Tag Archive | "LM317"

bbboost chapter five – the power supply module

[3 July 2010 – this project has been retired, but the posts left for reference]

Greeting again to followers of the bbboost journey. It has been a month since the last instalment, however the 20V DC plug pack took a long time to arrive from the land of China. Nevertheless, the project is moving forward. For my new readers, the bbboost is a power supply that can be assembled by a beginner, and can offer a smooth variable DC output voltage of between ~1.8 and ~20 volts – perfect for experimenting, breadboard, and generally saving money by not buying batteries. You can just make a PCB version, or mount it in an enclosure like a professional desktop unit. No mains voltage wiring is required, so it will fine for the younger enthusiasts. Follow the project from here.

This time I have breadboarded the power supply module, using the circuit described in chapter two.  Let’s have a bit of a look:

power-layout

power-layout2

These trimpots were ok, but it would be preferable to use the fully enclosed dustproof versions. Will order some and try ’em out.

trimpots

One trimpot (the blue and white one) is 5k ohm, – to adjust between the full range, so this is the ‘coarse’ adjuster; the other trimpot is only 500 ohms and changes the voltage selected by the coarse pot by around +/- 1.2 volts. The purpose of having two controls is to make it very easy to select your required voltage down to one-hundredth of a volt. The following video clip is a rough example of this type of adjustment in action:

This power supply will also be designed for installation into a nice enclosure, so in that case one would use normal-sized potentiometers for the coarse and fine voltage adjustment.

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bbboost chapter two – which regulator?

[3 July 2010 – this project has been retired, but the posts left for reference]

In the journey to create the bbboost, first we need to start at the core – that is, the voltate regulator itself. Searching for one that meets our specification was easier than expected, I just searched for “adjustable linear voltage regulator IC” in the Farnell website and listed the results by price. The likely candidate was the National Semiconductor LM317T. Hopefully most of you would realise that this was not a surprise, the LM317 is very popular. Limor over at adafruit industries uses a Micrel MIC2941, which is also an excellent regulator, due to the low dropout voltage, which means you can create 3.3V from 3.7v (for example).

However it is just too expensive, at $1.51 each for lots of 50. The LM317T is available individually for ~78 cents, or 58 cents in amounts greater than 100. Furthermore, the LM317 can provide up to 1.5 amps of current, greater than our intial spec for the bbboost. However to keep costs down, we will stick with the assumption of one amp, unless you choose to find a 1.5A plugpack. It also has short-circuit protection on the output, and thermal shutdown. This means if it overheats, it will turn off instead of becoming damaged. However, the maximum current available will decrease if the regulator becomes hot. Now there’s an interesting experiment!

Lots of interesting information can be found on the data sheet: LM317T data sheet

One of the good things about data sheets are the example circuits, of which we can make use of for the basis of our bbboost. So thanks to National Semiconductor, here is the hand-drawn base for our bbboost, with one difference – there will be two voltage adjustment potentiometers (variable resistors). The 5k (R1) will control the voltage, however R2 will be used as a fine adjustment control. Handy if you really need 8.45V and not 8.49V…

(Sorry for the hand-written schematic. I’m still working on using the software I have. Next time…)

We will decide on a value for R2 later on, after experimenting with the voltage display. So far, our list of materials is:

  • C1 – 0.1 uF 50V greencap capacitor
  • C2 – 1.0 uF 50V electrolytic capacitor
  • IC1 – LM317T linear voltage regulator
  • R1 – 5k potentiometer
  • R2 – very low value potentiometer
  • R3 – 240 ohm 1/2 watt resistor

C1 is used to smooth any ripples in the input voltage that can be created during the AC to DC conversion in the plug pack. C2 is used to improve the transient response (i.e. keep the output voltage nice and smooth).

So at this point we will put the project to one side – I’m waiting for the parts to arrive! Be sure to subscribe for updates (see the top-right) and explore the other posts on the blog. Bye for now!

Posted in bbboost, projectsComments (0)

breadboards and batteries… bbboost

[3 July 2010 – this project has been retired, but the posts left for reference. Not my finest work, but it may help someone]

During my life in the field of electronics study, research, and daydreaming many ideas (good, bad and dangerous) and projects have been constructed using the typical solderless breadboard that everyone has used at one stage or another. There is nothing wrong with this approach, except for the power supply situation. You could either buy an expensive desktop power supply ($40 upwards), use a fixed plugpack (if you have the right voltage) then build some power smoothing into your circuit, or even use a 7805 or similar regulator to get your +5 volts. Failing that, it’s back to batteries.

batteries-and-breadboard

You might as well just throw money into the garbage if you keep using batteries and have AC power nearby. I have had enough of worrying about all of this and have decided to conceive a desktop power supply that meets the following criteria:

  • cheap to construct
  • safe to use (not exposed to mains voltage)
  • can accept any voltage DC plugpack and offer a variable, smooth DC output of up to 1 amp
  • can be mounted on a small PC board with spacers to save money, or enclosed in a housing for a professional look
  • have a digital output voltage meter – so that it looks cool and is convenient. No more guessing with analogue meters and dealing with parallax error
  • increases the constructor’s knowledge of electronics!

Let us call it the bbboost – the bread board booster!

So over the next month or two we will do just that. If you would like to put forward ideas, suggestions or criticisms, please do so. Otherwise, get ready to say goodbye to breadboard batteries…

Posted in bbboost, projects, test equipmentComments (2)


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