switch needed to activate when PV producing

westbranch · April 2008

I know there was a string on something like this:

I am after a switching mechanism that will only work when the PVs are actively generating power, that is when the sun shines there will be 12v flowing past the switch. Kind of like a car solenoid but not as heavy duty nor as much current consumed.

What I hope to achieve is to provide 12 v power to a PDA using a (kickdown) 'automotive plugin type' charger, supply @ +- 5v, 500 mA.

This setup is a stand alone arrangement that may be left for several months, so in case something happens I do not want it to be a 'hard wired' on 24/7 setup.

Any ideas as to what I am looking for would be called? some kind of micro switch?

thanks,
Eric

mike95490 · April 2008

Re: switch needed to activate when PV producing

A diode, installed in-line, would do what you are asking.

A PDA (which normally only needs a couple of mA to run) will need a wide range regulator, with a heat sink. It will also shut down and loose it memory each night, and need to be re-configured each morning.

What is the PV array can you give us nameplate info ?

westbranch · April 2008

Re: switch needed to activate when PV producing

Mike, as per my sig line

Mitsubishi
PV MF120EC3
Voc 22.0 v Vmp 17.6 v
Isc 7.36 a Imp 6.84 a

forgot to add that, iff when we get it working, the PDA will be on a
wake - request data - sleep cycle, frequency to be determined,
probably 1 hour intervals, during the day if possible.

The need for charging to occur is to 'save data' form bring lost by PDA turning on when battery is depleted.

Still in the planning stages at this time.

thanks for the diode idea, as I understand it the diode would be in the + side and would disconnect when there is not enough power to 'activate' it therefore breaking the circuit?

cheers
Eric

mike95490 · April 2008

Re: switch needed to activate when PV producing

Are the PV wired in series or parallel ?

Anyway, your 12V PV's put out about 18V (or even 36V) if going into a MPPT controller.
You need at least a 50V, 10A Schottky Diode
http://www.vishay.com/docs/88966/v20100sg.pdf 100V, 20A, $1.20 @ digikey.com
http://search.digikey.com/scripts/DkSearch/dksus.dll?PName?Name=V20100SG-E3%2F4WGI-ND

This part has a forward drop of .5V at 5A, and is in a reasonable sized package you can wire into.

Inserted into the + lead before the controller, and if you sample off the PV side, not the controller side, you will only get power when the array is lit.

You can then use a 12V 1A regulator to power your cig lighter adapter.
LM340T-12/NOPB http://www.national.com/ds/LM/LM340.pdf $2
The LM34xx series is a bit higher efficiency than your 78xx series
You will be running VERY close to it's 35V max input voltage, if you have your PV's in series, and the charge controller "glitches" as it tracks the array voltage. If the load is removed, your array (if in series) WILL burn it out with 44V.
You could try an additional "pre-regulator" [ TI UA7824CKCS ] 24 volt but it's input is still only 40V max DC.

Or just measure what your 12V charger pulls from the 12v battery, and use that once a week.

westbranch · April 2008

Re: switch needed to activate when PV producing

Mike, I am running 12 v, panels are in parallel.

It looks like the diode will be very able to handle the max of 22 Voc and 14.72 A Isc pre MPPT

Not sure about the max voltage the 'car charger ' can handle. It is on its way... hopefully it will have specs attached.

I had been thinking that I should tap the charger into the system after the MPPT that way I would get a max of 14.3 v and use the pre MPPT position for activating the 'switch'. Is this possible using a diode?

Eric

mike95490 · April 2008

Re: switch needed to activate when PV producing

westbranch wrote: »

Mike, I am running 12 v, panels are in parallel.

I had been thinking that I should tap the charger into the system after the MPPT that way I would get a max of 14.3 v and use the pre MPPT position for activating the 'switch'. Is this possible using a diode?

Eric

NO ! No diode between the controller and the batteries. That will mess up your charge voltage (the half volt drop from the diode) If you use a standard diode, you will have nearly a volt drop, that's why I suggested the Schottky diode.

Panels in parallel, you can use the

LM340T-12/NOPB http://www.national.com/ds/LM/LM340.pdf $2

no problem at all. The controller will still properly manage the voltage.

westbranch · April 2008

Re: switch needed to activate when PV producing

Thanks Mike, slow on the uptake here,
if I have the correct vision of what you are trying to get thru to an electrical neophyte.... you are visioning that the power to the charger will be supplied pre MPPT. Correct? If so would this not be up to Voc? May be too high for the charger..?

What I am visioning is a dual circuit where the switch (diode ?) would draw power before the MPPT and it would control the flow of the power ( post MPPT) to the charger therfore max V @ 14.3v

Eric

mike95490 · April 2008

Re: switch needed to activate when PV producing

OK, I've had a couple of glasses of wine with dinner , but here's a sketch of what I think I'm trying to say: ( If I've botched something, someone let me know) If you can verify that your charge controller has a "isolation diode" inside it, you won't need the diode I've show -
Digi-Key Part Number V20100SG-E3/4WGI-ND $2 but you will need the voltage regulatror, [ LM340T-12/NOPB ] which is good for up to 35V input, and parallel panels are well below that limit.
Mike

westbranch · April 2008

Re: switch needed to activate when PV producing

thanks Mike, great minds think alike! especially after dos embotellar vino tinto. I was just looking for 'stock' schematics or diagrams to cobble together a sketch.

Yes my CC has a reverse flow relay, they claim is good for 10 to the power of 5 cycles, I may be dead by then... so I guess the diode is not necessary.

OK, got the 340t-12 located, also called the 3 lead TO-220. do all the spec's in the middle column apply? (19v in 12 v out column)

Can you explain the reason for not tapping in after the CC. Without the diode in the circuit there would not be the 1 volt drop you identified. I can see a drop in amps but would the voltage not stay at whatever the CC is sending out ? This part I really don't understand.

Eric

niel · April 2008

Re: switch needed to activate when PV producing

eric,
that is not a 340t and to-220 is in reference to a case style and is not exclusive to just that regulator, nor is it called to-220 as you infer.
these are straight forward fixed regulator ics and are very common. output voltages come in several popular fixed values of voltage. you may see more common part numbers as lm-78xx where the xx is the voltage the regulator works at. although these regulators are rated at up to 35v they do have a max power dissipation ability so lower voltages are advisable.
if putting the regulator on the output of the charge controller, do know that that is the same as tapping the 12v battery. these regulators need at least about 2-3v differentials between input and output to function properly so to place a 12v regulator at the 12v battery won't work, but something like a 5v regulator should work and it will not be dependant upon the sun being out to work as the 12v battery will provide it. if you are looking to not take power from the 12v battery, but only the pvs when the sun is out then place it as mike shows. if it is a 5v regulator you're looking for then the part number would be lm-7805 and do know it won't put out full power without a heat sink.
a good on/off switch and fusing is advisable too.

crewzer · April 2008

Re: switch needed to activate when PV producing

Voltage controlled switch: http://store.solar-electric.com/volconswit.html
DC powered flex timer: http://store.solar-electric.com/fldctico.html

HTH,
Jim / crewzer

mike95490 · April 2008

Re: switch needed to activate when PV producing

niel wrote: »

eric,
that is not a 340t and to-220 is in reference to a case style and is not exclusive to just that regulator, nor is it called to-220 as you infer.

The LM340T-12/NOPB is a low dropout regulator, which only needs a 1 extra volt, not 3.
Powering a PDA is not a large load and I expect it to do fine with only a modest heatsink.
20V from PV, [ Voc 22.0V] dropping 8V across the regulator @ 100mA (to recharge the PDA battery)
8V x .1A = .8W dissipated I'd use a heat sink rated for at least 3 - 5 watts, and then never have to worry about it. Then the cigar lighter adapter converts that 12V to 3V for the PDA. That likely has a LM7805 in it, and consumes nothing - just the series draw from the PDA.

You will also need 2 small capacitrors across the input & output pins of the regulator, go to the data sheet I linked earlier, just need 2 or 3 uF , input is 40V output is 20V or they can both be 40V, last forever.

westbranch · April 2008

Re: switch needed to activate when PV producing

Mike and Niel, thanks for taking the effort to explain these subtleties of these regulators to me... I think I have it now.

Heat sink ( TO-3 ?) right! and fuses in line, ~ 1 or 2 amp?

Need 2 capacitors in the Input and Output connection to ground...

Any recommendations as per switch or will an automotive type do?

Cheers

Eric

niel · April 2008

Re: switch needed to activate when PV producing

to-3? no, this is not a heat sink as that is another case style and that one is more expensive as well as harder to mount. the fuse should be the next up common value available from the max current you expect it to draw. i'm not sure of your pda's current requirements.
you can use a common electrolytic type capacitor up to 100uf with a voltage rating of 35v or higher and that would be 1 on the input and 1 on the output to clarify. any switch should do as you aren't going high voltages or currents.

westbranch · April 2008

Re: switch needed to activate when PV producing

Niel the AC charger I have puts out 4.1 v dc 0.4 A and 6.0 v dc 0.1 A (?) the battery is rated at 3.5v 550 mAh so I expect an average draw would be ~ 5 v to charge. Don't have the car charger yet to check max output.

Smallest fuse locally available is 1 amp - glass tube.

Eric

mike95490 · April 2008

Re: switch needed to activate when PV producing

Heat sink ( TO-3 ?) right! and fuses in line, ~ 1 or 2 amp?

NO, you need a heat sink for a 220 style package.

The fuse, depends on your other loads from your battery, of which I know nothing about. I included it as good general practice, showing that it's near the battery.

Need 2 capacitors in the Input and Output connection to ground...

Look at the data sheet for the part (link is in an earlier message) I believe they want a 1-5 uF cap located close to the actual part, to suppress oscillations. You have clean DC, no need to filter it.

Any recommendations as per switch or will an automotive type do?

Switch ? What switch. This circuit will automatically begin charge when the sun comes up and stop when it goes down. The big 12V battery will still charge and power it's own things as before. It will only steal a tiny bit of recharge current from the big battery.

niel · April 2008

Re: switch needed to activate when PV producing

addressing some of mike's responce to you.
>he and i are correct concerning the heat sink.
>the fuse should be the next commonly available value above the max current you will draw. there are commercially made fuses available for 1/2amp, 3/4amp, etc. these would be better to use, but if you have to go 1amp then do it. 2amps is too much.
>technically you could get away with not using caps on it and they normally specify tantalum caps, but personally i'd still go with a cap on the input and output as these ics can be sensitive to surge and transient pickup that could blow the ics.
>if you tap the pvs rather than the 12v battery for the power you could get away without using a switch, but i still recommend you have the ability to shut it off.

switch needed to activate when PV producing

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