for any help.

Vivek
__________________
Thank God for DIY audio.
  Reply With Quote
Old 11th August 2004, 08:31 PM   #2
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Vivek,

Just for the sake of clarity: you feel that there is some VOLTAGE at the heatsink, I guess?
If you had BOTH transistors not well insulated, it wouldn't work. It can be that ONE transistor is not well insulated, and that is a disaster waiting to happen. It would mean that the heatsink is at either +V or -V, and if you connect it to gnd or another point on the amp, pooof!
So, to verify, measure the heatsing voltage with a multimeter against gnd. If there is a voltage, take care of the insulation of the power transistors before you blow up anything.

Jan Didden

PS It is possible that there is nothing wrong, that the heatsink is at zero volts, but that you touch a voltage point with your other hand. Same effect, what you feel is voltage difference, not absolute voltage.
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham
Check out Linear Audio!
  Reply With Quote
Old 12th August 2004, 08:08 AM   #3
Vivek is offline Vivek  India
diyAudio Member
 
Join Date: Nov 2000
Location: Bangalore, India
Hi,
This voltage I `feel' in the heatsink is there on both channel heatsinks. I have checked the insulation and the mica washers are all there. There is no voltage between the heatsink and ground.
Vivek
__________________
Thank God for DIY audio.
  Reply With Quote
Old 12th August 2004, 08:18 AM   #4
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Vivek,

You can only feel voltage between two terminals. One apparently is the heatsink. What are you touching with the other hand? Are you maybe barefoot on the floor tiles? Arms on a chassis that is grounded or connected to mains neutral?

Jan Didden
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham
Check out Linear Audio!
  Reply With Quote
Old 12th August 2004, 08:22 AM   #5
Vivek is offline Vivek  India
diyAudio Member
 
Join Date: Nov 2000
Location: Bangalore, India
You are right. Voltage is across two points. But I feel this even if I touch it with one hand with the other hand not touching anything. I might be sitting with this stuff on the bed, on the sofa or standing barefooted on the mosaic-tiled floor but it is still there.
__________________
Thank God for DIY audio.
  Reply With Quote
Old 12th August 2004, 08:24 AM   #6
Jennice is offline Jennice  Denmark
diyAudio Member
 
Join Date: Jun 2003
Location: Planet Earth
Vivek,

It may be the case that you feel some potential relative to "absolute" ground (your mains supply ground entering the building you live in.)
Potential can couple capacitively and inductively, so the entire amp is not at ground potential. Maybe the entire device is at a potential. Try measuring the heat sink or enclosure potential relative to mains "earth", but do so with a voltmeter that has a high input impedance (digital multimeter or similar). The old fashioned voltmeters with a scale and pointer have much lower input impedance, and thus you will measure a lower voltage.

Jennice
__________________
I get paid to break stuff. My g/f gets paid to play with children. Life is good.
  Reply With Quote
Old 12th August 2004, 08:25 AM   #7
diyAudio Member
 
Join Date: May 2002
Location: Switzerland
Quote:
This voltage I `feel' in the heatsink is there on both channel heatsinks. I have checked the insulation and the mica washers are all there. There is no voltage between the heatsink and ground.
If you say ground, I assume you are talking about signal ground of the amp.
In this case there may be a voltage between the chassis and earth , that could be caused by an insulation failure. This can in fact be very dangerous !

If the chassis is earthed it could even becaused by the electrical installation of your home. So be very careful.

It could also be something less severe (like capacitive coupling in a transformer or mains filter), since such things are difiicult to judge remotely, but it always pays to be careful.

Regards

Charles
  Reply With Quote
Old 12th August 2004, 08:36 AM   #8
Vivek is offline Vivek  India
diyAudio Member
 
Join Date: Nov 2000
Location: Bangalore, India
Jennice,
I will measure heatsink to mains earth and let you know.

Charles,
I am talking about the signal ground of the amp. Actually, the heatsinks are not mounted on any chassis. I was just testing the amp and the heatsinks are kept separately. The PCBs are just kept in a metal chassis but on a piece of wood. Maybe I should replace all mica washers and check again.
__________________
Thank God for DIY audio.
  Reply With Quote
Old 12th August 2004, 09:22 AM   #9
diyAudio Member
 
Join Date: May 2002
Location: Switzerland
Jennice was a little quicker than me. Basically I wanted to say the same but maybe I was alittle less clear.

Regards

Charles
  Reply With Quote
Old 12th August 2004, 09:35 AM   #10
diyAudio Member
 
wuffwaff's Avatar
 
Join Date: Aug 2001
Location: Ingolstadt Germany
Vivek,

are you shure you are feeling a voltage? Sometimes touching the sharp edged heatsinks can give the sensation of feeling a voltage, especially when you let you fingers run over the edges.

William
__________________
een ooievaar is geen konijn want zijn oren zijn te klein!
  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
no current mirror, current mirror, cascoded current mirror, bjt or jfet VAS? maxpou Solid State 31 17th June 2008 04:31 PM
Current rating and tube current draw cbutterworth Tubes / Valves 1 13th October 2006 04:42 AM
Using Ixys current regulator as current sink. G Solid State 3 16th November 2002 10:23 AM
R21? and Current Source AC Current Gain macka Pass Labs 9 20th May 2002 08:58 PM


New To Site? Need Help?

All times are GMT. The time now is 11:54 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