Solar Panel + Diode + Supercap - Page 1
Forum section: electronics / general

author: Undweeber
author_rank: Regular Contributor
timestamp: May 13, 2018, 07:30:29 pm
content: hi everyone,I have got a little calculator sa3515s4 solar cell, it is able to power a calculator that requires 1.5V button cell to work.I also have a supercapacitor I want to try charging with this solar cell just for experimental purposes, no plans to do anything what so ever yet.Question: Is it dangerous for supercap to be directly hooked up to the solar cell? The discharge on this beast is 60A or a million A. could this damage the tiny solar cell?Problem:1. When I charge supercap with DC power supply, i do not use any diodes, and everything seems fine, the power supply handles the feedback voltage like a champ, so does my PC when I use USB power.2. Solar cell does not have any circuitry to protect it from 90A feedback from the supercap so I am worried that the solar cell will fail killing itself while charging the supercap.3. I have tried using a small diode with the DC power supply while charging the supercap, but that diode got so hot that i burned by finger, I have a beefy microwave oven diode that I could use, would that be advisable?? why did it heat up?4. The solar cell can power a 5mm white LED directly from light, but when i put the same small diode between LED and solar cell, the light coming out of LED is much dimmer, why is that?Give me some feedback please

author: ahbushnell
author_rank: Frequent Contributor
timestamp: May 14, 2018, 03:36:09 am
content: As long as the capacitor is not charged when you connect to the solar cell I don't think it will be a problem.

author: fourtytwo42
author_rank: Super Contributor
timestamp: May 14, 2018, 08:10:24 am
content: Quote from: Undweeber on May 13, 2018, 07:30:29 pm1. A power supply normally has a rectifier following the transformer and this prevents any reverse current flow, not sure what you mean by USB power.2. Solar cells are by there nature diodes so they will not conduct reverse current flow unless you exceed there reverse voltage rating that is usually considerably above VOC.3. That sounds like you were overvoltageing the supercap causing a large current to flow, very dangerious. Microwave oven diodes are very high voltage low current diodes and not suitable. You want a standard GP diode like an 1N40014. The diode introduces a voltage drop, usually in the range 600-1000mV so the LED becomes dimmer. It is normal to include a current limiting resistor in a LED circuit however in this case I think the solar cell is self limiting at a low enough value not to burn the LED.Do you posess a DVM ? that would help you understand whats going on

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 14, 2018, 03:20:11 pm
content: Use a Schottky diode for low voltage drop.

author: metrologist
author_rank: Super Contributor
timestamp: May 14, 2018, 03:51:51 pm
content: Quote from: fourtytwo42 on May 14, 2018, 08:10:24 am1. probably 5V from a USB port.2. I'd like to know more about that. I thought SP would discharge a connected cell.3. I'm trying to picture this - a very small calculator solar cell. The cap would have to a very low voltage rating? I still don't see such a small solar cell making enough energy (current) to heat a diode enough to burn flesh. I mean, if...4. the solar cell has a very limited current output.

author: Seekonk
author_rank: Super Contributor
timestamp: May 14, 2018, 04:13:37 pm
content: A microwave oven diode is a stack of diodes and will have a forward voltage of about 7.5 volts, more than your panel produces.

author: fourtytwo42
author_rank: Super Contributor
timestamp: May 14, 2018, 05:44:43 pm
content: Quote from: metrologist on May 14, 2018, 03:51:51 pm1. OK so current limited perhaps and limited to 5V, still dont know the voltage rating of this supercap of the OP2. Sorry I was mistaken in my thinking, I spend to much time building models of solar panels with diodes, in actual panels the reverse leakage is horrendios for dark cells hence the use of bypass and blocking diodes so certainly the OP needs a blocking diode to prevent discharge.3. The OP said in this case the source was a DC power supply.4. Indeed BUT were it larger bang goes the LED

author: metrologist
author_rank: Super Contributor
timestamp: May 14, 2018, 06:05:40 pm
content: well, it makes me want to play with my little solar panel and a couple of large caps. I need to look more at supercaps too. I found a couple low voltage varieties on some scrap PCB's that I have laying around...

author: Undweeber
author_rank: Regular Contributor
timestamp: May 19, 2018, 08:19:26 pm
content: 1. Max voltage of the supercap is 2.7V but it can go higher than this, i once charged it to 3.2 at which point it was hot and i quickly discharged it by turning a piece of wire red hot, from that time I watch the voltage closely and go to 2.7/8 at most. And by usb power i mean USB from my PC, i butchered a USB cable to have a red and black wire on the other end charging the cap, or I can plug in the cable into iPhone charging brick, i don't have an actual power supply with variable voltage/current (probably should get one)2/3. I just hooked up my 350F 2.7V supercap directly and monitoring it with an analog multimeter (i don't have a digital one lol ill have to get one too, my analog one sucks..). You guys say i should use a blocking diode to prevent discharge, but when i did this simulating solar cell with 1A power supply, the diode burned my finger, but since solar cell is small, current is low, it shouldn't heat up, but yeah, the problem is the supercap will still try to give the feedback.I think to test this i should charge the supercap to the 2.7v, then try to close circuit through the diode but in the wrong direction, if the diode doesnt heat up, good, if it does....4. at the moment the supercap is at ~0.4V hooked up directly to the solar cell, the solar cell is producing at the moment with fully overcast sky 3.1V and some amount of amps that i cannot decipher using analog multi-meter.i will keep checking the supercap voltage every 30 minutes and the temperature of the wires/supercap.4a. when I charge supercap with USB power supplies (from PC or apple power bricks for iphone) those put out from 1-2.4A and 5V or so, the voltage itself plays no role as long as you monitor it and dont let it charge beyond 2.7V, but the current.... it loves current, basically these are the stats and charging times @ standard 5.2V USB charger.0.4V - 2.7V @2.4A - 6 minutes0V - 2.7V @2.4A - 6:30 minutes0.4V - 2.7V @1A - 14 minutes0V - 2.7V @1A - 21:30 minutesCurrently undergoing test @ 3.1V and ?mA (i precharged the supercap to 0.4V because for some reason under 0.4V supercap is very hard to charge, but it goes fast after 0.4V