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.
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.
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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?
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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