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| Lab Power Supply - The Lost Current |
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| xavier60:
This is the schematic with changes that will allow it to work stand alone without the micro-controller. Potentiometers for the settings need to be supplied from a negative reference which can be the -8v rail. And it is possible to use a TL431 in a negative series pass regulator. Btw, those two nice blue ten turn Pots that I was using in my earlier mock up, both developed bad wiper to track contact. Drilling a small hole and injecting some PAO oil got them working again. It is important that both the CC and CV modes are fail safe, they must go low if a Pot goes open circuit. |
| radoczi94:
Interesting. A few questions. What is that part with the ptc kinda thing doing on the right? The ptc protects the protection diode from reversed voltage tied on the output? That fet helps to kill the output faster? As I see, the opamp opens q2 and shunting down the gate of the fet, right? Why is it better to use negative voltage as reference? |
| xavier60:
--- Quote from: radoczi94 on August 04, 2018, 06:23:18 pm ---Interesting. A few questions. What is that part with the ptc kinda thing doing on the right? The ptc protects the protection diode from reversed voltage tied on the output? That fet helps to kill the output faster? As I see, the opamp opens q2 and shunting down the gate of the fet, right? Why is it better to use negative voltage as reference? --- End quote --- Q5 forms a constant current sink which discharges the output capacitor when the output voltage needs to be reduced and also drains away the small amount of leakage through the main MOSFET. The diode and Polyswitch protect this circuit against a battery being reverse connected to the output, As far as I can tell, the rest of the power supply might briefly tolerate this kind of accident depending on the Current setting. The extra voltage added by the battery to the voltage drop across the main MOSFET could easily exceed its Safe Area Operating Rating. Q2 is off while the power supply is in CV mode leaving C1 disconnected from the CC op-amp's feedback path. In this state, the op-amp has full open loop gain. If the power supply is suddenly overloaded by a short circuit across the output, the CC op-amp will very quickly lower its output and take control of the MOSFET's Gate from the CV op-amp. At this point, the current that the CC op-amp will be sinking from Q4 will turn on Q2, connecting C1 into the op-amp's feedback path resulting in stable current regulation. The CC op-amp goes form open loop to Miller Integrator. When the op-amps are configured for a positive reference, it becomes an advantage for the CC side. It causes loading of the CV Pot's wiper which I try to avoid as it affects linearity and possibly setting stability. Although this concern does not apply to the the micro-controller version, I have not altered much. |
| Audioguru:
The original Greek Kit and the project at Electronics Lab used a TL081 as the voltage regulating opamp. It has a problem called "opamp phase inversion" where the output suddenly goes as high as it can if an input voltage gets within a few volts from its negative supply voltage, which happens in this circuit when the power is turned off then the negative supply drops instantly. So T1 was added to short the output of the voltage regulating opamp to the input 0V when the negative supply disappears. But this version uses a TL071 as the current regulating opamp that has the same phase inversion problem as the TL081 so its output goes high when the power is turned off and the remaining charge in the positive filter capacitors allows unlimited current to the load, BOOM! When I fixed the original kit and project 12 or 13 years ago I selected MC34071 and TLE2141 opamps that have a high 44V rating, no phase inversion problem and have inputs that work down to their negative supply voltage so that the negative supply is not needed (except to allow the diode to reduce the output voltage when the current regulation is used). A couple of years ago a few Chinese companies copied the original circuit and all its problems. I don't think they built one and tested it to see all its problems. I don't think they sell their horrible copies anymore. |
| xavier60:
I found the discussion here, http://www.electronics-lab.com/linear-lab-power-supply-digital-meter/. Does someone have a link to the schematic? In the design that I have been working with, zener diode D8 is meant to cause Q4 to stop supplying pull up current when the control rails drop. It would also help if the negative regulator has a larger input capacitor so that the positive rail drops first at power down. Is this it? https://www.instructables.com/id/Superb-Lab-Power-Supply/ |
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