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Regulated power supply Mod

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aramosfet

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Hi All,

I recently purchased a Regulated power supply kit from (https://www.banggood.com/0-30V-2mA-3A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-p-958308.html).

The schematic of the design is also provided.
Regulated Supply.jpg

My goal is to control the setting of output voltage and current limit with a microcontroller, LCD/LED display and rotary encoders. To that end, I tried to understand the functioning of the supply and have a few doubts.

From the schematic, I understand

1. R7 is used as the current sense resistor.
2. P1 controls the output voltage through U2 opamp. which drives the darlington pair Q2 and Q4.
3. P2 sets the current limit through U3. When current limit is exceeded D9 pulled to negative by U3 which reduces the drive to output transistors.
4. Q3 and D12 serve as current limit indication.

Questions.
1. what is the purpose of U1?
Both inputs of U1 are tied to ground (before the current sense resistor).
2. Why is U1 negative supply connected to ground instead of negative supply. Further, most references in the schematic are connected to load ground before the sense resistor rather than the input supply ground or bridge rectifier ground. why is this so?
3. As per the BOM, the negative supply is regulated by a 5.7V Zener. So the opamps have a positive rail that goes to 32V or more and the negative rail to -5.7V. Is this fine?

Finally, the intent is to drive the P1 and P2 potentiometer points with voltages to set the output voltage and current limit digitally with a microcontroller. What would be the best way to achieve it?

Thanks
 

SunnySkyguy

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Shunt R14 with another , Rc to get 0.6Vbe at Q1 to get 0 mA out or better use a diode to bias from V- to Vb with a programmable current source or opto isolator that becomes linear now with Vbe drop on Q1 with Rc limiting output current to 0 at max input program current.

RV1 is the input bias current null adjust for the integrator while U1 is the Zener limited comparator with positive feedback that regulates this hysteretic convertor. No Neg V- needed.

Injecting a suitable DAC control V and R into U2-3 would program an offset to the limited Vout Max. Disconnect to PSU should return to normal operation.
 

Vbase

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U1 and D8 generate reference voltage, the circuit is congigured to have constant current through the zener.
Negative output is used as sense so you regulate the output voltage ignoring the voltage on R7 that relates to current.
It is ok to supply the amps with +/- voltages that aren't equal.
Most uC don't have analogue output (DAC), which uC are you using?
 

Audioguru

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That circuit has been at www.electronics-lab.com for at least 11 years and was a kit from Greece.
It is not reliable because many of its parts are overloaded, for example its TL081 opamps have a maximum allowed total supply of only 36V but 2 of them operate with more.
It originally used an overloaded 24VAC transformer but the rectified and filtered voltage is too low for a regulated output of 30V at 3A.

A few of us fixed it using modern opamps that have a max total supply of 44V and a 28V or 30V transformer that can provide enough current. The main filter capacitor value was increased a lot. We added calibration trimpots.
The output transistor on the original circuit got so hot it melted so our fixed version uses two output transistors to share the heat.

I did not look at the datasheets for the Oriental transistors used in the Chinese kit to see if they are suitable.
 

aramosfet

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That circuit has been at www.electronics-lab.com for at least 11 years and was a kit from Greece.
It is not reliable because many of its parts are overloaded, for example its TL081 opamps have a maximum allowed total supply of only 36V but 2 of them operate with more.
It originally used an overloaded 24VAC transformer but the rectified and filtered voltage is too low for a regulated output of 30V at 3A.

A few of us fixed it using modern opamps that have a max total supply of 44V and a 28V or 30V transformer that can provide enough current. The main filter capacitor value was increased a lot. We added calibration trimpots.
The output transistor on the original circuit got so hot it melted so our fixed version uses two output transistors to share the heat.

I did not look at the datasheets for the Oriental transistors used in the Chinese kit to see if they are suitable.

Hi Audioguru,
Thanks for the pointer. I found the power supply design thread that explains all the questions.
I haven't decided on which microcontroller to use. But to get a decent resolution of voltage/current setting I will need a 12bit DAC atleast which has a 7mv resolution for 30V.
 

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