Circuit to guarantee minimum voltage from solar panel

Thread Starter

aaron_m

Joined Apr 27, 2014
13
Hey, I have a solar panel that can deliver up to 6V / 250 mA on a clear day. As the sun wanes, of course, the voltage drops off. I have my output - a small 5V LiPo battery charger - connected to an LM7805 so the charger should never get more than 5V. The circuit seems to work well enough in this case: my LiPo batteries get charged, eventually, depending on how much sun we have.

My question: how would I design a circuit that doesn't power the LM7805 until the voltage reaches at least 5V? I read through Hill & Horowitz (Art of Electronics), and Practical Electronics for Inventors, and simulated a circuit using zener diodes, but if the voltage goes below the zener cutoff level, it simply routes to the parallel circuit, bypassing the zener, so the load would still see any voltage < 5V. At a low enough voltage, of course, the LM7805 doesn't conduct, so it kind of works out, but I'd like to know how to implement this.
 

BobTPH

Joined Jun 5, 2013
9,116
An LM7805 needs 6.5 V in to put out 5V, so it will never work properly with your panel that outputs a max of 6V. Use a low dropout regulator. They can operate at maybe 5.5V or even lower. Feeding less than 5V to the charger is likely okay. It will cut off itself if the voltage is insufficient.
 

nsaspook

Joined Aug 27, 2009
13,413
That panel voltage value from the OP is likely just the MPPT point (or near it). The panel charging voltage is likely somewhere between the open panel and MPPT voltage points on the curve. The regulator here already provides the low voltage cutoff function. I don't see the need for another circuit at those low power levels if it's working now.
 

Vinod Pant

Joined Jul 21, 2018
1
Hey, I have a solar panel that can deliver up to 6V / 250 mA on a clear day. As the sun wanes, of course, the voltage drops off. I have my output - a small 5V LiPo battery charger - connected to an LM7805 so the charger should never get more than 5V. The circuit seems to work well enough in this case: my LiPo batteries get charged, eventually, depending on how much sun we have.

My question: how would I design a circuit that doesn't power the LM7805 until the voltage reaches at least 5V? I read through Hill & Horowitz (Art of Electronics), and Practical Electronics for Inventors, and simulated a circuit using zener diodes, but if the voltage goes below the zener cutoff level, it simply routes to the parallel circuit, bypassing the zener, so the load would still see any voltage < 5V. At a low enough voltage, of course, the LM7805 doesn't conduct, so it kind of works out, but I'd like to know how to implement this.
You can use a DC-DC converter module with adjustable voltage.
 

kaindub

Joined Oct 28, 2019
132
You're doing it all wrong. Just get a Tp4056 module and it's all done for you. The 5v usb chargers happen to be 5v because that's what usb supply is. The module I mentioned will do all the functions.
 

nsaspook

Joined Aug 27, 2009
13,413
You're doing it all wrong. Just get a Tp4056 module and it's all done for you. The 5v usb chargers happen to be 5v because that's what usb supply is. The module I mentioned will do all the functions.
That's not really what he needs to regulate solar panel voltage that can exceed 6v to the 5v that going to an existing battery charger.
 

camerart

Joined Feb 25, 2013
3,731
Hi A,
I think my solar panel chargers use capacitors and DC to DC up converters as #4 to charge 12V leasure batteries.
What appears to happen, is say the panel output is 3V when dull, then it charges a capacitor to 3V and uprates it to the correct voltage, to charge. If the output voltage is high 'say' 17V then it charges the CAPS to 2V in a split second, repeating 'very' fast.
I'm not 100% , but having watched then for years in different light, including under street lighting, they keep trying to charge.
C
 
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Thread Starter

aaron_m

Joined Apr 27, 2014
13
Ok, so this isn't the greatest answer, but it does appear to work - at least in the simulation. Output current levels aren't that great; might not even be useful. But it does meet the requirements.
Edit: Where the requirements center around, "Don't turn on until the voltage reaches at least 5v." This won't charge a battery, but it will stay off until the input is a minimum of 5v.
 

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