The Inner Circle

Okey, so my battery and solar panel have arrived. I’m just waiting on a 12V 300ma battery charger to arrive so that I can actually properly charge the battery for the first time before it begins it’s life of being floated. I gave them both a quick go to make sure they wernt DOA. The solar panel has already been modified to remove a silly blinking blue LED which was eating up 2.5mA on a 50% duty cycle while the panel was on. Baring in mind that I’ve seen it only do 12mA in overcast conditions every little helps. Maximum I’ve seen so far is 54mA in direct sun, although at the end of the day.

So I decided to look at why I’m “loosing” voltage when converting from AC to DC, I accepted that there would be a drop from the diodes in the full wave rectifier. Estimates suggests this would be around 0.7V for a diode and with two facing towards the positive rail this added up to 1.4V. Turns out that I had a voltage drop that was not quite consistent across the range of voltages I was expecting.

AC DC Diff
1.56 0.38 1.18
2.5 1.06 1.44
5.7 3.96 1.74
6.58 4.76 1.82

Hmmm, so not quite right and certainly this didn’t explain a 1.4V drop for the first result. After a bit more digging I read up that the reading my volt meter was giving me was most likly RMS. So by using:

AC * (2 * SQRT(2)/PI()) = DC

I got figures that always seemed to be around 1.2V above where they should be. I’ve surmised so far that in fact the drop across my two diodes is totaling around 1.2V. As the calculation to work out current from the voltage requires the AC voltage I think I need to reverse this in order to obtain correct readings.

However having since played and discovered that I’ve already received a voltage above the 5VAC that I had initially anticipated, I suspect I need to alter my burden resistor to bring this around a suitable value, although the 6.58VAC was with virtually every electrical thing in the house turned on although it suggests a total pull of around 20A. Which is possible I’m sure.

I need to have a good think about this tomorrow to actually work out what I really need.

Edit: Looking at the data sheet for the CR Magnetics split core current transformer the calculation is for VAC RMS and IAC RMS, so that answers that question. I suspect I need to tweak the burden resistor to cope with 32A which is the maximum of any of the house circuits, although the incomer more.

D.