This was one of those projects that should have taken an afternoon and didn't, however it's now complete and working rather nicely.
I already had a box from a defunct project that had the 4x20 LCD display installed and power switch, mains transformer and a mains IEC socket on the rear so I decided to re-task it for this.
Inside is a PIC18F25K22 running this show, and the device can monitor up to three channels. Each channel has a 5-pin din connector and cable that connects to a Maxim DS18B20 temperature sensor.
You can set the alarm temperature independently on each channel, and the project starts beeping and flashing the LCD backlight if a maximum is exceeded. I also included a relay that can be used as a power interrupter for the project under test. This way I can leave a project on soak but if it starts to over heat, the power can be cut automatically.
The reason it took longer than anticipated was for some reason the PCB never etched correctly and I had some messing around to do. The software only took around an hour to write and debug. I love Proton BASIC.
...and the reason it's three channels and not more, well I happened to have three 5-pin din sockets in my junk box. If I'd had more... who knows.
If anybody is interested I'll make available the circuit diagram, PCB foil and PIC firmware but this really is an easy project to design and build.
So, now that's up and running, I can get back to the original task of designing a decent voltage boost converter.
So, over the last few evenings I've managed to finish the first useful board for my new extendable PSU; a 5v board that can supply around 3 amps. It has a trim control that allows the output to be adjusted from 5v to 6v, and a couple of op-amps wired as voltage comparators that illuminate a couple of LEDs to indicate over or under voltage conditions (less than 5v or more than 6v).
Now all happy that I'd got my first board up and working I set about designing the next one that can provide a variable 0 to 30v at around 1 amp.
Happily playing with some ideas on my breadboard I suddenly noticed smoke coming from the circuit, a bang, and then a piece of the voltage regulator went flying past my head.
The problem was, besides a fault in my design, I hadn't noticed that the regulator was starting to fry.
I did have an amp meter in series with the board but I didn't see what it was reading.
What's needed I thought, is something that can alert me to when things are getting hot.
So, I've started on a new project. It will have the ability to monitor multiple temperature sensors and report audibly if one or more of them exceeds a pre-defined temperature. All this so I can eventually measure and record the discharge curve of a battery pack. See previous blog entries to find out more about that.
That's another breadboard with a half finished prototype put to one side, and the start of another project; multi-temperature sensor alarm project. I need to think of a more snazzy name than that. Whilst it would have just been simpler to pick up some cheap data logger off Ebay.... it wouldn't have been so much fun.
Oh.. and a word of warning. Electronics on the whole is a safe hobby. Baring the old soldering iron burn or stabbed finger when a screw driver slips, you should be pretty safe if you are sensible and carful. However, things do go wrong and most components will complain venomously if they are stressed beyond their design parameters; or just connected the wrong way around. I'm fortunate in that I wear spectacles and they offer a limited amount of eye protection, but you do need to be carful.
If you're ever present when a tantalum capacitor explodes, you will wish you had a gas mask and fire extinguisher handy never mind a pair of spectacles.
I'm currently working on a +5v plug-in module for my new PSU (see previous blog post).
I had several requirements for this module including the ability to withstand a dead-short circuit and the final output voltage to be trimable to +/- 0.5v. Because of this I decided not to use the existing +5v rail that's already present, instead opting to down convert the existing +12v.
Good old linear regulators are almost indestructible if a few sensible precautions are taken, but they give off a fair amount of heat when dropping a large voltage, especially if you are wanting to pull a couple of amps.
After some research I opted for the LM2576-ADJ (also because for some reason I have a stack of them to hand).
The above circuit is straight from the datasheet and I'm using this almost as is. The only component required that I didn't have was the 150uH inductor.
Like most seasoned hobbyists I've learned to keep things that are useful and I've a drawer full of old inductors, ferrite rods and toroids that I've salvaged over the years but I decided to wind my own inductor on a ferrite toroid.
Now I've never really given this much thought before, but when buying inductors they list the inductance value, the amperage and sometimes the resistance. A wire inductor is after all just a long piece of wire, usually wound around a former of some kind so it's bound to have resistance and the more wire you have, the higher resistance. Because it's wire it has a maximum current carrying capacity and hence a maximum working current.
However, whilst experimenting with winding an inductor for this project I noticed something that in hindsight is obvious, but that I'd never really thought about before.
I wound two different inductors and after some experimentation managed to get them both pretty close to the inductance required; and I've opted to use a slightly higher value of around 220uH for this project.
The left one was wound on a much smaller toroid; you can't tell from the picture but the larger core is also around double the thickness of the smaller one.
Both inductors work in my test circuit but the larger one is more efficient.
With the input voltages the same for both tests, the regulator circuit draws 472 ma under test with the larger inductor, and 493 ma with the smaller one.
I'm not sure how many turns there are on the small inductor, probably around 70, but you can see there are only 14 on the large one and even taking into account the physical size of the core, much less wire was required and hence it has a lower resistance.
Oh, and if you're wondering how I wind my cores and check their inductance, I'll create a blog entry about that when I get a chance.