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Old 02-15-19, 04:32 PM   #16
Robaroni
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Hi Poor Man,
I love these kinds of projects, they're so much fun!
OK, if the light is 5 watts that means the current it draws is 5 watts divided by 120 volts or 0.042 amps. BUT LEDs don't run on AC (Alternating current like your mains power). They run on DC (direct current), so this means that inside your bulb there is a circuit to convert AC to DC to run the bulb, also the DC voltage to run the LED is much lower than the 120 volts.

That's a good thing because the LED actually uses less current than the 0.042 amps we just calculated. What makes it better is that the PV module also runs low voltage DC like the LED. So why add an inverter to boost and invert the DC from the module when you can run things without doing most of that?

The trick here for people who don't know electronics is how to disassemble the bulb and tap the output of the bulbs electronics. There are a couple of ways to do this. If you just want a light without the auto off/on then finding the two wires that run the LED is a lot easier but if you want the auto off/on the you will need a meter to test the circuit and find the point where it powers the complete circuit.

Living without, at least at the beginning of your experimenting, isn't a bad idea because you can buy boards on eBay and I guess Amazon to drive LEDs. You can still use your light fixture but what you'll do is remove the standard bulb socket and put the LED driver circuit inside with a plain old LED or a made up unit like this (this is just a suggestion, this light is 18w and is probably a spot light but you get the idea):

https://www.ebay.com/itm/2x-4INCH-36...AV9:rk:13:pf:0

That runs on 12volts DC. So you'll need a battery, a small charger to interface the battery to your PV module like this:

https://www.ebay.com/itm/12V-24V-PV-...frcectupt=true

What I would do is to get the parts I linked or something like them and a PV module.

So, how big a module do you need?
OK, if you're using a 5 watt LED for ten hours you need ~50 watt hours. (5 watts for 10 hours uses 10 times 5 watts for one hour. we use watt hours to calculate our power needs)

Let's say you have an average of 5 hours sunlight for your latitude average daily for one year. ( you can look these up for your area) If you have a 75 watt panel it will put out in 5 hours 75 x 5 or 375 wh. You want some 'overhead' because you'll also need a battery, something like an SLA (sealed lead acid). BUT the battery has something called an amp hour rating. That means how many amps it will put out for an hour but this is misleading. You really don't want to drain your battery more than about 25% to be safe and to have it last. Batteries are rated in cycles, each time you drain it and charge it up is a cylce. The deeper you drain it the fewer cycles it will have in its life. So let's say we have a cheapo 12v - 7aH, SLA battery. It has to run all night (10 hours). a 5 watt LED = 5/12 or ~416 milliamps (milliamps are amps divided by 1000; 416 mA = .416 amps, for example) just so you understand amps and watts. OK, our battery has to supply 50 wH and our battery puts out 7aH x 12v or 84wH. This is not enough because we want to only use 25% of our battery. We need a battery of 50 x 4 (which is 4 times our 25% to make 100%) or 200 wH. 200/12v = 16.6 aH thus we need a battery of this size at 12 volts for our 5 watt LED.

So there you have it. A 5 watt LED, a 75 watt module (60 will probably work), a small charger and a 16 aH 12V SLA battery.
Rob
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