After contacting Electronics 2000 Admin in regards to adding circuits to build in the Forum, he was in full agreement. There will also be a possibility that a new Forum topic will be created just for this very reason.
Thus this first project will be the kick-start and will probably be many more I will be adding in the near future.
If other members have projects of their own, they would be more than welcome to add theirs here. Or if you've built a project and are having difficulties, by all means discuss it here.
OK, first up, my disclaimer:
In no way will the Forum or myself be responsible for any mistakes, misprints with diagrams, wrong parts and unintentional incorrect advise.
You may build this project at your own risk. We cannot be responsible if it doesn't work, as we cannot monitor the way you will build it, whether you use parts we recommend, orientate parts around the wrong way, poor soldering skills etc.
Well, enough of that, lets get into it.
Below you will see the schematic diagram of a 1.25V to 15V variable DC power supply which is fully regulated.
Direct Link to Schematic Just hover your mouse pointer over the schematic and you should see a few icons to the upper-right, click on the Magnifying Glass icon to enlarge it...
Looking at the schematic diagram, you can see the input voltage being 18 volts DC (CN1). You would need to supply an 18 volt regulated plug-pack or another type of 18 volt regulated DC supply that can supply at least 1 Amp of current.
The main reason for using a regulated voltage source over an unregulated type is that unregulated supplies have a tendency to output more than the indicated voltage value (as printed on the plug-pack or transformer), by around 6 to 9 volts. Thus, when wanting to be able to vary the voltage in the above circuit, the higher the input voltage compared to the output, will generate more heat in the LM317T voltage regulator. Thus the need for a substantial heat-sink which would be needed to attach to the LM317.
If you like, you could use a 9 or 12 volt unregulated plug-pack, as this should give you the right input to output voltage ratio, without having to use a large heat-sink.
It would be advisable at this point that you should use a reasonable sized heat-sink regardless. Fitting a heat-sink will allow you to push out a little bit more current from the device, plus it will keep it running much cooler.
With no heat-sink attached you may be able to obtain around 400mA at 15 volts. Adding a heat-sink will push this figure to around 600~700mA, perhaps even a bit more.
Here are some examples of the type of heat-sink which I would recommend:
Heat-sink Type 1 This next heat-sink will require you to remove the solder tags: Heat-sink Type 2
These types would be even better: Heat-sink Type 3
A good sized one: Heat-sink Type 4 Though with this one, you'll have to remove the two pins for it to fit on the board. But you could always drill a couple of holes in the right spot and fit it that way. I would leave that up to you.
An 18 volt supply was chosen due to the voltage drop of the LM317T adjustable regulator. it will drop ~1.25 volts. Its always good practice to have the input voltage greater than the maximum output by around 3 volts.
If you decide to fit a heat-sink to this project and leave the circuit exposed, then you may need to think about isolating the heat-sink from the voltage regulator IC. Reason being is that the tag on the regulator is at output potential and you wouldn't like to accidentally contact the heat-sink to another part of the circuit. Don't worry, this circuit will not give you an electric shock even if you touched the heat-sink or tag directly. Not enough voltage.
Maplin has a range of isolation washers and bushes you would need.
If you do decide to put the circuit into a case, then there's no reason to isolate the heat-sink.
You will notice in the schematic that there are two diodes in circuit. They are there to protect the voltage regulator from any possible short circuits if the input or outputs are shorted to ground.
By the way, C3 is not essential to the circuit to maintain good regulation and filtering, therefore you can omit it. I'm just too lazy to correct all the diagrams.
Next is the circuit layout:
Direct Link to Circuit Layout Same as above, look for the Magnifying Glass icon to enlarge it.
If you notice in the circuit image, I've allowed some room for a heat-sink.
Next, we need the circuit tracks, via's and pads to be the only things exposed so you can etch your own printed circuit boards (PCB's). See the next picture:
Direct Link
If you decide you want to etch the PCB yourself, then here's where it gets a little hairy.
You need to right click on the picture as it is and save it to your PC. Name it what ever you like.
You can print it in Micro$oft preview, but you must maintain the aspect ratio of 1 to 1. Otherwise it won't print out to its actual size.
When you do select print in preview, make sure 'Fit Picture to Frame' is checked.
Try and force your printer to darken the print as much as possible.
OK, you have two methods of etching the PCB:
(1) You can buy blank PCB's that have "Press and Peel Film' type of board that you will need to use a photocopier or laser printer to print the tracks onto, then using your mums clothes iron, you press the image onto the blank. Then using a stripper/developer, then an etchant like Ferric Chloride or Ammonium Persulphate to actually etch the board until just the tracks, pads and via's are left.
I personally haven't used the "Press and Peel" type, as I feel it doesn't produce a very good result. I much prefer the next method:
(2) Purchasing printable Transparencies, you would print the tracks to a Transparency; just like the Transparencies used for overhead projectors and the like.
You would then need to buy a pre-photosensitized blank PCB.
Using a UV light source or just the sun, you would place the printed transparency ove the PCB and expose it to UV light.
After a pre-determined time, you would place the exposed PCB into a stripping solution that will remove the exposed areas and leave just the track, pads and via's.
You then would etch away the unwanted copper using either Ferric Chloride or Ammonium Persalphate.
I have to admit the above is a costly and messy procedure, though I do use this method when making my full blown projects and on the odd occasion, prototypes.
I usually use my breadboard or the more permanent method of using veroboard.
Most of you wouldn't want to go through the trouble, or have the knowledge on making your own PCB's, therefore we'll go in the direction of using veroboards as mentioned above.
Unfortunately at this moment I don't have time to produce a tutorial on it, but over the next few days, I'll make one up and post it under this one.
I hope the above has sparked some interest and will be a gateway to further, more complex projects.
Regards,
Relayer