PSU Console

Dead Simple Power Supply

At work I'm quite spoilt, there is an abundance of equipment in the labs, and if I need, I can borrow something for home use.  Very handy when ding something specific (like the thermocouple I used here) but it comes with a down side.

If I realise I need something, it means dropping work for a few days until I can borrow the equipment (if it's in use at work, I can't have it until it's free) and as I catch the train, it also means bringing the car in if it's too big to fit in my backpack.

When it came to powering my projects, I have a very basic dual rail Power Supply (I built one just like in in Uni, tortured it to death over the coming years by running a car amp from it in my bedroom) that I inherited from a guy-who-got-it-from-a-guy-from-a-garage-sale deal.

The updside: It's got dual isolated rails, 1.25V to 15V range and 'up to' 1A output.  

The downside: It's so old the pots are scratchy, so setting an output voltage is a chore, and there isn't any current limit.  And you need to whip out your multimeter to set the voltage / measure the current in use.

So I got cracking on this:

Multi Channel Meter, Part 2

From Idea to Implementation

What I really love about my hobby projects is that I can develop answers to questions that I can't explore at work.  20kW HF transmitters, Radar Installations all over the world, and Cutting Edge elemental analysers?
No worries.

Digitally Controlled Gain, Isolated PSU Choices?  Fergedaboudit.

In part 1, I detailed my path from the concept of a multi-channel volt meter, to a working system based on individual modules.  Here I'd like to talk about how I refined the design into my first prototype and then look at where I'm going from there.  Like Art, Engineering is never finished, you just chose a point to stop...

Multi Channel Meter

So after all that background...

Recently I've blogged about DIY Isolated Supplies and Microcontrolled Analogue Gain, as these have been necessary building blocks for my latest project - a multi channel voltage / current meter.

Microcontrolled Analogue Gain

Digital Controlled Gain

For a long time I've wanted to implement a gain stage where I was able to alter the gain under digital control.

Why?

Back in my Uni days, I have to build an Automatic Gain Control circuit, and was greatly dissapointed that the trick there wasn't variable GAIN but variable ATTENUATION on the front end.  At the end of the day, that approach works well where i used it, but I always wanted to have true programmable gain.

But this was the case of a solution looking for a problem, until I decided that I wanted to build some test equipment that required programmable gain!  So, to test the theory I cooked up this:

It Worked!  Eventually!

Upcoming

Coming Soon

I know I've been a touch quiet on the blog, mostly because I've been sidelined with other projects.

Blog entries for all coming but here's some preview pics:

Multi Channel Meter

Isolation Options

Isolated PSU

So if you find out you need to isolate your circuit, you will need to power it over the isolation barrier, and there are options out there to do that.

Finding myself in that position, and being a notorious tight-arse I started to look at DIY options.

Measure Twice, Cut Once, Get a New Ruler....

History Repeats

With past projects, I've made some mistakes that are common to a lot of projects.  Who hasn't mixed up D+ and D- on a 485 driver?

One thing I've added to my workflow is using excel to help build my new packages.  In Altium you can copy pin information from a pdf into a table, massage it as needed and then pate it onto Altium.  I've configured 100 pin devices like this in a lot less time it would take to edit them one by one.

New Boards!

New Boards

My next bunch of boards from Seeed studio have arrived.  Here's some shots of parting out the prototypes from 10 x 15 cm panels.

Build and test info coming... as soon as I build and test.

Finishing Up

Final Tests and Code

Listening to Others

I've made no secret that I took my inspiration from Lior's Alarmino project, and I was quite pleasantly surprised when he contacted me on one of my posts.  He suggested that I take a look at the value of the filter cap I'm using on the 4V rail supplying the SIM900.

In his design, the bulk storage cap was charged by a linear regulator that delivered a maximum of 1A, but as my design uses a switch mode that's (nominally) rated to 3A I figured I could go smaller with my filter cap as my circuit can deliver a higher rate of charge - even with the substitution of the eBay sourced LM2596 circuit.

Thank You eBay!

Design and Test, The Third

Filter Testing and Options

IN THEORY

In my previous posts, I'd debugged my power supply, and got my Arduino Leonardo clone up and running.

I'm getting closer to finishing but there's still one functional block that I'd not yet tested and it's my active filter.  Again, a big hat's off to Lior for the inspiration behind my build, but I have done some things my own way.

For example I used a Sallen-Key low pass active filter to get a 1.5kHz Low Pass Filter.

Thanks to the internet, rather than hand crank the numbers, I used this on-line calculator from OKAWA Electric Design. From that design I whipped up this LT-Spice File.

LT-Spice Circuit (Active Filter)

Design and Test, Part Two

Testing Day Two

As mentioned before, I'd tested and debugged my hardware to the point where it's time to start integrating the systems.  Or in other words get the micro talking to the rest of the board.

Notice Something Strange?

Design and Test

Testing, Day One.

It's been a while (a large part was waiting on parts - I'm looking at you Futurelec!) since I posted about my Alarm Monitoring system but as I've finally been able to get some build and test underway I thought I'd share the progress.  Which is a nice way of saying I'm not finished yet.

Step By Step

With this design I've incorporated a fair few new circuit blocks I haven't tried before, being:

1. FET Current Limiter Circuit
2. 4V, 3A (TWELVE WATTS BABY!) Switch Mode power supply
3. AVR microcontroller (yes, never used 'em before)

So to start, I loaded only the power supply section before placing other parts.  This is a good practice when prototyping as if you stuffed something up, you don't blow up the rest of your board.  I've actually designed in zero ohm links on the power supply rails which could also prevent the same destruction but through experience I prefer to do it his way.

Zero ohm links are good for when you are setting up production boards.  You can adjust anything that needs adjusting in isolation before installing the links.

But for prototypes - Why load everything on your first prototype when a SNAFU in any area could make the whole exercise pointless?

Partially Loaded

Power Supply Protection for Raspberry-Pi

Defining the Problem, Again

The power rails of the Raspberry-Pi are bought out on the GPIO header as well as the IO pins.  This is both a blessing and a curse.

The advantage of this is that you have 5V and 3V3 rails available to power your circuitry that you are hooking up to your GPIO pins, but again, as the Raspberry-Pi doesn't have any protection circuitry you run the risk of killing your Raspberry-Pi through:

• Short Circuiting the Rail
• Reverse Biasing the Rail
• Over-Volting the Rail

GPIO Protection For Raspberry-Pi

Or How Not to Let the magic Smoke Out

Previously I've written about my love / hate relationship with Polyswitches, and discussed some input and output pin protection methods.  I was looking into this so I could make a breakout board / protection device for experimenting with my Raspberry-Pi and not let out the magic smoke.

I could say it's to help kids / beginners protect themselves from basic mistakes, but even after practicing Engineering for nearly 20 years I have learn that Murphy still bites, so it's also a lesson in self preservation :)

Testing

Why We Test

The Magic of FETs

I've never had a lot to do with FETs before so I've been keen to experiment with and learn about FETs on my home projects.

As I wrote before in Basic Protection I like the simplicity of using a P-FET to provide reverse bias protection, and with my Alarm Interface I was planning on combining this reverse bias protection with current limiting.

As usual, I got it wrong the first time....

Cutting out the Middle Man

Home Alarm Monitoring

I have a home alarm system, and it's reliable and the monitoring company provides a good service.  For a bit less that a dollar a day, if the alarm goes off, they give me a call.  

Each and every time it goes off.  

My system has multiple zones, and my thinking is that if only the one zone is triggered, it's probably not a burglar in the house.  And I don't want my long weekend away spoiled by my wife continuously worrying about the single zone alarm that went off 2 hours ago and we are not going to be home for two more days.

I've requested with the monitoring company to *not* call me when it's only one zone, but policy is policy and they need to call.  Fair enough.