Super-simple solar regulator - Page 1
Forum section: electronics / projects

author: Siwastaja
author_rank: Super Contributor
timestamp: June 27, 2015, 07:37:39 am
content: Hi,Sometimes I like to do some super-simple, "one-day" projects from parts I find. This time I found a nice 0.5W solar panel, around 5.5V open-circuit voltage, I guess the best power point is somewhere under 4V.So I soldered some pogo pins on a PCB clad to make a connector to charge a small camera li-ion cell.I though that a simple zener controlled shunt regulator would be easy and good enough, so I drove a NPN base with that (didn't have beefy zeners), with a few resistors as a load between collector and +. And instead of zener, I actually used a series combination of a 2.1V low-tempco reference diode and a green LED, because that got me what I wanted.But as we all know, these diodes do have a curve that is not steep enough. The charger does work under full sunshine, but anything less, and the regulator burns any charge when the cell is still at about 3.9 volts. I don't need to get the cell completely full, but it's not okay if it starts to discharge back to less than 60-70%, or never gets over that point. The sun rarely shines for the whole day here.I could design any complex system, and a small MCU-driven DC-DC could do MPPT and stop charging at exactly 4.2V, but I don't want to do that now.Any quick, dirty, super-duper easy ideas to make the shunt regulator curve more steep, or anything else? I also have a schottky preventing power flowing back to the panel, and replacing that with active solution would be great (because the power point is already a bit unoptimal, and extra drop makes it worse), but I can't come up with any really simple way.I'm thinking about using a pass transistor (low-drop FET) controlled by two comparators, one would shut down the pass transistor when the battery is at 4.15V, another would shut it down if the solar cell voltage is less than battery voltage (kind of active diode controller), but I don't think I have any very low power comparators in my parts bin right now, and this sounds a bit complex for a one-day project, especially because I don't expect the active diode controller to be that easy in practice (I have done it before and it's not trivial).Any brainstorming is welcome, even outside this exact problem.

author: djQUAN
author_rank: Frequent Contributor
timestamp: June 27, 2015, 09:03:33 am
content: Use a TL431 and a power transistor to dissipate the excess. The TL431 is basically a comparator with a built in reference.

author: ralphd
author_rank: Frequent Contributor
timestamp: June 27, 2015, 01:34:14 pm
content: Maybe a PNP voltage follower with a 3.6V zener between base and Gnd. 3.6 + .6 gives you the 4.2V you want.

author: Seekonk
author_rank: Super Contributor
timestamp: June 27, 2015, 04:31:02 pm
content: Don't think you can do better than a 431 and two high value resistors. At that power level you likely wouldn't even need a dropping resistor although one could be thrown in.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 27, 2015, 05:26:50 pm
content: Yes, I forgot about the TL431; it's just right for the job, and it can shunt 100 mA so I won't need an external transistor. Probably going to put some low-value resistor to share the power dissipation between the chip and resistor, which is what I did before with the zener + NPN configuration.Of course some discrete cheap-ass solution would have been nice, but 431 is luckily common and old enough so it should be stocked at a local supplier.I'll do just that. And I'll forget about active rectification because it'll get too complex.Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: June 27, 2015, 05:57:40 pm
content: You need to add a diode in series with the battery to prevent it from discharging when it's dark but you'll need to increase the regulator's voltage to compensate for the voltage drop.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 27, 2015, 06:04:40 pm
content: Quote from: Hero999 on June 27, 2015, 05:57:40 pmTL431 should have low enough quiescent draw (with high enough resistor values) so that the diode can be on the input side - this way, the end voltage doesn't depend on current like it does with diode on output; though, the difference is small, so it might still be best to put the diode on the output side to make the dark leakage even smaller, and adjust the voltage as you said.

author: Zero999
author_rank: Super Contributor
timestamp: June 28, 2015, 08:13:29 pm
content: Quote from: Siwastaja on June 27, 2015, 06:04:40 pmWhat battery capacity are you connecting to it?The TL431 is not specified at currents below 1mA.Even when VREFis below 2.495V, causing the output transistor inside the TL431 to turn off, the op-amp will still draw some current. Figure 2 and page 5 of the datasheet suggests the current will still be 100s of µA when VREFis below 2.495V. Note that this is when the cathode voltage equals VREFand at 25oC, at higher temperatures and cathode voltages, it will be worse.https://www.fairchildsemi.com/datasheets/TL/TL431.pdf

author: Siwastaja
author_rank: Super Contributor
timestamp: June 29, 2015, 03:53:16 pm
content: Found MAX921 so used it to avoid buying parts. And it really was one-day project, which I am happy about.Now the regulation looks approximately like this:4.12V and below -> 0.0 mA4.13V -> 10 mA4.20V -> 70 mA4.25V -> 130 mAPanel is specified at 100 mA.So it still has a slight curve, as expected, due to the oscillation, which is something I expected. With capacitance added on resistor divider, the PWM works at 200 - 300 Hz and is very stable, which is great.Thanks for all the ideas!The battery is from 600 mAh to 1200 mAh.

author: Kalvin
author_rank: Super Contributor
timestamp: June 29, 2015, 04:23:43 pm
content: If you want to still improve the efficiency, you can get rid of the schottky diode at the input by using a P-MOSFET instead.http://www.ti.com/lit/an/slva139/slva139.pdfEdit: May not be a good idea after all, as the battery might discharge through the solar panel if the solar panel voltage is lower than the battery but high enough that the P-MOSFET is conducting. Something more clever is needed.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 29, 2015, 04:43:02 pm
content: Quote from: Kalvin on June 29, 2015, 04:23:43 pmActive rectification is not that simple. That paper only describes reverse voltage protection. It doesn't help if the panel produces 3.5V and the battery is at 3.6V, where the current is reversed but there still is positive voltage that keeps the FET on.

author: Kalvin
author_rank: Super Contributor
timestamp: June 29, 2015, 04:46:20 pm
content: Quote from: Siwastaja on June 29, 2015, 04:43:02 pmYes, I just noticed that myself too and edited my message.