Need help with solar outdoor walkway LED light specs

Thread Starter

Aleksey Shurtygin

Joined Dec 21, 2018
71
I am planning to repurpose our solar outdoor light and power them from an outlet. Does anyone know any specs for these LEDs or any datasheets? Are they similar to regular LEDs? I want to see how much I can increase voltage to get max brightness, or if I can at all.

See photo. The black piece is a 5252F solar led driver and whole thing seems to be powered by 1.5V battery. According to other posts that battery may be actually lower, more like 1.2V when fully charged. I've seen couple of other posts but none seem to refer to LED specs.

Thank you!
 

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wayneh

Joined Sep 9, 2010
17,152
Are they similar to regular LEDs?
Yes. I have not encountered one that is not.
I want to see how much I can increase voltage to get max brightness, or if I can at all.
Well, there may a little room for increase but the brightness you see is proportional to the time-averaged current. A typical LED can survive 20mA but will last longer at 15mA and below. So I would not expect to get much brightness increase compared to what a fresh battery gives. Of course that's a lot brighter than these things are as the battery runs down.

The little circuit in these things pulses the voltage so that an LED that normally requires 3V to light can be run off a single cell. It's actually off much of the time and the time-average voltage is roughly identical to the battery voltage. If you put your multimeter leads on the LED, that's what you'll see. But in fact it's pulsing on and off. A higher battery voltage will brighten the LED.

You could absolutely power the LEDs off a DC power supply and bypass the boost circuit.
 

Audioguru again

Joined Oct 21, 2019
3,171
The circuit is designed to use 1.4V maximum so forget about increasing its supply voltage.
You need to replace the circuit board with a series resistor that limits the current to 20mA. Use 150 ohms, 1/4W for a white LED.
One white LED needs about 3.2V to 3.6V. Use a 6VDC wall wart with a current rating close to the number of LEDs you will power.
Each LED will draw (6V - 3.4V)/150 ohms= 17.3mA.
 

Audioguru again

Joined Oct 21, 2019
3,171
Old solar garden lights used an AA size Ni-Cad battery cell with a low capacity of 250mAh.
Most of my newest solar garden lights came with a weak AAA Ni-MH battery cell that is marked 300mAh but it performs like it is only 200mAh. So that it lights for a few hours its battery current is low by having an inductor in the voltage booster has a high inductance. The inductor is small like a 1/4W resistor.

I throw away the garbage battery and use an Energizer AAA Ni-MH battery cell made with Eneloop chemistry and is 800mAh so I use a lower inductance for a brighter LED. The solar panel charges well when the day is sunny. It charges poorly on cloudy days.
 

Thread Starter

Aleksey Shurtygin

Joined Dec 21, 2018
71
Yes. I have not encountered one that is not.
Well, there may a little room for increase but the brightness you see is proportional to the time-averaged current. A typical LED can survive 20mA but will last longer at 15mA and below. So I would not expect to get much brightness increase compared to what a fresh battery gives. Of course that's a lot brighter than these things are as the battery runs down.

The little circuit in these things pulses the voltage so that an LED that normally requires 3V to light can be run off a single cell. It's actually off much of the time and the time-average voltage is roughly identical to the battery voltage. If you put your multimeter leads on the LED, that's what you'll see. But in fact it's pulsing on and off. A higher battery voltage will brighten the LED.

You could absolutely power the LEDs off a DC power supply and bypass the boost circuit.
Thank you!
 

Thread Starter

Aleksey Shurtygin

Joined Dec 21, 2018
71
The circuit is designed to use 1.4V maximum so forget about increasing its supply voltage.
You need to replace the circuit board with a series resistor that limits the current to 20mA. Use 150 ohms, 1/4W for a white LED.
One white LED needs about 3.2V to 3.6V. Use a 6VDC wall wart with a current rating close to the number of LEDs you will power.
Each LED will draw (6V - 3.4V)/150 ohms= 17.3mA.
Thank you! So if I wanted to run it off 12V line I'd need to use 470 Ohms resistor, right? (12V - 3.4V) / 470Ω = 18mA
 

Thread Starter

Aleksey Shurtygin

Joined Dec 21, 2018
71
Is there a reliable graph for current vs brightness? Or can I assume that any graph that I find out there for same color can be applied to these LEDs as well?

I am planning to run a line for about 40 feet with about 7-10 lamps and I am expecting a 0.2-0.3 V drop at the other end of the wire. So, 12V/470Ω gives me about 1mA difference between closest and furthest points. While a 5V/150Ω gives me about 2mA difference. I wanted to see if that is going to make a big difference in brightness across the whole line. Also would appreciate if someone can confirm if my thinking is correct or if I have missed something.

Thanks!
 

wayneh

Joined Sep 9, 2010
17,152
Is there a reliable graph for current vs brightness? Or can I assume that any graph that I find out there for same color can be applied to these LEDs as well?

I am planning to run a line for about 40 feet with about 7-10 lamps and I am expecting a 0.2-0.3 V drop at the other end of the wire. So, 12V/470Ω gives me about 1mA difference between closest and furthest points. While a 5V/150Ω gives me about 2mA difference. I wanted to see if that is going to make a big difference in brightness across the whole line. Also would appreciate if someone can confirm if my thinking is correct or if I have missed something.

Thanks!
Brightness is roughly proportional to current. Drop the current 10% and the perceived brightness drops about the same amount. I doubt very much you'd notice that unless the LEDs are side-by-side.
 

Audioguru again

Joined Oct 21, 2019
3,171
Thank you! So if I wanted to run it off 12V line I'd need to use 470 Ohms resistor, right? (12V - 3.4V) / 470Ω = 18mA
Yes, and a 1/4W resistor will be very warm but not too hot.
You can also connect two LEDs in series and use a 270 ohm resistor.
White, blue and bright green LEDs are about 3.4V. If the LEDs are red, pink, yellow or old green then their voltages are about 2V and the resistor value must be increased to 560 for one LED and 470 ohms for two series LEDs.
 

Thread Starter

Aleksey Shurtygin

Joined Dec 21, 2018
71
Yes. Bulk LEDs often ship with 'free' 470Ω resistors for this reason. Note though that you could put 3 LEDs in serial and then use a smaller resistor. That arrangement wastes less power in heating the resistors.
Its actually not a bad idea. If I am going to remove all the guts I'll have more real estate for couple of extra LEDs. However for now I am planning to use existing ones for now but if it proves to be too dim for my purpose that will be a viable option.

Thanks!
 

Thread Starter

Aleksey Shurtygin

Joined Dec 21, 2018
71
Yes, and a 1/4W resistor will be very warm but not too hot.
You can also connect two LEDs in series and use a 270 ohm resistor.
White, blue and bright green LEDs are about 3.4V. If the LEDs are red, pink, yellow or old green then their voltages are about 2V and the resistor value must be increased to 560 for one LED and 470 ohms for two series LEDs.
It's a warm white LED. It will be warm because current is ~18mA and voltage drop on the resistor is 8.6V which equals 0.15W and it is close to 1/4W. Is that correct? At what power level resistors would start heating? Is there a graph that can show that or its really depends on a manufacturer (for the same power rating)?

Thanks
 

wayneh

Joined Sep 9, 2010
17,152
It's a warm white LED. It will be warm because current is ~18mA and voltage drop on the resistor is 8.6V which equals 0.15W and it is close to 1/4W. Is that correct? At what power level resistors would start heating? Is there a graph that can show that or its really depends on a manufacturer (for the same power rating)?

Thanks
A rule-of-thumb is to always choose a resistor with a power rating at least double the expected actual power it will be dissipating. So 1/4W rating for 0.15W continuous duty is technically below borderline. Probably OK, especially for a low risk scenario (garden light versus a moon landing), but a concern for long-run lifetime. Using a 0.5W eliminates the concern. But I'd rather just add a 2nd LED and run them both at <15mA. More light, no heat.
 

Audioguru again

Joined Oct 21, 2019
3,171
20mA in an ordinary little 5mm white LED does not produce enough light to light up a walkway but it indicates the border. If it is close to you and you look at it at night then it will seem to be extremely bright.
 

Thread Starter

Aleksey Shurtygin

Joined Dec 21, 2018
71
A rule-of-thumb is to always choose a resistor with a power rating at least double the expected actual power it will be dissipating. So 1/4W rating for 0.15W continuous duty is technically below borderline. Probably OK, especially for a low risk scenario (garden light versus a moon landing), but a concern for long-run lifetime. Using a 0.5W eliminates the concern. But I'd rather just add a 2nd LED and run them both at <15mA. More light, no heat.
That makes sense. Thank you!
20mA in an ordinary little 5mm white LED does not produce enough light to light up a walkway but it indicates the border. If it is close to you and you look at it at night then it will seem to be extremely bright.
Yeah, my requirement is really to provide only some indication of the borders and/or have some light sources on the backyard so its not pitch black at night. At the same time wanted to see what I can get out of it if I switch solar to power outlets. Thanks!
 
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