Adjustable Voltage Regulator Problems

So yesterday I had no lectures, so instead of spending my time working on assignments that I don’t have, I decided to go into the engineering lab and prototype the MIC2941 adjustable voltage regulator circuit.
The datasheet for the 2941 contains a example circuit with maths to go with it. This circuit should be capable of outputting between 1.2v and 26v, perfect for what I want to do.
Datasheet example circuit

So in the lab I found the components I needed, and set to work.

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In order to calculate the values on R1 and R2 I used the equation provided, Vout=Vref(1+(R1/R2)) re-arranging so that, R1/R2=(Vout/Vref)-1. With this I then had a ratio that I could scale to any values I wanted.

Due to the output voltage being limited to between 1.2v and Vin – 1v I chose to aim for a output of 15v initially, because the power supply unit’s (PSU’s) my university have on workbenches only go up to 20v.
So with that value in mind I calculated R1/R2 = 11.195 to 3dp, therefore I selected a pre-set 2kΩ for R2 and a variable 22kΩ for R1. In theory I should then be getting approximate output voltages of:

14.76v at full rotation
7.995v at centre point
1.5375v at low rotation

ImageAs seen in the video below, with the reference voltage at 1.2 – 1.3v (poor resolution on the power supply’s behalf) and 19.9v on the input, the output voltage seems to fluctuate between 6.6v and then back up to 15.1v and back down.

My plan in the next couple of days is to hock it up to the oscilloscope and investigate the input current, which was limited to 0.5A by the PSU.

Delivery!

So the MIC2941AWT‘s have arrived! These are the adjustable 1.2A Low dropout voltage regulators which I will be using to provide a variable voltage supply. I will only be using 1 in my project which means I will have 2 spares.
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To provide a readout of the supply voltage, current and power, I intend to buy a pre-packaged module off ebay. This module is sold in 3 different variations, the 5A module, 10A module, and the 20A module. The 10A module has the best current sensing resolution while still capable of sensing a large range and costs about £20.

Alternatively I can buy a Voltmeter and Ammeter module for £18 which also provides an adjustable output voltage and an aluminium heat sink.

We’ll wait and see which module I decide to go for.

After my mistake was pointed out that I needed fuses on my output terminals, here they are.

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Converting a old PC psu

Most decent workbench power supplies cost quite a lot, at least for a student. Whereas I have and old computer PSU lying around, so I figured I’d have a bit of fun converting it.

Taking inspiration from various other projects including the DIY Bench Supply from TG’s builds and the RepRap PCPowerSupply I’ve ordered a MIC2941AWT from Micrel, this is a 1.2A Low-Dropout Voltage Regulator that I’ll be using to supply a variable voltage. In addition to the variable supply I’ll be including terminals for the voltages supplied directly from the old psu.

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