Categorized | bbboost, projects

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!

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John Boxall

Person. Author of http://arduinoworkshop.com Director of http://tronixlabs.com.au Rare updater of http://tronixstuff.com VK3FJBX

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