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Excuse the simpleton schematic, but I don't understand how I should use/wire a P or N channel MOSFET to disconnect a load.

I am using this MOSFET, this solar charger, this PV solar panel and this LED driver.

During daytime, a PV solar panel shall charge a battery (works). During nighttime, the battery shall power an LED driver (works). I assumed I can use a P channel MOSFET as shown to automatically disconnect the LED driver during daytime and only be powered at nighttime. What would I have to do to do it right?

Edit: No matter what the insolation (0-7V output by the solar panel - covered, indoor, in full sun), the LED remains off at all times (the driver receives near zero voltage). I checked the MOSFET and tried it with another of the same type.

Schematics Circuit

Systembolaget
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  • Adafruit part numbers and links to product pages? – AnalogKid Jun 10 '19 at 18:27
  • I added links to the parts. – Systembolaget Jun 10 '19 at 18:37
  • Please try to avoid issuing "open" & "closed" when referring to transistors & switches. Interpreting what you mean can become very context-sensitive (novices often use open/closed a if they're referring to a gate in a fence, which has the opposite meaning when used with a switch). "On" & "off" are almost always unambiguous. – brhans Jun 10 '19 at 19:04
  • Ok, so what is to be used en lieu of "open" and "closed"? If I say "off", it means the MOSFET does not conduct between D and S... or is "conducts" and "does not conduct" the even more unambiguous option? – Systembolaget Jun 10 '19 at 20:24

2 Answers2

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What you show is correct, a p-channel power MOSFET in series with the + output, source to the battery manager, and drain to the load (the LED driver). The question is, what is the amount of daylight when the solar panel output voltage drops MOSFET's gate voltage to the point where it no longer is fully enhanced and starts to "turn off". It might be lighter or darker than you want.

Separate from that is the amount of time it takes for the FET to go from fully on to fully off. during this time it can dissipate significant power and get hot enough to destroy itself. Consider adding a comparator that snaps the FET from on to off.

An added benefit of the comparator is it makes the on/off daylight level adjustable.

AnalogKid
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  • Thanks, yes, "it might be lighter or darker than you want" is true... I was also reading something about "hysteresis", meaning that for example a prolonged dark daytime cloud should not result in the LED to light up. Since I am a MOSFET-novice, how would one, in the next step, add a comparator do achieve what you suggest? An adjustable level sounds very useful. – Systembolaget Jun 10 '19 at 20:27
  • Well, first I should put the parts together as shown, and to see how that goes, and then, if it rudimentarily works, learn from it what does not work compared with that suggested comparator solution. – Systembolaget Jun 10 '19 at 20:29
  • No joy. Whether the solar panel is fully covered or in full daytime sunlight, the LED driver is not powered, see image above. – Systembolaget Jun 11 '19 at 08:04
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All right, a wrong MOSFET type it was (regarding the voltages involved).

A NDP6020P P-channel MOSFET arrived.

I put that one into the circuit instead and now everything works as intended. Only in late evening, the LED comes on and goes off already in the very early morning. The P-channel MOSFET does not heat up detectably at all.

I may now up the charger from 500mA to 1A charging as per Adafruit's instructions and use a 10,000mAh Li-Poly battery for full lumen output over 7 hours.

Thank you all for your input.

Schematic Activation at various battery voltages

Systembolaget
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