Installed the mosfet connected the preamp circuit and repeated the measurements:
Output regulator 23V
Noninverting opamp input 3V
Inverting input 0,65 V
When disconnecting the noninverting input of opamp i measure the 5v regulated by Lm4040-5V
These measurements are done after i replaced the led with 5v ref recommended here LM4040-5V and RP2,3 resistors. In the first phase i measured 40V so it worked in the first phase but after doing some work on it stopped working as described.
Output regulator 23V
Noninverting opamp input 3V
Inverting input 0,65 V
When disconnecting the noninverting input of opamp i measure the 5v regulated by Lm4040-5V
These measurements are done after i replaced the led with 5v ref recommended here LM4040-5V and RP2,3 resistors. In the first phase i measured 40V so it worked in the first phase but after doing some work on it stopped working as described.
What values are you using for RP2,3, and 4?
I've seen the 23 V condition in simulation. It seems to happen when the circuit doesn't "turn on" on start up. The initial current is high so the voltage drop across RP1 is high, this lowers the output voltage so much that after the zener PD there is only ~3-4 V on the op amp and it cannot start up normally. Without the op amp powering up, there is no way to increase the output voltage and no way of powering up the op amp. The circular dependency is what causes the issue. Powering the error amp off the regulator output is always a potential problem is this respect.
I'm afraid I don't know how to solve that. Anyone? Connecting PD to before PR1 and increasing its value to 33 V or so might work, but performance would take a hit. Maybe some sort of kickstarter circuit could be added, an additional connection to PSU_IN that turns off after the reg starts to work.
I've seen the 23 V condition in simulation. It seems to happen when the circuit doesn't "turn on" on start up. The initial current is high so the voltage drop across RP1 is high, this lowers the output voltage so much that after the zener PD there is only ~3-4 V on the op amp and it cannot start up normally. Without the op amp powering up, there is no way to increase the output voltage and no way of powering up the op amp. The circular dependency is what causes the issue. Powering the error amp off the regulator output is always a potential problem is this respect.
I'm afraid I don't know how to solve that. Anyone? Connecting PD to before PR1 and increasing its value to 33 V or so might work, but performance would take a hit. Maybe some sort of kickstarter circuit could be added, an additional connection to PSU_IN that turns off after the reg starts to work.
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It might be due to bias cancellation circuitry inside the opamp. A guess, given the low input voltage and current noise specs. Some bipolar opamps don't get enough current to start up and latch up on power up. The lm4562 will do that with around 65K of input resistance in certain circuits - guitar pedals.
Change it out for a OPA1641 or other FET input opamp.
Change it out for a OPA1641 or other FET input opamp.
The non-inverting should be 5V. What resistor do you have feeding the ref?
You may also have to place a diode from non-inv input to inv input to help start-up.
Jan
This is actual schematic i use now.
An externally hosted image should be here but it was not working when we last tested it.
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See the two red lines.
An externally hosted image should be here but it was not working when we last tested it.
Doesn't the LM4040 Reference need to be reversed (cathode to inv input)?
kind regards, Daniel
@atupi
I'm quite sure you have everything connected and working properly. The problem is not with you, but with the circuit itself when it turns on. There's a race condition: if the output current draw increases past a certain point before the opamp is ready to turn on to regulated that current, the op amp cannot turn on to regulate the current. Classic catch-22.
Try increasing RP2 to ~17.2k (45V output).
I haven't been able to figure out exactly where the problem is, but I am inclined to agree with disfunctionalshadow in that it is likely related to the tendency of certain opamps to latch on startup when their input voltages are out of range.
I'm quite sure you have everything connected and working properly. The problem is not with you, but with the circuit itself when it turns on. There's a race condition: if the output current draw increases past a certain point before the opamp is ready to turn on to regulated that current, the op amp cannot turn on to regulate the current. Classic catch-22.
Try increasing RP2 to ~17.2k (45V output).
I haven't been able to figure out exactly where the problem is, but I am inclined to agree with disfunctionalshadow in that it is likely related to the tendency of certain opamps to latch on startup when their input voltages are out of range.
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The startup of the regulator, when it works, looks something as shown.
The input and output voltages ramp up. While that happens, the voltage on the op amp is zero until until the Zener conducts, about half way through the sequence.
The voltages on the op amp inputs are meanwhile 5V and about 2.5 V respectively. When the output reaches 40 V, Vin+=Vin- and the opamp turns on the MOSFET. The turn on is very sharp.
This all seems fine. The op amp is powered up and controlling everything by keeping the MOSFET closed as the output voltage approaches the regulation value.
Perhaps Vin+ ended up latched to the positive rail V_opamp? That would certainly throw a spanner in the works.
The input and output voltages ramp up. While that happens, the voltage on the op amp is zero until until the Zener conducts, about half way through the sequence.
The voltages on the op amp inputs are meanwhile 5V and about 2.5 V respectively. When the output reaches 40 V, Vin+=Vin- and the opamp turns on the MOSFET. The turn on is very sharp.
This all seems fine. The op amp is powered up and controlling everything by keeping the MOSFET closed as the output voltage approaches the regulation value.
Perhaps Vin+ ended up latched to the positive rail V_opamp? That would certainly throw a spanner in the works.
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