This is a little invention of my own which i used to increase the output range of wind turbines that i use to power my workshop, but could very well be used on other power supplies like solar or hydroelectric, that have a varying output. The origianl idea come around when i was making my dual stator wind turbine, ( http://www.instructables.com/id/Smart-Drive-Wind-Turbine-Tutorial/ ) which has two stators on the same shaft. The idea was to make something that would give me a lower charging RPM but without sacrificing the high end performance. This is where this little circuit came in.
When using these renewable sources of energy, there is always days when there is not enough wind to charge your batteries, its cloudy or your steam is running low. This little circuit, the gen stepper as i have named it, helps when there is power been generated but not yet enough to charge you batteries. E.G. when the winds enough to turn your wind turbine over, but the voltage is yet to reach charging voltage.
The gen stepper takes advantage of these low output times by splitting your power source into two or more section. In my case this is dual stators on my smart drive wind turbine. This could be with a bit of re-wiring two half's of the same stator or with three phase alternators, you could switch from a delta to a star arrangement.
The circuit then has the two half's in series in low winds or when the voltage output is low, so the voltage reaches the charging voltage quicker so you have greater time charging, then when the wind gets up, it switches the two half back into parallel so you still get your maximum output. Having the two halves of your system in series in low winds means there is less power overall but your batteries will be charging for a much greater period of time, using more of the available resources where it would be usually sitting idle.
Step 1: How it works
The gen stepper is based around the picaxe 08M micro-controller. They are relatively easy to get and are very simple to use and program. Programming is done via serial port or an adapter can be used and USB works too. The programming software is PICAXE programing editor and is found at the picaxe website http://www.picaxe.com/Software/ .
For my wind turbine which had alternators on it, the input that determined when the circuit switched from series to parallel was AC and was counted as pulses but this could be done the same just with a vane-optocoupler to measure the RPM. The voltage can't be used as it is clamped by the battery bank if you were thinking it would be easier than RPM.
The workings of the circuit is simple, the input is read, if its lower than a pre-determined value then it will switch a relay into series then if the value of the input rises above that limit it will switch to parallel.
Because the circuit is running on a variable power supply, a 5 volt regulator is used and because the max voltage of my generator is almost always clamped to the battery voltage, i didn't have to worry about burning anything out. After some trails i found this to be a bad idea as if the wind turbine was disconnected for even a short amount of time it could burn out the circuit. I recommend you use a separate power-supply to prevent this.
Step 2: Circuit Diagram and making
For a solar application connect a LDR from 5v to ADC input 4 and earth it with a 10k ohm resistor.
When i made this circuit i used software to place components on stripboard, but have lost all that information so if anyone designs a PCB or strip board diagram for this, i would be happy to put it up for other users. the software i used for making the stripboard diagram is http://www.marlwifi.org.nz/other/stripboard-magic . It is easy to use, you just draw in your circuit and it places components. My board isn't the most compact because i wanted it a certain size so it can be made smaller. Once you have the design all that is required is to solder the components on. make sure you don't over heat any of the components in this step, especially the picaxe chip. Always check hat you have all the components the right way round and that there is no shorts in the board, otherwise you may have to re-do some of your hard work. During soldering do not have the picaxe chip in its mount, this will help over heating of the chip and potential damage. The electrical diagrams in this step are modified from http://www.thebackshed.com/Windmill/Contents.asp and have been really useful so check them out.
Step 3: Code
The code should all work fine when uploaded, i have tested the AC input code but have only done computer simulation of the solar one. It should still work fine and i would love to hear any feed back of anyone who makes either of the circuit variations.If you find any bugs or improvement you think would be good, please comment as i am always trying to improve my projects and I'm sure others would fine it useful.
To change the point at which the circuit switches from series to parallel, change the two values in the "Main" part of the code which i have set to 580 and 540. these are the RPM values and are set to switch when the parallel voltage of the generators is above about 13 volts, which is good for charging. The best way to find these values is by trial and error. At first i had the circuit down the bottom of my tower so i could measure voltage and rpm, and see the circuit tripping and adjust it. Then once i had got it about right, i installed it in the top of the wind turbine, and only adjusted it occasionally.
Input from AC or pulse measurement
if RPM>580 then SwitchToParallel ' Set these values to determine the switching values
if RPM goto main
' ########## Switch to parallel
' ########## Switch to series
'Always keep the switchtoparallel and switchtoseries values 10 or more apart to prevent fluttering of relay
input from LRC
readadc 4, LIGHT
if LIGHT>4 then SwitchToParallel ' Set these values to determine the switching values
if LIGHT goto main
' ########## Switch to parallel
' ########## Switch to series
'Always keep the switchtoparallel and switchtoseries values different to prevent fluttering of relay
Once the circuit was complete and tested i installed it between the two stators of my wind turbine so that i only needed to run down two power cables and not four. It meant that programming was a little harder but i had a 3 meter long serial cable that i sat on the roof and plugged into my wind turbine up the tower so i could do adjustments while it was running. It was hard to find a relay of adequate size to handle upto 1.6 kw of power at 12 volts, so i used a smaller one just to try it out and it seemed to work fine.
The circuit hasn't failed me yet, now my batteries are getting trickle charged from about 9am in the morning, when the wind starts to get up, till about 10pm at night when it dies down again, compared to before when it didn't start in-till around 11 am. this means i can keep power coming on some of the duller days we have here.
I would love to hear any improvements or people who have used this instructables and please vote in the contest if you like this.