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Old 26th July 2013, 06:12 PM   #1091
bcmbob is offline bcmbob  United States
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Fine, if I use the 12V zener I have, can I expect enough of a range from the pot to get the bias I need, or should I just wait for the correct 9.1V diode? I do get 25.6 VDC at all pads (both sides of PCBs) from neg rail - on both bias boards.
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Old 26th July 2013, 06:17 PM   #1092
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You will have even more voltage to work with. The pot takes the zener voltage and divides it down and presenst it to the gate of the mosfets. This sets their Dc bias and final outptu current value. The lower bias net will not work without being hooked up tot he output boards, as it needs to sense the voltage across the fet source resistors. That being said, I would not hook up the output boards until you have adjustablility on both of top bias nets. This setting will ultimately control the offset as it will cause the top fet to contribute an equal current to the ouput when compared to lower Cs. You know this is true when offet is zero or couple mV away.
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Old 26th July 2013, 06:27 PM   #1093
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OK, Thanks for the detailed explanation and your patience. I'll just get some new 9.1 Vs, as I'd rather stay within specs during this learning phase. I'll be back.......
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Old 26th July 2013, 06:28 PM   #1094
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the value is irrelevent. If yu have 12, use them. It has no bearing on outcome.
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Old 26th July 2013, 06:29 PM   #1095
bcmbob is offline bcmbob  United States
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Understood, I'll give it a shot.
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Old 26th July 2013, 06:45 PM   #1096
bcmbob is offline bcmbob  United States
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Perfect. Have 11.7 VDC now. Two more questions: should I replace the other 9 with a 12? And, at the risk of being super redundant and thick headed, with the output boards attached - all I'm looking for is 0 VDC across ground and the "out" pad on the output board - correct?
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Old 26th July 2013, 06:50 PM   #1097
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It is irrelevant. It has nothing to do with final output value. You should be able to adjust the voltage across ground and D+. If you cant and youhave 9-12v across the zener, then you may have a bad pot.
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Old 26th July 2013, 07:10 PM   #1098
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Both trimmers are working. Values match. Now what?

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Old 26th July 2013, 07:38 PM   #1099
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Turn the voltage down to 3V on the trimmers. Dont want the fets coming up hot. This assumes IRF outputs. Others may need lower voltage for safety. Hook up ouput board and power up. Check voltage across bjt, by probing neg rail and D-. THis should read about 5V. Check D+ again, May have dropped some. If all is OK, slowly bring up bring up D+, watching voltage across Rs.
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Old 26th July 2013, 07:42 PM   #1100
bcmbob is offline bcmbob  United States
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In progress - will report.
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