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Old 12th July 2009, 04:19 PM   #4491
tinitus is offline tinitus  Europe
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4U may work, depending on air temperature
Heat sink is 300x165x40

My guess is that its Fischer heatsink
Sk501 sem to fit the specced size used in modushop box
Increasing from 150mm to 200mm height doesnt seem to make significant improvement
Its about 0.3K/W in both sizes
Curve is almost horisontal at 150mm height, so no further improvement with increased size, it seems

But 40mm fins is on the small side and not the most effective
SK157 from Fischer has 83mm fins, that seem to be very effective
Its close to 0.2K/W at 200mm, and still more to gain, it seems


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Old 12th July 2009, 04:23 PM   #4492
massimo is offline massimo  Italy
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Quote:
Originally posted by pro
Hi Massimo, I think i'll go for 4U 400, so I'll have a bit more room for the PS that is a bit overkill (ingombrante dir poco)

The only problem with the 400 mm unit is that each side/heatsink is devided into two 200 mm pieces mounted side by side.
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Old 12th July 2009, 04:31 PM   #4493
tinitus is offline tinitus  Europe
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Quote:
Originally posted by massimo


heatsink is devided into two 200 mm pieces mounted side by side.

good post

And I obviously only looked at 300mm case
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Old 12th July 2009, 04:33 PM   #4494
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Quote:
The only problem with the 400 mm unit is that each side/heatsink is devided into two 200 mm pieces mounted side by side.
So, just one piece per mosfet
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Old 12th July 2009, 04:39 PM   #4495
udailey is offline udailey  United States
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With one side split into two heatsinks you actually get a tiny bit more efficiency out of the heatsinks.
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Old 12th July 2009, 06:45 PM   #4496
massimo is offline massimo  Italy
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To get the max efficiency, one should fix the two mosfet exactly at the center of each heatsink. That means about 200 mm apart and at 82 mm high
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Old 12th July 2009, 07:25 PM   #4497
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A question regarding j-fet.
Is it normal that while k170 at 6ma bias, on 30 units, I have found a Vgs which varies between 20 and 80 mV while the J74 varies between -500 and -1500 mV?
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Old 12th July 2009, 07:37 PM   #4498
EUVL is offline EUVL  Europe
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> while the J74 varies between -500 and -1500 mV?

Did you mean between -500 and -1500 mV or between 50 and 150 mV ?

If you meant -500 and -1500, then it is not normal. Firstly I expect your Idss to be larger than 6mA, in which case Vgs should be POSITIVE and not negative for 2SJ74, and vice versa for 2SK170.

But suppose that Vgs is positive, and even with the V-grade at 16mA Idss, the Vgs should not be more than +400mV at 6mA bias. Please refer to Page 2 of datasheet.

If you indeed measured -1500mV (i.e. the gate voltage is lower than source), you are for certain forward biasing the JFET, and I would have expected a much larger current than 6mA.

Or I misunderstood you completely.


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Old 12th July 2009, 08:02 PM   #4499
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Hi Patrick. Thank you for the answer.
I arranged this small schematic, and the values I have read between ground and source are from -500 to -1500 with the most, on 30 units, between -800 and -1300 mV. Only one is -60 mV.

Click the image to open in full size.
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Old 13th July 2009, 12:16 PM   #4500
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I can't understand (may be the warm weather) your schematic: gate-drain is 50v? No load? Source resistor = 3870R? Source at 0V ?
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