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Old 26th July 2005, 07:31 AM   #1
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Question mj15003 running at 80 degrees

Hi,
Is this to hot for those transistors. What would be recomended temperature for them - I am running them in hood amp 21-0-21V 1.5A. Heatsinks are at 50-52 degrees.
best regards
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Old 26th July 2005, 07:44 AM   #2
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Heatsinks are at 50C., Why/how is MJ at 80C?

It's good for 200C but derate from 25C i.e. 250W x 120/175.

Cheers,
Greg
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Old 26th July 2005, 12:16 PM   #3
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I will try to explain.... My mj15003 are connected to the 5mm aluminium L profile. That profile is connected to the 8mm 330x150mm aluminium plate that hold 12 small heatsinks together as one heatsink. picture attached
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Old 26th July 2005, 12:19 PM   #4
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picture from the top of the heatsink..... the problem is bad airflow perhaps - because of the small heatsinks - they have their radiators in the wrong direction (the don't go from the side plate to left and right but they go along the side plate of the housing)..... Also, probably - i have that aditional 8mm alu- plate that represents aditional thermal resistance.... I will try to put small heatsins on the top of every mj15003 to aditionally cool them also.... maybe this would work to turn the temperature down.....
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Old 26th July 2005, 12:50 PM   #5
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I would say that that plate you have sandwiched between th L shaped metal and the heatsink probably doesnt help.

The more gaps you have to bridge using heat sink compound the more thermal resistance you get, and if your transistors are much hotter than your heatsink then their is definatly a thermal resistance issue.
They should be of a much closer temperature if well mounted.


Craig
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Old 26th July 2005, 05:38 PM   #6
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Quote:
Originally posted by Craig405
I would say that that plate you have sandwiched between th L shaped metal and the heatsink probably doesnt help.

The more gaps you have to bridge using heat sink compound the more thermal resistance you get, and if your transistors are much hotter than your heatsink then their is definatly a thermal resistance issue.
They should be of a much closer temperature if well mounted.


Craig

I know, I know- but that plate is holding my heatsink together - I could only put other transistors like some to247 types or something - but I have allready buyed mj15003...... I would try to put some heatsinks on the mj15003. Maybe that will help...
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Old 26th July 2005, 06:50 PM   #7
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It's too hot if you can't hold your fingers on it!

IMO TO3 transistors are durable and reliable, but hard to put on heatsinks. I've done the L-bracket thing before, and the heatsink will be only warm, while the bracket and transistors are scalding hot.

MJL 4281/4302 are 230W Transistors TO-247. They are easy to mount compared to TO3, and handle nearly as much power.
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Old 26th July 2005, 09:17 PM   #8
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Quote:
Originally posted by EWorkshop1708
It's too hot if you can't hold your fingers on it!


thats what i thought..... i must think of something

btw... thnx for the tip regarding mjl**** --do You know, maybe, how do they sound in hood amp??
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Old 27th July 2005, 09:18 AM   #9
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You need to reduce the losses at your junctions...ideally you would have the surfaces where they meet machined so that they make as perfect contact as possible, and use heatsink grease at those junctions.

At a minimum, you need to sand the surfaces as best you can, and use heatsink grease, and a slew of bolts to hold the assemblies firmly together (drilled and tapped holes to bolt everything together also need to be ever-so-slightly countersunk to account for the rise in the material caused by drilling the holes).
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Old 27th July 2005, 10:10 AM   #10
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Quote:
Originally posted by EchoWars
You need to reduce the losses at your junctions...ideally you would have the surfaces where they meet machined so that they make as perfect contact as possible, and use heatsink grease at those junctions.

At a minimum, you need to sand the surfaces as best you can, and use heatsink grease, and a slew of bolts to hold the assemblies firmly together (drilled and tapped holes to bolt everything together also need to be ever-so-slightly countersunk to account for the rise in the material caused by drilling the holes).
Here is how I have made it..... everything is o.k. and connected very good with silicon pads between the trans and the L profile
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