Discussion in 'The Projects Forum' started by solarnovice, Apr 12, 2008.

1. solarnovice Thread Starter New Member

Apr 12, 2008
3
0
low budget trial to adapt existing trickle charger.

Purpose: USB charger direct from solar panel. (I know, battery inbetween is plenty good. but USB gadgets have internal batteries and charge logic built in, so usurp them!)

My solar puts out 1.5Watts 15V 100ma hope the numbers are right.

I looked, I found. DC voltage regulator circuits. Using LM317T and resistors to regulate the Volts to 5.3V. No problem there. So the DC regulator cuts the volts from 17.5 max (variable) to 5.3V (steady).

I know, many have misunderstood the relationship between Volts/Amps/Watts including me. I am there to a degree. So the resistor resists the flow and dissipates some as heat. Seems a waste of all those volts since the panel generates them anyway - that is probably a stupid thing to say but wait a minute!

My question is this. The panel puts out 100ma. at 15V. right. I reduced Volts to 5.3V. Can I use the extra available (i might add - freely available) Volts to generate more Amps ? say i wanted 300ma at 5.3V - is there a circuit out there that could use the unused volts to pump up the amps ? Yes, i know. again. silly question. but I have no electronics background - maybe one of yous know more and I missed something ?

Thanks.

call me a numbnut if you will, but your answers could save me months of searching. Thus making me jack of many trades but king of none!

2. mrmeval Distinguished Member

Jun 30, 2006
833
2
A linear regulator *wastes* the power. You would want to look at an ultra high efficiency DC-DC convertor. Try http://www.national.com This is the power management page http://www.national.com/appinfo/power/

If 15*0.100=1.5W then 5.3* A =1.5W if you have perfect efficiency. This is 280ma at best.

I plugged the numbers in to their page and got a 90 percent one which means about 250ma or so. YMMV

LM25574
Topology BUCK
Max Current 0.5 A
Typical Efficiency 90%
On/Off Pin Y
Error Pin N
Price \$1.35
Frequency 1000 kHz
Design Note Simple Switcher(r) Regulator

It's a nifty little part, the only exotic part is the diode.
http://www.national.com/pf//LM/LM25574.html

3. solarnovice Thread Starter New Member

Apr 12, 2008
3
0
that answered my question. thanks mrmeval.

4. Audioguru New Member

Dec 20, 2007
9,411
896
The output from a solar panel depends on a sunny day, the time of day, how close you are to the equator of earth and how well it is aimed at the sun.

I tested my 1.8W solar panel behind a clear window. Its current was half what it is rated and outside it was only a little more. In the morning and in the late afternoon its output was much lower.