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Old 6th October 2011, 07:17 AM   #1
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Default Help with Super Cascode Power Supply

I wonder if any of you gents can help me. I have a couple of circuit boards I purchased a while ago from White Noise Audio. I'm trying to configure one to give 3.3V and 5V selected by the jumpers. I have attached the supplied documentation.
In the documentation it says that each output can be set to any value from 3V - 35v dc by altering the value of two resistors. The formual given is :

Vout=6.9(R7 + R8)/R8 or Vout=6.9(R10+R11)/R11

Now i'm not a mathematical genious but even I know that Vout can not drop below 6.9V given the above formual. Looking at the schematic 6.9V is the value of RF1 and RF2, so what I thought, would happen if I changed this to 2.5V and used the above formula to derive the values for R8 and R9 (6K5 and 3K3). Using a 12V 0.5A transformer the output was 9V - not what I was hoping for.
Can anyone help in configuring this circuit to output the desired voltages. My knowledge of electronics won't streach to this. I think possibly I might have to change Z1 and Z2 as well.
Attached Images
File Type: jpg SCPSU1.jpg (744.7 KB, 224 views)
File Type: jpg SCPSU2.jpg (577.5 KB, 190 views)
File Type: jpg SCPSU3.jpg (780.5 KB, 161 views)
File Type: jpg SCPSU4.jpg (739.9 KB, 144 views)
File Type: jpg SCPSU5.jpg (636.8 KB, 126 views)
File Type: jpg SCPSU6.jpg (289.6 KB, 25 views)
File Type: jpg SCPSU7.jpg (317.1 KB, 128 views)
File Type: jpg SCPSU8.jpg (235.2 KB, 127 views)

Last edited by stonedowner; 6th October 2011 at 07:42 AM.
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Old 6th October 2011, 08:04 AM   #2
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Just added a better copy of sheet 7
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File Type: jpg SCPSU7.jpg (289.9 KB, 136 views)
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Old 15th October 2011, 08:24 PM   #3
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can anyone help please?
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Old 15th October 2011, 09:00 PM   #4
DF96 is offline DF96  England
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It looks like the output voltage also acts as the supply voltage for IC1, so this might set a minimum. I think you are right to assume that you can't go below the reference voltage.
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Old 17th October 2011, 05:14 PM   #5
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Replaced Z1 with a 3.3v zener and am now getting 4.2V and 4.98V out, Seem to be getting there. Will try a 2.5V zener tomorrow.
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Old 18th October 2011, 01:24 PM   #6
AndrewT is offline AndrewT  Scotland
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5mA to tr5 base seems a bit mean.
The D44 range have fairly low gain and may require more than the 5mA provided since some of this must go the alternative route through tr7 and the opamp.

You could probably use a LED instead of a Zener as the voltage raiser.

BUT do go and read Walt Jung and Janneman on self starting and getting the opamp to work consistently.
Can your opamp work with a voltage way down there?

The tracking pre-reg only applies ~2V of Vce to tr5. Since you are at very low voltage on the output what about raising the pre-reg to say 4V above the output voltage and then modifying the PCB slightly to use the pre-reg output to power the opamp?

Anyone see problem/s with those ideas?
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Last edited by AndrewT; 18th October 2011 at 01:35 PM.
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Old 19th October 2011, 06:51 AM   #7
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Replaced Z1 with a 2.7V zener which made no difference. Checked the supply voltage range for the AD843 op amp. It's from 4.5V to 18V so it won't work down to 3.3V which is a problem. Can anyone suggest a suitable op amp that will work down to 3.3V?
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Old 19th October 2011, 08:09 AM   #8
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Looks like a OP162GSZ will do the job. Will order a few and see how it goes.
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Old 19th October 2011, 08:23 AM   #9
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The OP90GP DIP8 looks ok and I won't have to use a SOIC to DIP adaptor
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Old 28th October 2012, 07:50 PM   #10
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An LM317 will make voltage sag by about 3v and therefore the transformer needs to supply over 15vac to get 12v out
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