Solar pandel and voltage divider problem, please help

[Geebbon]

Hello,
I have a 87.5W solar panel which should provide 5.68A at 15.4V (18.48V open circuit).
I want to connect 2 devices on it. One operates at 15V and can take up to 10A the other one operates at 12V and takes 0.2A.
I have tried to build voltage divider which gives me when measured 15V and 12V.
But when i connect device on 12V branch it does not work. If i measure voltage with device connected it shows only 1.8V.
This is diagram of the wiring:

solar.png

All Rs are 10k Ohm.


When i connect both devices only device1 runs.
When i connect only device1 it runs.
When i connect only device2 it does not run.
When i connect device2 to 15V it runs.
When i connect both devices parallel on 15V just device1 runs.

As i wrote earlier, i measured branches with voltmeter and 1st shows 15V second shows 12V. Yet nothing runs from second branch.

Im eletric newbie, could you please give me some hints what am i doing wrong? I though of putting battery (with some charge controller) before the devices but it gets expensive and i dont require battery, i need this only to run when its sunny.

Thank you for any insight.
Geebbon

[sensij]

Think about your design... For the 12 V circuit, how much power dissipation did you calculate for those 10k resistors? Why would any current flow through a 10k resistor when it can go to device 1 instead?

[bcroe]

Hello,
I have a 87.5W solar panel which should provide 5.68A at 15.4V (18.48V open circuit).
I want to connect 2 devices on it. One operates at 15V and can take up to 10A the other one operates at 12V and takes 0.2A.
I have tried to build voltage divider which gives me when measured 15V and 12V.
But when i connect device on 12V branch it does not work. If i measure voltage with device connected it shows only 1.8V.
This is diagram of the wiring:
All Rs are 10k Ohm.
When i connect both devices only device1 runs.
As i wrote earlier, i measured branches with voltmeter and 1st shows 15V second shows 12V. Yet nothing runs from second branch.
Geebbon

Welcome to Circuits 101. The voltage from your divider will change as soon as you draw current, because the circuit is changed.

Look at Device 2. At 12V and 0.2A, it could be considered a 60 ohm resistor. In parallel with the 4 10K resistors, the load
appears as 59.91 ohms. That load is being fed by the single 10K resistor to 15V, forming a voltage divider with an intermediate
voltage of 0.0893V. The reason you're are getting even 1.8V, is that Device 2 contains some non linear devices (diodes, etc)
that don't conduct till about 1V is applied. Bruce Roe

[Sunking]

It is called Kirchoff's Law you know nothing about.

[Geebbon]

Well when i think of it yeah.. why would it...
And when i think of it i guess 10k resistor even dampens the current so that device2 dont even run alone...
hmmm

could you point me to some direction that would actually work?

[bcroe]

Well when i think of it yeah.. why would it...

could you point me to some direction that would actually work?

You need a series element to drop 15V to 12V. The simplest way might be 4 to 6 diodes in series, wired in
the conduction direction. There are an infinite number of ways. Bruce Roe

[Geebbon]

It is called Kirchoff's Law you know nothing about.

Obviously :-/


Guess i need to balance the resistance on both branches. Is that correct? But that would be hard when device1 takes whatever amp i feed it up to 10 and with "variable" source such as solar panel.

If i could stabilize input voltage by voltage regulator to 15V and i know that panel gives 5.68A then i guess i could calculate draw from device1 by substracting draw from device2 and calculating precise resistance values...

Would that be work?

[Geebbon]

You need a series element to drop 15V to 12V. The simplest way might be 4 to 6 diodes in series, wired in
the conduction direction. There are an infinite number of ways. Bruce Roe

But this way if i connect device1 on 15V before the diodes woudln't it still go through the device1 instead of going through diodes?

[bcroe]

But this way if I connect device1 on 15V before the diodes woudln't it still go through the device1 instead of going through diodes?

If your source can maintain 15V, the circuit to Device 2 will work regardless of Device 1. With voltage applied, current goes anywhere
and everywhere there is a path; it doesn't chose. Bruce Roe

[Geebbon]

If your source can maintain 15V, the circuit to Device 2 will work regardless of Device 1. With voltage applied, current goes anywhere
and everywhere there is a path; it doesn't chose. Bruce Roe

I cuurently have just 3 diodes. Which drop around 0.5V. I will try it tomorrow if i manage to go to the store after work. I will let you know if it worked. Thanks for suggestion.

[Sunking]

Obviously :-/


Guess i need to balance the resistance on both branches. Is that correct?

No, none of the above. Use voltage regulators. I am not going to help you any further as it is way beyond your knowledge and capabilities.

[DaveDE2]

Hello,
I have a 87.5W solar panel which should provide 5.68A at 15.4V (18.48V open circuit).
I want to connect 2 devices on it. One operates at 15V and can take up to 10A the other one operates at 12V and takes 0.2A.
Geebbon

If you have one device that operates at 15v and 10A, that's 150W, more than your panel can provide. A non-starter no matter what you do.

[Mike90250]

Use a zener diode on a heat sink, rated for the desired voltage, and 2x the amperage of the PV panel. (Simple, 2 parts, diode & heat sink)

But first, you need a PV panel of the right size, should be at least 2x the load you want to power, to insure it will work beyond solar noon.