Designing a good PCB requires some knowledge and good calculator. First one can be obtained gradually by making tons of PCBs and learning from failures. Different thing is calculator. You cannot have all things in mind when tracing crucial traces in your PCB. What is high voltage is present? Or if there are large currents floating. How one or another configuration will behave when high frequency signals are used. There are always a tradeoffs between materials used and price. So measuring things right may save time and cost.

Saturn PCB Design Inc. knows these problems and put their experience in a nice free calculator which covers many designing routine calculations. Here you are be able to calculate things like via/trace resistance, capacitance, inductance. By changing inductor width and thickness find its power dissipation. Also you can do quick thermal calculations that will allow to decide if you need a heat-sink. Other things include planar inductor parameter evaluation, spacing between traces on different voltages, crosstalks and more. Units can be selected between imperial and metric. Calculator is refined with many releases and can be downloaded here.

Writing formulas and equations for some publication or websites is time consuming task. Usually there is LaTex syntax used to enter formulas. But when formulas get more complex it gets really hard to remember all short-codes and it is easy to loose end in a long line. At school we are used to write equations by hand so why not to transfer that experience in to computer.

Here is a nice small script that simply converts hand written equation in to LaTex and MathML XML codes. All you have is to open a MyScript demo website and start drawing. Algorithm analyzes hand drawn formulas and tries to convert them in to code. It previews your progress so you could redo last steps to match your needs. I should say it does job really great. So if you do lots of formulas you may give a try – could be faster than typing.

[Alex] decided to build a solar charger for his car battery. He had an 18V solar panel able to provide up to 83mA. You cannot connect panel directly to battery because charging voltage cannot exceed allowed safe limit and also solar panel may become as load for battery in dark time and this way discharge it.

So he ended up with simple circuit utilizing LM317 and couple resistors setting voltage so that battery would be charged at recommended 13.2V. In order to prevent back supply a Shottky was used. Of course it adds some voltage drop (0.7V worst case). This was taken in account while calculating voltage adjust resistor divider. As a test [Alex] left solar charger for three days connected to his battery and it charged up to 12.35V which is about 75% of capacity. Not bad at all.