Solar Panel Testing
Hi,
I was looking at the small solar panel car battery maintenance charger I bought awhile ago and thought, how useful would this be to charge a mobile phone. So I did some tests.
As can be seen above, the specs of this panel are 12-16VDC, 4W and 300mA.
I put together a quick 5V USB charging circuit on a breadboard following the typical application from the 7805 datasheet.
Above shows the solar panel charging a mobile phone at a measly 171.3mA.
With the 7805 linear regulator the input current must equal the output current therefore, following Ohm's law, the input power is always higher than the output power and the higher the difference in voltage, the worse the efficiency will be.
Output = 171.3mA @ 5V - 5V * 171.3mA = 0.856 ~ .86W
Input = 171.3mA @ 12V - 12V * 171.3mA = 2.055 ~ 2.06W
η = Output Power / Input Power
η = 0.86W / 2.06W = 0.417 ~ 42% efficient
Now, out of interest, lets examine the thermal properties. The LM7805 datasheet specifies the junction to air characteristic as 65°C/W. 2.06W - .86W = 1.2W dissipated in heat from the 7805. 65° * 1.2W = 78°. Ambient temp was 37.3° (Shown on the meter below and confirmed with historical data).
Therefore, 78° + 37.3° = 115.3°. This figure is confirmed by the thermal camera image below.
I also tried charging a JBL speaker.
This time we have a charge current of 184.6mA. Doing the calculations again:
Output = 184.6mA @ 5V - 5V * 184.6mA = 0.923 ~ .92W
Input = 184.6mA @ 12V - 12V * 184.6mA = 2.21 ~ 2.2W
The efficiency remains the same.
Next I tried the same tests but using a cheap switching regulator.
As can be seen above, we're getting more charge current this time. Unfortunately at the time I didn't measure how much power the panel itself was generating so I can't do accurate efficiency calculations. But anyways:
Power = 223.9mA * 5V = 1.1W
Better than the 7805 but still not great, however, when I plugged the JBL speaker in again...
We got:
Power = 332.5mA * 5V = 1.67W
Curiously a much better result. For those interested, the thermal's of the switching regulator were:
Much, much better at 35°!
So there you go. Nothing unexpected just thought it was an interesting little test of efficiency and thermals between two different voltage regulator technologies.
Thanks
Justin
I was looking at the small solar panel car battery maintenance charger I bought awhile ago and thought, how useful would this be to charge a mobile phone. So I did some tests.
As can be seen above, the specs of this panel are 12-16VDC, 4W and 300mA.
I put together a quick 5V USB charging circuit on a breadboard following the typical application from the 7805 datasheet.
Above shows the solar panel charging a mobile phone at a measly 171.3mA.
With the 7805 linear regulator the input current must equal the output current therefore, following Ohm's law, the input power is always higher than the output power and the higher the difference in voltage, the worse the efficiency will be.
Output = 171.3mA @ 5V - 5V * 171.3mA = 0.856 ~ .86W
Input = 171.3mA @ 12V - 12V * 171.3mA = 2.055 ~ 2.06W
η = Output Power / Input Power
η = 0.86W / 2.06W = 0.417 ~ 42% efficient
Now, out of interest, lets examine the thermal properties. The LM7805 datasheet specifies the junction to air characteristic as 65°C/W. 2.06W - .86W = 1.2W dissipated in heat from the 7805. 65° * 1.2W = 78°. Ambient temp was 37.3° (Shown on the meter below and confirmed with historical data).
Therefore, 78° + 37.3° = 115.3°. This figure is confirmed by the thermal camera image below.
I also tried charging a JBL speaker.
This time we have a charge current of 184.6mA. Doing the calculations again:
Output = 184.6mA @ 5V - 5V * 184.6mA = 0.923 ~ .92W
Input = 184.6mA @ 12V - 12V * 184.6mA = 2.21 ~ 2.2W
The efficiency remains the same.
Next I tried the same tests but using a cheap switching regulator.
As can be seen above, we're getting more charge current this time. Unfortunately at the time I didn't measure how much power the panel itself was generating so I can't do accurate efficiency calculations. But anyways:
Power = 223.9mA * 5V = 1.1W
Better than the 7805 but still not great, however, when I plugged the JBL speaker in again...
We got:
Power = 332.5mA * 5V = 1.67W
Curiously a much better result. For those interested, the thermal's of the switching regulator were:
Much, much better at 35°!
So there you go. Nothing unexpected just thought it was an interesting little test of efficiency and thermals between two different voltage regulator technologies.
Thanks
Justin
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