for my studies I have to complete and analyse an electrical project; I am doing a solar panel charger, the solar panel will be connected to a USB power pack to store the electricity generated for phones etc to be charged.

I am aware I need a regulator, but my research has me stuck for which one is the most appropriate. The solar panel will be 80w 12V being converted down to 5v. Do I want to use a linear LM317 or is a DC buck converter a better choice? It is going to be on the inside of a tent, so heat from the circuit must be minimal and need the most efficiency as solar panels do not produce good efficiency as it is.

Im a complete newbie to all of this, and I am completely stuck! So I would appreciate any help!! Thanks in advance :)

EDIT: The USB power pack which I wish to use is 13,000 mAH with the specs:

2x 10W (5V, 2A) USB outputs 1x 10W (5V, 2A) Micro-USB input (flat white Micro-USB cable supplied) auto-on four-LED status system no passthrough charging

I am going to buy a pre built voltage regulator as I am running very short of time, just need to know which type of one and what size I need.

  • 1
    \$\begingroup\$ You canbuy low cost off the shelf (ebay, other ...) that will do what you want far easier and at lower cost than you can do it yourself. \$\endgroup\$
    – Russell McMahon
    Commented Apr 13, 2017 at 15:30
  • 1
    \$\begingroup\$ But the whole point is I dont know how to do it myself :) \$\endgroup\$
    – Walker746
    Commented Apr 13, 2017 at 16:37
  • \$\begingroup\$ You can use a LM317 to make a switching buck regulator, so... \$\endgroup\$ Commented Apr 13, 2017 at 19:52
  • \$\begingroup\$ Your requirements are ambiguous. ALL information from you should be edited into the question. It's OK to provide information in comments as long as you then promptly update you question. | You say: " ... I dont want to use anything completely pre built as I need it to be as high of a level as I can make it. ..." -> Does this mean that the converter MUST be built from components by you? Or ios it acceptable for you to use a purchased converter as part of your overall system? If a prebuilt converter is OK then that would be by far the best path given your state of knowledge. ... \$\endgroup\$
    – Russell McMahon
    Commented Apr 15, 2017 at 9:19
  • \$\begingroup\$ You also need to specify what the power bank consists of. Must this be built by you or may a prebuilt unit be used? If prebuilt is OK then why also not the converter. You MUST specify battery capacity in mAh (NOT mA) and battery voltage. And you should provide a link to the battery spec OR if it is internal to the powerbank we need the max powerbank current. A powerbank usually contains its own 5V to battery charging converter and has a max charge rate so this will totally change your spec. Few powerbanaks would charge above 3A at 5V and most below that. So at 3A 5V 15W OUTPUT from ... \$\endgroup\$
    – Russell McMahon
    Commented Apr 15, 2017 at 9:22

3 Answers 3


A linear regulator works by wasting exactly so much input power, that the output is at the right voltage. If you go from 12V to 5V, it means that you will be wasting over half of your power, always! Apart from the fact that it is very inefficient, there is another problem here - if you are really getting the rated 80W out of your solar panel, it means you will have to dissipate more than 40W in your LM317. This is on par with desktop computers, and there is a reason those have big heatsinks with fans. Even if you made some very fancy cooling system to get rid of this heat, most LM317s in a TO220 package are rated to 1.5A. 80W at 12V is about 6.5A of current - so your LM317 would be damaged no matter what. In fact, I don't know of any linear converter that can handle that much current (There are however linear regulator controllers and such that can do it, but they need big external transistors to do it).

A switching regulator however moves around charge or current, and by doing so, it will be more efficient. Switching regulators can get more than 95% efficiency (given it's a good regulator, working at it's optimal point, ...).

If budget isn't too much of a problem, I would suggest looking at pre-built DC/DC convertor modules. They aren't too expensive, and they are a lot easier to use - building a switching converter is a bit more complicated as they often require external components (switching elements, capacitors, inductors). On top of that, things like placement of components, imperfections in the components and their effects become quite important.

A lot of the prebuilt modules are also protected against overload and short circuits.

I would also like to point out that there are likely DC/DC converters designed for solar panels. These can make sure that your solar panels are always operated in their optimal working point.

  • \$\begingroup\$ I appreciate your help! Would lowering the high wattage of the solar panel be a good idea? I chose a high one as I want the power bank to be charged quickly, but if I chose a 40w for example, or even lower, would this make the choice of regulator different? Also I dont want to use anything completely pre built as I need it to be as high of a level as I can make it \$\endgroup\$
    – Walker746
    Commented Apr 13, 2017 at 12:44
  • \$\begingroup\$ You are always going to want to use a switching regulator, since you don't want to waste too much power. If you are going to charge the powerbank over USB, there is little use in going so high power - most USB powerbanks will only take 1A, maybe 2A at 5V, or 10W. Ofcourse, a higher-wattage panel can potentially create that 10W with less sunlight. If you are charging in some other way, it depends on how fast you can charge your batteries. Even then, 80W is a lot of power, and you are going to need big (or specialized) batteries to be able to charge at such a high power. \$\endgroup\$
    – Joren Vaes
    Commented Apr 13, 2017 at 12:51
  • \$\begingroup\$ So if a 20w 12v panel was used, would a standard DC buck converter actually work? Dosnt need to be the most efficient-just needs to actually work. I have not worked with switching regulators before, and I have very little time (left it to the last minute because Im an idiot). Also, thank you I very much appreciate your help! \$\endgroup\$
    – Walker746
    Commented Apr 13, 2017 at 13:00
  • \$\begingroup\$ You cannot stick ANY BUCK onto any current source (MPT controllers use VI hunting, see my answer for another method, that results in a simple solution. \$\endgroup\$ Commented Apr 13, 2017 at 13:02
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    \$\begingroup\$ "they often require external converters" - should that perhaps be "external components" (or perhaps "external capacitors")? \$\endgroup\$
    – psmears
    Commented Apr 13, 2017 at 15:58

An LM317 is unsuited whatever approach you take as it has a large minimum differential voltage drop compared to Vout so efficincy loss in the regulator is high.

The other choices are a Vpanel >> Vbattery and a down converter OR a panel whose loaded voltage is reasonably well matched to your battery voltage.

If using the existing 2A x 5V power input your max power demand is 10W so max panel wattage needed for a panel that is reasonably well matched for direct connection or using a buck converter is say 15-20W.

If you can charge the battery directly currents of 6.5A max are probably acceptableand 13A may be, depending on battery used.

I was thinking of a 13,000 mAh battery pack, 13000mAh power bank.
The specs are:
2x 10W (5V, 2A) USB outputs
1x 10W (5V, 2A) Micro-USB input (flat white Micro-USB cable supplied)
auto-on four-LED status system
no passthrough charging

2A x 5V input is 10W.
This could be 2A to battery or if they use a converter say average of
5V/3.6V x 2A x 90% say = 2.5A to battery.
3.6V is LiPo mean V - at min its say 3V
so Ibat = 5V/3V x 2A x 90% = 3A.

For a 13000 mAh = 13Ah cell The battery can PROBABLY take up to 6.5A to 13A (C/2 or C) depending on battery model. So using USB at 2A is a very slow charge compared to battery capability. IF you are prepared to bypass the internal charger you can get much faster charge at the cost of needing to control charging yourself.

A charger that is limited to 6.5A max and 3.8V to 4.0V max (whichever limits first) would be "simple enough" and should be OK.
DO NOT EVER "float charge" LiIon or LiPo at 4.2V - they die rapidly.

4V x 6.5A = 26 Watts.
A PV panel rated at 5V to 6V loaded at max power and clamped to 4V max after output diode would work. Panel wattage for max demand needs to be
Wmp > VMP_panel/4V x 26 Watts or say 30+ Watts for a 5V Vmp panel. If you REALLY want abs max performance then a panel that delivers in lower sun is needed and a say 40W to 50W panel MAY be useful.

If you use a panel rated at 12-18V Vmp and a buck converter the panel wattage ratings are similar. Note that panels above about 15W are only needed if you directly charge battery rather than using the USB power input.


edit: let me be perfectly clear


It must be a Buck converter that matches impedance and voltage , Linear losses would reduce output to 25%.


An Ideal voltage source is defined with zero (0) source impedance. An ideal current source is defined by infinite source impedance. A PV is a non-ideal current source with some Zener-like Limit Voltage called V open circuit (Voc).


Thus only a Buck Regulator designed for PV's is efficient since it tracks input current AND output charge voltage AND output current for LiPo's or SLA's to prevent over charging. Many are design with what is called Maximum Power Transfer algorithm.

- So look for a MPT Solar charger, which cost a bit more but if you want most of your 80W why setting for 10~20%?

Other ticky tacky technical details

Since PV is a non ideal current source, the equivalent Zsource varies with solar intensity, Z is complex power source

.eg. 100% is P=V²/Rs or Rs~V²/Pd then at 25% solarity Rs is 4x.

To get 80% of Voc (open cct) then Load impedance by impedance ratios can be computed, right?

While impedance of series Buck L to PV is ZL=2πfL / duty cycle roughly if sinusoidal but slightly different when switched at 50% in a triangular current in continuous mode.

A 5V battery is like an infinite Cap or >> 1000Farads with known ESR.

My concept for you,... is such that the Buck regulator always loads the PV to 80% of Voc +/5% + full power, so if rated for 80W 12V , I assume it as an no load or Voc = 1/80% *12V or 15V approx. which rises with full sun and lowers when 10% sun power about +5/-10%.

It must be a Buck converter that matches impedance and voltage , Linear losses would reduce output to 25%.

Since Zsource varies with solar intensity, Z is complex .eg. 100% is P=V²/Rs or Rs~V²/Pd then at 25% solar Rs is 4x, while impedance of series L to PV is ZL(f)=2πfL /d.c. ( duty cycle)... rough simple model for triangle current

Thus 80W , @ 12V @ 100% solar input Rs=1.8Ω so to get 80% of Voc down to 12V for Maximum Power Transfer P=V*I, (aka MPT)

Now consider matched impedance for maximum power transfer theorem.

BUCK down converter must be 1.8Ω at 100% solar input (Ps) then we have 3 equations;

  • Rs~V²/Pd
  • Vmp=80% Voc +5/-10% ( your mileage may vary with PV chemistry , quality, dust etc)
  • ZL=2πfL / duty cycle for continuous current mode (CCM) choke
  • to lower PV Zs further with harmonics of fundamental, we need to add a Cap where Zc=1/(2πfC) to shunt PV impedance and protect for battery reversal.

Can you solve now for CCM Buck converter to get 90% efficiency?

Next we need voltage regulator feedback for 5V battery to adjust PWM duty cycle to prevent over charge and impedance of MOSFET and choke must not be lower than Zs of panel under any conditions.

To avoid further complexity I have avoided ESR of PV , DCR of L and ESR of Caps and battery also RdsON of FET. But these are also relevant for efficiency improvements as well as I max rating for choke to prevent saturation, where lower ESR is better.

  • \$\begingroup\$ Thanks for your answer! Im not an electrical student, Ive not studied it before, so I got very lost very quickly. Im going to change the solar panel to 20w 12v, is there a simple and stupid way of finding which regulator I need? Or could you tell me what lines I need to look down? Thanks \$\endgroup\$
    – Walker746
    Commented Apr 13, 2017 at 13:10
  • \$\begingroup\$ OK we dont do shopping questions but obvious an efficient 80W 12 to 5V solar converter with MPT, without MPT it becomes very inefficient, like riding a bike up hill in 10th gear. PWM is like continuously changing gears but instead of regulating V out , Solar chargers must regulate both Iin and Vout \$\endgroup\$ Commented Apr 13, 2017 at 13:11
  • \$\begingroup\$ Tony - he is powering a "USB power bank" so the output is fixed 5V nominal which makes a buck converter set to Vout = ~= 5V acceptable. These tend to have iMax limited by their internal converters over most of the charge cycle (say the CC portion plus perhaps some of the CV) so load is moderately constant most of the time. A quite simplistic solution is liable to work well enough for him. \$\endgroup\$
    – Russell McMahon
    Commented Apr 15, 2017 at 9:28
  • \$\begingroup\$ It does not regulate peak input current and just average limited by watt rating, so if a large cap was added to input..., maybe under varying solar conditions 50% of MPP on average. and worse without this,. Yes works but but how much storage or peak power under adverse conditions .. who cares?? \$\endgroup\$ Commented Apr 15, 2017 at 15:39

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