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Old 28th April 2012, 10:12 PM   #391
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Step 2
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Old 28th April 2012, 10:14 PM   #392
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No problem, thanks.
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Old 28th April 2012, 10:29 PM   #393
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TO-220 MOSFET drivers will dissipate app. 1,5 W at 50 mA driver's bias current (1,1 V voltage drop on 22 ohm resistors), so be prepared they'll be hot on these heatsinks, but no problem, they'll last easily at 75C for testing period hehe

I had to lower 2,2 k gate-source resistor to 1 k since we need at least 1,2 mA compound input bias current to operate properly. Input bias current will split evenly through both input BJT's, app. 0,6 mA through each BC550C/560C.

For pos/neg CCS current drive I put 2 k MT trimmer for easier setting of 2 mA through LED.

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Old 28th April 2012, 10:49 PM   #394
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Latfets are robust, sure.
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Old 28th April 2012, 11:04 PM   #395
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I have only these as high Ugs J-Fets. They should work although regulation is not that tight then with high Idss units.
2SJ103Y, 2SK246Y
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Old 28th April 2012, 11:15 PM   #396
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They'll be OK, especially if you make high Idss (at Ugs=0) selection and take the highest ones. Choosing Idss=10-14 mA would be 10 times more than operating conditions, so enough room for good current regulation.

Ohhh yeah, now I saw Y: 1,2-3 mA

Nothing you can do, just take 3 mA ones

Last edited by Lazy Cat; 28th April 2012 at 11:17 PM.
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Old 28th April 2012, 11:40 PM   #397
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I will use 2SJ103BL and make BL compounds from 3 Y SK246.
See also home made 1kOhm power resistors.
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Old 28th April 2012, 11:41 PM   #398
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I will work until 3.00.
When i need longer then that i make the rest tomorrow.
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Old 28th April 2012, 11:47 PM   #399
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Again few words about MOSFET Darlington gate-source resistors:
- ideal conditions would be to have 1,5 mA input bias current (0,65 mA + 0,85 mA Sziklai pair) and 2,2 k resistor value (high OLG), so voltage drop 3,3 V
- Urg=Ug1+Ug2=2,2 V + 1,1 V=3,3 V looking to graphs of drivers and outputs it means in this case drain current will be in amps, that's probably too much, so I lowered Rgs to 1 k
- basic condition is that you don't go below 1,2 mA for Sziklai bias, that's the most important
- start with 1 kohm, than you will see (according to 50 mA driver's bias and the output bias) if you can higher the Rgs to 1,5 or 1,8 kohm and still maintain 1,2 mA
- both positive and negative side gate-source resistors must be the same value
- output bias/DC offset conditions are set only with CCS currents-trimmers
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Old 28th April 2012, 11:50 PM   #400
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Quote:
Originally Posted by Joachim Gerhard View Post
I will use 2SJ103BL and make BL compounds from 3 Y SK246.
See also home made 1kOhm power resistors.
Yes good idea, they parallel very well

Resistors are now low inductance type

OK, 3:0 AM
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