I have built a solar charger by using a linear voltage regulator (LM7805) that outputs 5V from a solar panel which can deliver a maximum of 18V. It works fine, but the current is very low and there is a lot of heat coming out from the voltage regulator.

I read here and there that a better option for this would be to use a DC/DC converter (or buck converter) which, according to Wikipedia is a device that rises up the current while dropping the voltage.

Now, it would be great to have a constant 5V output like I do now, but I'm afraid that the oscillation in the current is not so good for charging a phone, but I base this on pure intuition, therefore I would like to hear an opinion on this.

  1. Will a buck converter have a large current interval as the sun goes behind the clouds?
  2. Can this interval be regulated?
  3. Is it harmful to have a large interval of current for charging a phone?
  4. What type of buck converter should one look at if one plans to buy a bigger solar panel in the future?
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    \$\begingroup\$ "I have built a solar charger by using a voltage regulator that outputs 5V", is that voltage regulator a linear voltage regulator? (I feel it's something that's worth mentioning for some reason). \$\endgroup\$ – Harry Svensson Aug 8 '17 at 17:31
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    \$\begingroup\$ use a super cap to brighten up cloudy days, or just keep the 5v steady. an adjustable boost+buck would be even better than "just" a buck smps \$\endgroup\$ – dandavis Aug 9 '17 at 1:39

The voltage output of a buck converter will be constant, as long as there is enough input power to supply your load. If there isn't, the output will either sag, or the buck will go into undervoltage lock-out and shut down. With a switching regulator Power in = power out with some losses from conduction and switching.

The current drawn will obviously depend on your load. If the load is constant and the buck is in regulation, the current will be constant. Not sure what you mean by "a large current interval".

The better way to use all the available power from a solar panel is to use an MPPT controller (Maximum Power Point Tracking). This part optimizes the current draw from the panel over operating conditions to extract the maximum possible power from the panel.

If you Google MPPT you should be able to find lots of information on the subject.

  • \$\begingroup\$ Thank you very much for the answer and for the advice. Very helpful. As for the "large current interval", I mean if I have a converter that outputs max 2A, the interval will be from 0 to 2A, right? The concern that I have is what happens if this interval occurs in a very short time due to some clouds. I'm afraid of peaks like that. I'm checking MPPT. \$\endgroup\$ – Physther Aug 8 '17 at 17:57
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    \$\begingroup\$ @Physther what happens when you pull out the charger from the socket while charging a phone? The phone will.... stop... charging. And then when the clouds go away, the phone will... continue... charging. \$\endgroup\$ – Harry Svensson Aug 8 '17 at 17:58
  • \$\begingroup\$ Good point. But what happens if you plug in and plug out continuously? Will this affect in any way the phone? \$\endgroup\$ – Physther Aug 8 '17 at 18:00
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    \$\begingroup\$ It will not affect the phone. Phones have sophisticated charge management systems built in, and will seamlessly handle presence/absence of a charging voltage on the input. \$\endgroup\$ – John D Aug 8 '17 at 18:04


Vload/Voc open cct. needs to be 75 ~82% depending on solar power. You are using 5V/18V=27.7% so your output current cannot be increased, but output power is optimal with above ratios. So choose a 5V SMPS stepdown regulator with a large C or 13.7V SLA battery or 14.2V LiPo with V limiter. this will boost the output current by (correction) Vin/Vout * efficiency of SMPS. e.g. 14.2/5V * 90% = 250% increase in current , neglecting battery current , by assuming fully charged. SHort term current boost depends on Ah capacity and SMPS power rating can be much higher. Reverse V blocking diode is necessary for PV.

  • \$\begingroup\$ Small batteries are > 100 Farads , large caps are in mF , important factor is Zc(f) << R Load. \$\endgroup\$ – Sunnyskyguy EE75 Aug 8 '17 at 18:01
  • \$\begingroup\$ Thank you, Tony. Interesting information. I will look into that \$\endgroup\$ – Physther Aug 8 '17 at 18:03

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