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Old 19th November 2012, 11:18 AM   #2631
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hi,
I finally got my boards working with 2n3055, so thanks to all that helped with advises
Now I got a few more questions, that I can't figure out:

I got some radiators that I bought for the TO3 case style, but they aren't very big, and the transistors get really hot, like 130 Celsius. Is that to be expected? And should I get bigger radiators, or could there be a problem with the circuit? - it sounds ok, as ok it can considering my source is a PC.

Second, I left VR2 at maximum resistance, If I start to decrease it's value, do I risk to get the Q1 and Q1 hotter? The specs say it can withstand 200 deg Celsius, but I really don't want to risk it.

tanks.
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Old 19th November 2012, 11:28 AM   #2632
Mooly is offline Mooly  United Kingdom
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Great to hear you have it working.

Anything over around 50 C in use is too hot for long term reliabilty. The 200 C will be the temperature the silicon can stand with no current or voltage applied. For the transistor to run at its max electrical loading and within its safe operating area then you are looking at keeping it around 25 C.

It sounds like your heatsinks are far to small. The heat problem, as your are finding out is the big big drawback to class A operation.
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Old 19th November 2012, 12:35 PM   #2633
AC439 is offline AC439  United States
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Quote:
Originally Posted by haiduc32 View Post
hi,
... and the transistors get really hot, like 130 Celsius. Is that to be expected? And should I get bigger radiators, or could there be a problem with the circuit? .....
Too hot. As I said in my previous post, I use a silent fan for added cooling. Post a pic of your heat sink so that we can tell if it is under-rated. Class-A amps usually have massive heat sinks. I have my Iq set at 1.2A, which is not as high as some of the others set to. But I still have to add a fan. My amp draws 150W from the mains.
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Old 19th November 2012, 12:48 PM   #2634
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Hi, now I am starting to get a picture of how under rated my sinks are..
here is what I got :
http://michaelcarden.net/luxeon/heatsink.jpg
I'll try to put them under a PC CPU fan, to see if I can get any improvements, but I guess I should start looking for something bigger. I was looking into CPU heat sinks, but it seems too complicated to drill them.

AC439, any chance of a picture of your heat sinks?
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Old 19th November 2012, 12:53 PM   #2635
AC439 is offline AC439  United States
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Quote:
Originally Posted by haiduc32 View Post
Hi, now I am starting to get a picture of how under rated my sinks are..
here is what I got :
http://michaelcarden.net/luxeon/heatsink.jpg
I'll try to put them under a PC CPU fan, to see if I can get any improvements, but I guess I should start looking for something bigger. I was looking into CPU heat sinks, but it seems too complicated to drill them.

AC439, any chance of a picture of your heat sinks?
Oh no ! Your heat sinks are way too small !!! See my thread below with pics:-

My JLH 1996 EVO....output transistors questions....
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Old 19th November 2012, 01:00 PM   #2636
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Thanks!
You saved me the trouble of hacking another project of mine in order to get the fans.
That means back to component hunting!
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Old 19th November 2012, 01:09 PM   #2637
AC439 is offline AC439  United States
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Yeah, you need much better heat sinks. Also, CPU fans are smaller, spin faster and make more noise. They are not good candidates for active cooling in amps (which we want no distracted noise from anything).

As you can see, my heat sinks are bigger but still needs a fan. I will be building another JLH amp in the future and the first thing I did was to get bigger heat sinks. I have collected four beautiful gold color heat sinks the size of 8.5" x 4" x 3". Each of them can install 4x TO-3 transistors.
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Old 20th November 2012, 08:10 AM   #2638
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Default Still too small

Quote:
Originally Posted by haiduc32 View Post
I was looking into CPU heat sinks, but it seems too complicated to drill them.
Still too small. You're pushing out c. 50W of heat, you want to keep your heatsinks below 55C

If ambient is c. 30C then you need below 0.5 degrees/watt

Find something like this Click the image to open in full size. W=300mm, H=75mm, R=0.36d/w

(From Conrad Heatsink MF30-1F-75)

Last edited by thoglette; 20th November 2012 at 08:11 AM. Reason: Add dimensions
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Old 20th November 2012, 10:17 AM   #2639
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by thoglette View Post
Still too small. You're pushing out c. 50W of heat, you want to keep your heatsinks below 55C

If ambient is c. 30C then you need below 0.5 degrees/watt

Find something like this Click the image to open in full size. W=300mm, H=75mm, R=0.36d/w

(From Conrad Heatsink MF30-1F-75)
it's worse than that.
50W into a 0.5C/W sink gives a predicted 25Cdegrees deltaT.
Look up the sink manufacturer's data for deltaT de-rating and you may find that the DF ~1.2 to 1.3.
Apply this to the 25Cdegrees to get a closer prediction for deltaT of ~30C to 33C
Now add this to Ta of 30degC to get Ts ~ 60degC to 63degC.
Then add deltaT for the Rthc-s. Tc = ?
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Old 20th November 2012, 12:17 PM   #2640
Bonsai is offline Bonsai  Taiwan
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If you look at my sx-Amp here Ovation sx-Amplifier you can get some idea of the heating requirements for a class A amp - in this case 15W per channel. My heatsinks are 0.4W per channel and run at between 60 and 65 C. The quiescent current is 1.4A on 22V rails.
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