Calculate the current through your fixed resistor, or find out the voltage across the fixed resistor.
Determine the required voltage across that fixed resistor location to give the output voltage you require.
Calculate the parallel resistor that needs to be added to the existing fixed resistor to give the new resistor value.
Add in the temporary parallel resistor and check the output voltage. This temporary parallel resistor can be clipped in place with longish leads, or soldered in place across the existing fixed resistors leads. This paralleled method is far quicker than swapping out a single resistor on a trial and error basis. It is also far less likely to damage the PCB.
BTW, the paralleled resistor can be used to fine trim the output voltage and left in place permanently. It just does not look so neat, (unless the PCB has provision for a paralleled trimming resistor).
Finally learn to do some arithmetic. We got taught it in school to help us cope with life beyond school.
Determine the required voltage across that fixed resistor location to give the output voltage you require.
Calculate the parallel resistor that needs to be added to the existing fixed resistor to give the new resistor value.
Add in the temporary parallel resistor and check the output voltage. This temporary parallel resistor can be clipped in place with longish leads, or soldered in place across the existing fixed resistors leads. This paralleled method is far quicker than swapping out a single resistor on a trial and error basis. It is also far less likely to damage the PCB.
BTW, the paralleled resistor can be used to fine trim the output voltage and left in place permanently. It just does not look so neat, (unless the PCB has provision for a paralleled trimming resistor).
Finally learn to do some arithmetic. We got taught it in school to help us cope with life beyond school.
Last edited:
Finally I ended paralleling 15R with 55R = 11R78 to have 5.25Vout (thanks AndrewT a very good idea to get the Voltage neede without damage the pcb), no ripple with scope🙂
Now I understand why is necessary a scope to check the ripple, we can have the voltage but if there is ripple the sound is worst than without ripple.
The BC550 contributes 0.6V, the 1N4007 you used another 0.6V, the rest is all the resistance you got in series times your associated K117 current. If the things don't add up normally for K117GR IDSS range then there is something wrong with it.
Is Q103 the jFET with 0.6mV across D to S?
If that is so, then the Idss of 4.3mA measured at 10Vds will be much different from Id when 0.6Vds is applied.
Expect Id ~ 50%Idss, i.e. around 2mA to 2.5mA.
The output voltage will be Vbe of the BJT plus the Voltage across the Vref string.
The Voltage across that Vref string will be due to the Id passing that jFET of about 2mA to 2.5mA.
If the Vref string were a diode + a Zener (5.6V) + fixed resistor (1k0) then the voltage drop due to 2mA of current flow is ~ 600mV for diode + 5.6V for Zener (if Pz >10% of Pmax) + 0.002 * 1000 ~= 8.2V
One can short out (link across) or add as many components to the Vref string as needed, to get the desired Vout.
Just measure the in circuit Id of the jFET by measuring the voltage drop across the fixed resistor. That resistor can be 10r. 2mA and 10r would read ~20.0mVdc using a 199.9Vdc scale of a dmm, good enough accuracy for our purpose.
It really is that simple, some simple arithmetic combined with an understanding of what the circuit is doing.
If that is so, then the Idss of 4.3mA measured at 10Vds will be much different from Id when 0.6Vds is applied.
Expect Id ~ 50%Idss, i.e. around 2mA to 2.5mA.
The output voltage will be Vbe of the BJT plus the Voltage across the Vref string.
The Voltage across that Vref string will be due to the Id passing that jFET of about 2mA to 2.5mA.
If the Vref string were a diode + a Zener (5.6V) + fixed resistor (1k0) then the voltage drop due to 2mA of current flow is ~ 600mV for diode + 5.6V for Zener (if Pz >10% of Pmax) + 0.002 * 1000 ~= 8.2V
One can short out (link across) or add as many components to the Vref string as needed, to get the desired Vout.
Just measure the in circuit Id of the jFET by measuring the voltage drop across the fixed resistor. That resistor can be 10r. 2mA and 10r would read ~20.0mVdc using a 199.9Vdc scale of a dmm, good enough accuracy for our purpose.
It really is that simple, some simple arithmetic combined with an understanding of what the circuit is doing.
Last edited:
K117 loses a bit IDSS under only 0.6V but not as much as 50%. Measure live in circuit to know each sample in each Vref.