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Topic: How to modify the voltage of solar charger? (Read 5945 times) previous topic - next topic


The  deadline I set to myself is tomorrow - for a prototype that works.
Can be done in 10 min.
Connect the solar panel through amp meter directly to batteries, if the current is no more than 200mA you ok for 4 hours, then disconnect the pamel.



There is the "mechanical" part as well to be completed.


- frame of the panel = 90% done,
- concept of the wiring= 50% done (pending: LED support, design of the components to the perfboard, wiring to the switches)
- selection of components = 50% done (switches not yet decided, 4.7 1.3W Zener gone where all the lost components go, limiting resistor to the LED - to be decided by trial and error or to stick on 200 Ohm for no reason).
- overall frame: 0% (maybe two PVC profiles together will do the job, but I shall see)

I will post the pics in the coming days.


Jun 01, 2018, 09:30 pm Last Edit: Jun 01, 2018, 09:41 pm by falexandru
Here it is the final stage of the electronic part of the prototype - demo photo-voltaic charger.

In place of voltmeters, at this stage, I use an ON-OFF switch, to bypass the charging /lightening circuit.

Now, there is something that I cant figure out:
- if the  ON-OFF-ON switch that selects either to charge the batteries or to power the light is on OFF, then operating the ON-OFF switch will let the current from the PV panel to flow to the LED - or not - that is clear and ok
- if the ON-OFF-ON switch is on either one of the two ON position, then what is the value of the coltage and current reaching the LED in each case?

In my view, if the ON-OFF-ON switch is closing the left side of the circuit (charging) then the voltage is 4.7V- drop on Shotky.

But if the ON-OFF-ON switch closes the right part of the circuit (namely powering LED from batteries), what is the current and voltage reaching the LED this time?

Thank you very much for enlightenint me! I just cant focus on this second case.  


The battery will be charged to zener voltage minus drop voltage of schottky diode 4.7 - 0.4 = 4.3V. Typical current of the LED is 20mA, depends to the LED resistor value can be different, put 300 ohm and measure the current shouldbe around 20 mA,



I mean, when I close the ON-OFF while the ON-OFF-ON switch closes the right side of the circuit.

It looks like it does not make any difference in voltage and current to the LED whatever left or right side of the circuit is closed.

Did I get it correctly?

My point is not to blow up the LED while operating the switches.


Zener is 4.7 V/1.3W, R is 220 Ohm/0.25W. LED is 5mm white.

Panel is rated 6V.

Batteries are 3 x 1.2V NiMhd AAA (in series).


My 5mm LED and 150 ohm resistor @ 4.3 V - current is 9mA and LED is bright.
Use name of the switches SW1 , SW2.
To be save, put  10 ohm resistor with zener



SW1 closed.
SW2 closes right side.
PV in sunbright.

My understanding is that in this case the Voltage will still be 4.7V minus Shotky drop, but the current will ad up:

I(R+LED) =  I(PV) + I(Batt).

Now, providing I got it correctly, if I put a 10 Ohm in series with the Zener, the excess current will follow the resistor, thus less being available to the LED. So in a certain reasonable range of current, there would be little chance to reach values that can kill the LED.

Namely, in cases like the above (worst case scenario).


R LED is limiting the current of the LED, with R= 500 ohm you ok up to 8V, so LED will not die and you will have a light at 3.6V also.


Jun 02, 2018, 06:01 am Last Edit: Jun 02, 2018, 06:07 am by falexandru
Thank you, @Ted!

I will experiment by modifying the value of R LED alone then:

- from 220 Ohm now, Ill check with the R values I can find in my boxes within  range 220 - 600 Ohm.

I still have to solve out:

how to mount one single switch and one single LCD to alternatively show voltage of the PV and voltage of the batteries (some solutions found, but not sure)


Thank you Ted!

Few days ago I managed to get a stripboard.  Not yet use it. Nevertheless is almost double price than a regular perfboard. And available in one place only :-(.

Old wiring in air looks promising as well.


Experiment on small breadboard:

- 4.7 V/1.3 W  Zener
- 1 N 5819 Shotky
- white LED
- various resitors to LED:
  - 220 Ohm
  - 460 Ohm
  - 560 Ohm

Powered, alternatively, by:

3.6 V NiMHd 800 mAh
7.2 V LiIon 18650
PV 6 V 200 mA

Now, the really strange phenomenon is that the 460 Ohm resistor caused the Zener to get hot. While 220 Ohm and 560 Ohm did not. This happens to both batteries sources, but not with the PV.

No explanation so far. I just put the 560 Ohm for the time being.

Two improvements, thanks to guys helping (and encouraging) me on this thread:
- stripboard (veroboard) instead of perfboard - this makes a lot of sense seeing the geometry of the wiring
- solid core wires in place of multi-threads ones to local wiring -easier to solder.

On minor improvement is to use a barrel socked soldered to vero-board in place of the connector mounted on wire.

Circuit OK on the breadboard. I will complete it on the veroboard soon. I will also use a larger panel this time (some 400mA, still 6V). And I will post the pics.



You need R1 10 ohms or more as on post  # 34


Jun 03, 2018, 08:45 pm Last Edit: Jun 03, 2018, 08:51 pm by ted
If you put 7.4V in the same place as 6V = battery can deliver 2A current compared to 200ma by PV, if you going to use strong PV you need 200mA current limiter.

Circuit with zener has limitations, the proper circuit is;
4.2V voltage regulator output of it to 200mA current limiter, output of  limiter to battery.

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