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Topic: How to modify the voltage of solar charger? (Read 5939 times) previous topic - next topic


OK. I will put R1 Resistor back in circuit. See what I can find in the range 10-50 Ohms.

I used the 7.4 V Lion only to check the circuit (I had it on my desk by chance).

The real circuit will be with NiMHd 3.6 V AAA 800 mAh which will be charged by a 6 V 384 mA PV. The NiMHd uni will power the LED.


The lowest resistance resistor you have is ?


Jun 03, 2018, 09:09 pm Last Edit: Jun 03, 2018, 09:27 pm by falexandru
Here it is the new version of the schematics:

1) PV panel 6 V 384 mA  1 pc
2) 4.7 V/1.3 W  Zener 1 pc
3) 1 N 5819 Shotky 1 pc
4) R1 resistor ? (still digging im my boxes) - 1pc
5) 1.2V AAA NiMHd 800 mAh units in series  - 1 pc
6) case for 3 x AAA batteries - 1 pc
7) 560 Ohm R2 Resistor - 1 pc
8) SW1 ON-OFF-ON switch  - 1 pc
9) SW2 ON-OF switch - 1 pc
10) 5mm LED white - 1 pc
11) Case for 5 mm LED - 1 PC
12) morfets 2 connections - 2 pcs
13) barrel connector to board - 1 pc
14) veroboard (stripboard) - 1 pc
15) wires (solid core) - various


Circuit is ok, for R1 use a few resistors connected in parallel for example 5x 100 ohm


I found 10 Ohm/0.25 W  and 56 Ohm /0.25 W for R1.

Perhaps they are other values around - but not lucky so far.



Then I shall mount the 10 Ohm one?

I am a bit concerned about resistor having only 0.25 W. For a 200 mA (say) at 5.5. V = 1.1 W - if my calculus is correct



Are you complaining about the zener getting hot?
It's supposed to get hot.

It 's either the power of the solar panel as charging current into the batteries, or burned off in the zener.
That's why I adviced a long time ago a super-zener made of a zener and a big transistor.

A more complicated regulation is series regulation (voltage regulator IC) instead of simple load-dump (what you asked for).


Jun 04, 2018, 01:06 am Last Edit: Jun 04, 2018, 01:13 am by ted
If you are using 6V/200mA PV .
0.2x4.7 = 0.94W = 1.5W zener ok.
If you are using 7.4 V battery which can deliver 3A - 10W zener ......

For new PV you can calculate as above.
And you can't charge AAA battery with more than 200mA


I know you are having a lot of fun, but why not just get a solar yard light and see how they do it?



Theoretically, you are perfectly right.

This particular case however it is a demo project for kids.
Therefore, I must find easy the components (no "exotic" or particular or mysterious circuitry), use as few as possible and being able to explain what is there.

To be honest, it is a nice way for me to learn more. :-).

And I have to couple the wind turbine to the same charging circuit (+ additional small  interface to match the particularities of the wind energy).



I am planning to improve the device later on, so I am moving along a modular approach. :-).  There are two ideas pending so far:
- mini-voltmeters
- more efficient charging

For the second one, I am thinking of a sort of add-on to the already existing circuit.


Jun 15, 2018, 12:43 pm Last Edit: Jun 15, 2018, 12:48 pm by falexandru
The device built as above (post 77) works very well.

It passed the transportation test (aka being transported in a plastic bag, circuit in sight). 

It is still in the "screws stage", next phase I will glue it up for ever.

Its "sister component" - aka the wind turbine is in the incipient stage - still to adapt a brushless motor as generator.

I will post pics of the photovoltaic device later on.

Thank you very much for your help that made possible the device!


Questions I face now is how to electronically command the photovoltaic circuit to either charge or supply electricity?

Or, in other words, how to replace the manual switches by electronics?

The problem is I must use only diodes and maximum one bipolar transistor (no Mosfet) and passive components. No solenoid either.

These restrictions are from the fact that the project is a demo for kids, so I will have problems to explain more than that.

The solution I plan is to collect  the small current from a photodiode (or a light sensor), to amplify it to the point it can close one branch of the SW1. And then find a way to close the other branch in a similar manner.

I could not find schematics for such a circuit. Except for something similar that I hope I can adapt- see bellow. I think I can replace the Source ("Voltage")  by a photodiode.


My question is: is it possible to adapt the circuit in the way I plan to? 

Thank you very much for your kind, patient and wonderful help!https://forum.arduino.cc/index.php?action=dlattach;topic=549219.0;attach=262360


It is not clear what you want.
Manually you can turn switches any time, in 1 hour or tomorrow.
What factor will decide to do that electronically ?

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