D5 LED is not just reversed, but Veb of Q1 won't be enough to generate light. U1 is not a comparator, it is an OPAmp. It has to be frequency compensated, otherwise will oscillate.
BTW: OPAmp inputs are swapped. There isn't negative feedback.
how do i calculate the new values on R8 and R9 neglecting R7?
2,49v = (+V).(R9/(R8+R9))
I am using OP177 op amp (PN: OP177FPZ), can you explain your claim that the op amp inputs are swapped?
Despite all the comments, the schematic still doesn't look right to me:
I assume Q1 is a constant current source to help D6 (whatever it is) maintain a constant reference voltage. The LED is obviously reversed but even then it doesn't make sense, are you sure R4 shouldn't be in Q1 emitter pin. That would let the LED maintain a constant base voltage and hence emitter current of around (Vled - Vbe)/4700 = 0.2mA. That assumes a red indicator type LED.
I think for clarity the schematic should be corrected and some part numbers added before we go further.
Brian.
Hi,
One approach is to divide required output voltage by reference voltage to get the resistor ratios needed. I wouldn't omit the trimpot from the equation, I'd divide it equally across Ra and Rb, but anyway.
e.g. a upper R, b lower R in feedback divider:
Ra
Rb
((12/2.495) - 1) = 3.8096192384769539078156312625251
Then say Ra is 47k, subtract the 0s:
47/3.8096192384769539078156312625251 = 12.337190952130457653866386112572
Multiply 12.337190952130457653866386112572 by the number of 0s removed from previous step = 12k337
12337/59337 = 0.20791411766688575425114178337294
0.20791411766688575425114178337294 * 12V = 2.4949694120026290510137014004753
Calculated directly, 2.495/12 = 0.20791666666666666666666666666667, notice marginal/trivial difference in feedback ref value using a hypothetically and impossibly perfect 12k337 and 47k as divider resistors.
I've used this to find TL431 Ref pin resistor values quickly rather than by repetitive iterative approach.
No it isn't, it is uses as a crude voltage reference but even if flipped it can't possibly have enough voltage across it to light up. That's why I suspect R4 should be in the transistor emitter pin.the Red LED does not turn on, i will try to flip it in the morning, it's just an indicator let's forget about it anyway.
I am not sure just failing to see how having this voltage on Op-Amp's input is going to give us 12v on output
- True, but they do help if copy/pasted into a calculator to get the same result. Anyway, I'll try not to do that again, it's a bad habit, I know...All numbers after the third decimal are useless, they just cause confusion and reduce readability.
I see your point.- True, but they do help if copy/pasted into a calculator to get the same result. Anyway, I'll try not to do that again, it's a bad habit, I know...
No, LED cathode is closer to positive potential than anode. It won't light.
Please, add the arrow to the MOS to mark its type, which has to be P-channel. It is not a pain, and also not a pain to use dots on crossing wires to mark that they are connected together. Not just at certain wires, like on your figure at the output.
Provided you set up overall negative feedback, means R7 wiper connects to OP non-inverting input, you'll experience that the circuit misses necessary feedback frequency compensation and will most likely oscillate. That's because the OP is internally compensated for unity gain but not for the additional gain provided by the MOSFET in common source configuration.
It neither works as a voltage reference if connected this way. Presently R3, D5, Q1 are useless and can be replaced by a wire powering the reference.it does not light and i guess Brian was right about it being a voltage reference.
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