Electronics > Beginners

Linear lab bench power supply - Advise Needed

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UltraRC:
A week ago I posted on reddit asking for some advice but i figured here would be a better place.  I think i'm getting close to putting this thing together but I still have a few questions/problems. I am aware that there are likely many problems with the way this is drawn/designed and that when I posted on reddit, people suggested that I simply buy a pre-made regulator unit off of eBay or something, but that seems dull and I want to learn how to build it my self.

What I have drawn so far is at the bottom of the post.

The circuit is based around an IRFZ44N mosfet used as a series pass device and regulated by an opamp. The current control is essentially a copy paste from the eev blog PSU design series although a few of the resister values will need changing as i'm using two 1R resistors in parrallel as the current sense so its 0.5R. When the current gets over a threshold set by R9 the mosfet gate is pulled low by Q2, BC547. I'll probably also add a resistor in series with R9 to limit the max current to some current value which will depend on how hot the heat sink gets while testing (probably 1A).

While I was doing some bread board prototyping the opamp/mosfet voltage regulation combo did a good job of regulating the output and closely followed the voltage of the pot, but stating at max voltage and slowly lowering the voltage with the pot, when the output approaches 0V it quickly jumps up to near max output voltage even though the pot is set to 0V. I think it has something to do with how the opamp is configured but i'm unsure whats causing this.

I am planning to use an arduino board to measure and display the current set, current sense and voltage sense values on an LCD but otherwise not involved in the control circuitry. Can I just use a few of the analog pins to measure 0-15V? Or is that too high of a voltage for the chip to deal with?

I will add current limit and power on indicator LEDS to the design later but other than that  was just looking for more experienced eyes to point out any issues or suggestions for the design before I start soldering it together.

Thanks, Reuben

xavier60:
What type of op-amp are you using? It needs to have an input common mode voltage range that includes its negative supply voltage(pin 4).
Some common types are LM358 and LM324.

UltraRC:
Yep its an LM324N

rstofer:
There's an 'oopsie' known as inversion under certain input conditions.

http://www.ti.com/lit/an/sloa277/sloa277.pdf

Section 2.2 starts to talk about this.

xavier60:
I can't see why it should have that problem. I can see some unrelated problems. R8 and all op-amps should be powered directly by the 15V.
The CV feedback loop is likely to be unstable and oscillate.
Take some voltage readings on the op-amp.
An oscilloscope will often spot problem causes that a DMM can miss.

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