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Old 23rd July 2012, 08:21 AM   #5181
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Quote:
Originally Posted by merlin el mago View Post
To avoid drift measures resistance in the trimmer & change for a low ppm fixed resistor. Do you have a scope? if yes see if with the 47uF near the DAC you have ripple or oscillates, if I remember well all shunt regs designed by Salas don't like a cap between the reg & the load.
thanks!
caps have been removed as adviced. Unfortunately do not have any scope, planning to get one in the following month...

Regarding the voltage, not sure if swapping trimmer to fixed resisitor will help as I have also negative rail with the same pot and there is no voltage raise at all. Also notice that with closed lid of the DAC, case becomes very hot (about 50-55 c inside) and the output voltage raises up to 5.25vdc, though negative rail stays the same...

what could that be?!
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Old 23rd July 2012, 08:25 AM   #5182
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JFET's Tc in your hot box. Try BF245A Q3.
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Old 23rd July 2012, 08:36 AM   #5183
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P.S. You may need 1k trimmer with such a fet if it has under 5mA IDSS.
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Old 23rd July 2012, 10:53 AM   #5184
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Originally Posted by Salas View Post
JFET's Tc in your hot box. Try BF245A Q3.
thanks, I'll try it.
could you please elaborate a bit more about 5mA IDSS, not sure if I got it
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Old 23rd July 2012, 11:20 AM   #5185
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You get a range of samples, each one runs different max current when tested with 9V battery when VGS=0. That max current is IDSS, and its a central characteristic to JFETs.
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Old 23rd July 2012, 11:58 AM   #5186
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Some drift more (temperature coefficient) especially the low noise ones with strong gain. Since you got a hot box inside you may try compromise a bit with the one I suggested. Its less affected although not that strong but adequate.
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Old 23rd July 2012, 12:03 PM   #5187
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You may have chanced to one sample that counteracts better the other semis Tc in the negative. Or it resides in a cooler spot.
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Old 23rd July 2012, 01:04 PM   #5188
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thank you for the help, Salas!

I have some dozen of sk170, so will try to match idss for all 3 jfets in the reg. Alternatively use suggested alternative! Found here how it could be done very easily.
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Old 23rd July 2012, 03:01 PM   #5189
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When matching FETs be sure battery have 9V or values will be not accurates.
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Old 23rd July 2012, 07:38 PM   #5190
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measured few of spare sk170s and all appeared in the range from 2.2-4.6mA... then I realized that I have GR version that as written here is limited to 6mA unlike BL that goes up to 12mA. So I'll try to find BF245A and see if it helps from voltage raises.
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