usb solar panel charger

A

antok86

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The output from a solar panel is highly variable. You'll need to ensure the resulting voltage you're powering a device with is within a certain tolerance. Often a battery is charged with the solar panel's output, with the battery acting as a buffer.
 
You can place a 100uF or so electrolytic in parallel with the panel to help smooth its output.
 
antok86 said:
shouldnt it be safe as long as it doesnt go over 5v?
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Did you design the device that this is going to be charging? If not, there is no way to answer this - the USB spec allows 4.75V - 5.25V, and there is no way to know if the device was made to function with anything less then 4.75V. Using a diode will reduce the voltage by 0.2V - 0.7V, depending on the type used.


Since the panel output is unregulated & already close to where it needs to be (5V), you would ideally use a SEPIC or Buck-Boost converter, which will both be able to provide regulation regardless of the input voltage being above or below 5V. You also may be able to get by with a simpler Buck or Boost converter, depending on operating conditions (both very plentiful on ebay). Using a diode to reduce the voltage could ensure that the output is always below 5V, thus a Boost converter could be used, but some power will be wasted in the diode as heat.
 
im just planning to use it as a usb device charger I.E cellphones and ipods/mp3 players. i thought it would be safe since the panel is at 5v. well if the spec drops below 5v wouldnt it stop charging since usb is at 5v? also i would be using the diode to stop it from sending power back to the panel so that power is one way from panel to device.
 
antok86 said:
im just planning to use it as a usb device charger I.E cellphones and ipods/mp3 players. i thought it would be safe since the panel is at 5v. well if the spec drops below 5v wouldnt it stop charging since usb is at 5v? also i would be using the diode to stop it from sending power back to the panel so that power is one way from panel to device.
Click to expand...

USB isn't exactly 5v though - the specification for the USB standard says that it can vary from 4.75-5.25v. If your panel puts out less than 4.75v due to lack of sun, there's no telling how the device will react. It may simply cut off, or it might have issues powering a circuit, causing it to lose all data in memory.

Yes, that's what diodes are generally used for. However, diodes are not perfect conductors, so there's some loss even when electricity is passing through in the normal forward direction. Normal silicon diodes are about .7v drop. Even if your panel is putting out a perfect 5v, adding a diode in will drop that down to about 4.3v. Since that's out of the specified range for USB, your device may never actually see enough voltage to even charge at all.
 
ill have to see what the readings pull once i have it. but as far as my setup it should be fine right?
solar panel >diode>usb. what can i do to maintain 5v as much as posssible even with vdrop. i mean the diode is necessary is there an alternative to using a diode
 
I certainly wouldn't connect any of *my* devices to an unregulated solar panel. The output voltage of a solar panel depends not only on how much light is hitting it, but also on how much current is being drawn from it. On a sunny day, the open-circuit voltage might be 7.5V. The "5V" is only a nominal value.

In other words, you need a boost-buck converter, like PM650 said.
 
Mohonri said:
I certainly wouldn't connect any of *my* devices to an unregulated solar panel. The output voltage of a solar panel depends not only on how much light is hitting it, but also on how much current is being drawn from it. On a sunny day, the open-circuit voltage might be 7.5V. The "5V" is only a nominal value.

In other words, you need a boost-buck converter, like PM650 said.
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ok so your saying if the panel is rated at 5v? i see panels ratings for various voltages. so lets just say its rated for 5v, it can still draw over that? is that what your saying?

got a good link on boost-buck converter? couldnt i use a voltage regulator?
 
antok86 said:
ok so your saying if the panel is rated at 5v? i see panels ratings for various voltages. so lets just say its rated for 5v, it can still draw over that? is that what your saying?

got a good link on boost-buck converter? couldnt i use a voltage regulator?
Click to expand...

The buck-boost converter can provide a regulated output voltage from an input that actively varies above & below the output voltage. You can't use a voltage regulator because you can't guarantee that the solar panel voltage will always be below 5.25V (not taking into account diode & voltage reg. dropout voltages), unless you're willing to take a huge hit in efficiency. Even a low-dropout linear regulator has a dropout voltage of 0.5V, so it's not really an option unless 5.5V is guaranteed. Using a higher voltage array + LDO regulator will work, but efficiency will decrease with increasing voltage.

I wasn't able to find any cheap, non-diy solutions on ebay. One thing you could do is use two converters: first boost the output voltage of the solar cell above 5V with a boost converter, then follow with a buck converter to reduce to 5V. All for $16:
boost
buck

I'm not so sure you would need a diode to prevent the device from attempting to supply power back into the power supply. You could try measuring the outer pins of the USB cables of said devices while unplugged to see if this is a problem.
 
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ok let me ask you this...i have a minty boost kit that i made a while back
looks someothing like this
Minty_Boost_4.jpg
instead of the battery holder can that be replaced with the solar panel?
 
Certainly looks like it would work, but you would still need to make sure the input is always under 5V.
 
i still dont understand..if the solar panel is rated for 5v...on how it can draw over 5v
 
The 5V is nominal....it's not the maximum or minimum rating. Wait until you get the solar cell and measure the output in various light conditions - if it never outputs more than 5V, then it can be used as-is.
 
ok ill have to check that out...on that instructables link it looks like they just connect the solar panel to the LiPoly Charger PCB for the purpose of charging the battery. so if i just bypass the LiPoly Charger PCb and connect it directly to the main mintyboost board it should work. right?
 
Look, this is all based in the type of solar cell you have. If you have one rated for less than 5V in all conditions, then you would only need a boost converter. Likewise if it was rated for over 5V in all conditions, a buck converter would suffice. Since the cell you've described is 5V nominal & the desired output is 5V, there is no simple solution to this.

That guide is for a much more complex, and presumably more expensive, device that includes a Li-Po battery & charger for extended use - both the battery & the solar cell used in that project provide less than 5V in all conditions, which negates the use of a buck converter to reduce the voltage. Just for completeness, their solar cell is rated for 4.5V, but actually provides 4.95V with no load, still less than 5V.
 
Possibly worth noting, building that instructables project could potentially be dangerous. The charger used is only capable of monitoring the charging of a Li-Po battery, but discharging must also be monitored to ensure that the cell voltage never reaches below 3V under load, or else the battery itself could be damaged. No protection is present, as the battery is connected directly to the boost converter, which will just keep working at full steam, even when the battery reaches 3V & lower.
 
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