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Old 22nd May 2004, 02:07 PM   #1
Wagener is offline Wagener  South Africa
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Default isolating power transistor from heatsink

I am building two leach channels.

I will be mounting 4 power transistors on one piece of heatsink.

Which transistors is mounted together? I suppose it is 2 x MJ15003 and 2 x MJ15004 on each heatsink?

How do I isolate the collectors of the MJ15003's from the collectors of the MJ15004?

In the construction details Dr. Leach mentions that you don't use plastic isolation caps when mounting the transistors.

I am concerned about the the two npn and pnp transistors shorting the +VCC and -VCC when the collectors are connected.

I need to connect my collector wire at the back of the heatsink where I also connect base, emitter.
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Old 22nd May 2004, 03:17 PM   #2
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you can get isolating kits where you need ato drill a slightly bigger hole (to fit the isolating thingy) and then use a lug with it's screw and nuts...to get to the collector...if you don't want such hard work...I suggest going TO-220 (Flat pack) style...it's a lot easier...since you only need a mica or silicon pad...
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Old 22nd May 2004, 07:25 PM   #3
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Default Here is too hot, somedays goes to 35 degrees

This way, i am always afraid of insulators, with mica or other materials, using thermical compound or not, normally the heat transfer is a problem... sometimes you burn your finger on transistor case and the heatsink to too hot, this way, can melt... when we have hi temperature in case, you all knows that, chip temperature is really a lot of degrees up!.

Knowing very well the risk of electrical injuries, i prefer to put transistor directly into heatsinks with a lot of thermical compound having sure if both surfaces are real flat, if whole surface is touching one each other. I used to insulate with wood heatsinks one related to the other, and insulate heatsinks from chassis with wooden strips... if internal is better because risk diminishes related to accidents.... i use always a fan working with 8.5 dc volts to not make noise and a small transformer, rectifier and filter specially to fan blowers (use always two, if one stops....).

I am completely neurothic related temperature, if i perceive temperature in heatsinks bigger than a human hot fever, i increase size and fan speed!

My "neurotronic heat transfer process" never failled (hahaha)

Mica is wonderfull, but weather here is terrible!

Carlos
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Old 22nd May 2004, 07:45 PM   #4
lucpes is offline lucpes  Romania
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As a rule of thumb, for 'taking' temperature with hand, 40 cellsius is warm but ok, 45C begins to be uncomfortable (you 'have' to remove your hand after 5-6 seconds and starting with 50C it is too hot).

Make sure the heatsink is below 45C.

good readings:

http://mitglied.lycos.de/Promitheus/new_page_2.htm

http://sound.westhost.com/heatsinks.htm
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Old 23rd May 2004, 01:02 AM   #5
K-amps is offline K-amps  United States
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I hear death Valley CA and Ben Ghazi Libya get up to 55C on a pleasant summer afternoon.... Sheesh Kabob!!!

So what temp does the amp maker there consider equilibrium?
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Old 23rd May 2004, 01:28 AM   #6
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Default Related our computers

The people say that when CPU reach 50 degrees centigrades is not good, better to try to improve ventilation and keep some two or three degrés less..... you know, there are program to monitor main chip temperature.
Here i openned holes on computer back, and i put one more fan into the microprocessor, and also another in the cabinet enclosure....holes one side and fan sucking air the other side... chip temperature reduces to 45 degrees celsius.... my home temperature is around 29 degrees whole day and reduce to 25 at nigth with hard winds that makes sound of flute when passes the aluminium doors and windows... i leave in front of the beach, in Northeast of Brazil, looking at the brazilian map, south american map, you will see an eastern point that is the nearest point related to Africa..... the small distance to cross ocean.... here, the wind is constant and hy speed wind, also temperature do not change too much all over the year.
I suppose that computers in an 55 degrees environment, will reach 71 degrees centigrades in its surface.... i think it will fail or may use some kind of control..... alike switch on and off, as interruptions not to consume too much energy, and work slowly to avoid overheating... things alike that..... do not know.
But is a very good question, i am waiting some expertise to came here and put some like in our darkness (mine darkness i guaranteed, your only you can say)
regards,
Carlos
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Old 23rd May 2004, 04:15 AM   #7
K-amps is offline K-amps  United States
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North-East Brazil eh Carlos? Lucky B**tard!
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Old 23rd May 2004, 05:16 AM   #8
jaycee is offline jaycee  United Kingdom
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You can mount all transistors on one heatsink provided they are all isolated.

The bolts must have insulating washers that protrude down into the holes in the transistor and extend to cover the top of the bolt going through them. The lug where the connector is connected must of course be arranged so that it is in contact with the case of the transistor, but not the bolt.

I prefer to use silicone impregnated pads instead of mica washers as they avoid having to spread on heatsink paste which is messy and dries out after a few years of use at high temperature.

You could also build the amp with MJL21193/94 or MJL4281/4302 which are TO-264 flatpack transistors and only require an insulator between it and the heatsink.
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Old 23rd May 2004, 10:08 PM   #9
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Default K Amps, good afternoon, boa tarde

Boa tarde para você também... good afternoon to you too... yes!...lucky.

Carlos
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Old 23rd May 2004, 10:44 PM   #10
K-amps is offline K-amps  United States
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Default No spkida de Portuguese.

Carlos,

How far are you from Salvador or the real Carnival fun?

K-
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