Kent,
Thanks for the great feedback. Your final question is key. I do want it to close at a fairly low voltage (low irradiance). I need the load to run pretty reliably from just after dawn to just before dusk. I think I'll dig deeper and find a relay that a) fits, b) tolerates higher voltage, and c) is low cost. The higher pull in voltage with a resistor in series would probably make the load kick off too often. Thanks again, Jason Szumlanski Fafco Solar Operations Manager 239.574.1500 office 239.410.4985 cell http://fafcosolar.com <http://fafcosolar.com/> Visit us on Facebook! <http://www.facebook.com/pages/Fafco-Solar/93677317294> From: [email protected] [mailto:[email protected]] On Behalf Of Kent Osterberg Sent: Tuesday, February 01, 2011 1:05 PM To: RE-wrenches Subject: Re: [RE-wrenches] Photosensor for 12VDC System with load controller Jason, I think that you would be pushing your luck a little with a 12-volt relay directly connected to a 12-volt PV module. I don't have the specs for the relay you have in mind so I looked up the coil spec for Omron's G2R relays: 12-volt coil: 275 ohms 24-volt coil: 1100 ohms Both: must close at 70% of rated voltage, or below must open at 15% of rated voltage, or above max voltage 170% of rated voltage (assuming 23°C ambient temperature) These are probably pretty typical specs for small relays. The 24-volt coil is not guaranteed to close until 17 volts, so that is out of consideration. The 170% of rating means the maximum for a 12-volt coil is 20.4 volts. Considering that the C40 is charging a battery, the module voltage normally won't be much higher than the battery voltage - unless the battery is fully charged. But in the event that the load were disconnected allowing the battery to reach full charge the relay could see a voltage close to the module Voc, around 22 volts maybe 25 volts if is cold. You are left with putting a resistor in series with the 12-volt coil. If a 120-ohm, 1/2-watt resistor were in series with the 12-volt coil for the Omron G2R, the contacts would close at, or before, 12 volts; it would open at 2.5 volts; and it would tolerate 29 volts without overheating the coil. You should get similar results with your relay if you select a series resistor that has a resistance of 40% to 45% of the coil resistance. The question is: should the load operate on days when the battery is charged but there isn't enough irradiance to activate the relay? Kent Osterberg Blue Mountain Solar, Inc. Jason Szumlanski wrote: Does anyone know if an automotive style 12V relay coil can handle the solar panel voltages common in a 12V nominal panel? This would be a low cost and small size solution that might work best in my case. A $6 relay can switch 40A - well over what I need to do. Jason Szumlanski Fafco Solar From: [email protected] [mailto:[email protected]] On Behalf Of Mick Abraham Sent: Monday, January 31, 2011 9:43 AM To: RE-wrenches Subject: Re: [RE-wrenches] Photosensor for 12VDC System with load controller Maverick's suggestion would give the opposite action to what the original poster needs. The C12 "night light" function turns the load on after dark but Jason needs the opposite. The solar array can serve as your "photocell", Jason. The PV +/- terminals in the C12 controller have no power on them at nighttime (check me on that with your voltmeter), but they are energized when there is sunshine. A relay coil connected parallel to the the PV +/- terminals on the C12 should fire when the sun is up and that coil would then release at sundown. Use a "single pole single throw normally open" relay with a DC coil, and connect the positive leg of your DC load across the normally open connection screws. Make sure the relay coil can accept a wide range of DC input, since it may encounter 20 volts across a 36 cell PV module when the sun first comes up. Jolliness, Mick Abraham, Proprietor www.abrahamsolar.com Voice: 970-731-4675 On Mon, Jan 31, 2011 at 6:52 AM, Jason Szumlanski <[email protected]> wrote: I have a customer with a Xantrex C12 charge controller with LVD controlling a 5A load. Their battery and PV are not large enough to run the load 24/7, and they don't need it to run at night. I am thinking about recommending a photocell to disconnect the load at dusk and turn it back on at dawn. They are not interested in upgrading the PV and battery (too costly). Can anyone recommend a photosensor source, and maybe a model number? Other ideas welcomed... Jason Szumlanski Fafco Solar ________________________________ _______________________________________________ List sponsored by Home Power magazine List Address: [email protected] Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
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