OK, so speed control is not required. This is indeed an unusual application. I am guessing that the solar cells are mounted in the picture frame somehow. One possible choice might be powering the motor directly from the solar cells.

There was a similar thread about a similar application here some years ago.

 

OK, so speed control is not required. This is indeed an unusual application. I am guessing that the solar cells are mounted in the picture frame somehow. One possible choice might be powering the motor directly from the solar cells.

There was a similar thread about a similar application here some years ago.

the motor used to be powered directly by the cells, but I know want the motor to keep running for a while after the cells stop outputting enough for the motor, hence the liion battery and the bq25570

 

OK, then you are using daylight and not in a lighted gallery?

 

Even works in with strong ambient light

 

The challenge is that the small number of solar cells only produce a small amount of power, and sharing that power between battery charging and motor spinning involves compromises. Making it more challenging is that any additional circuits are less than 100% efficient. Better motor efficiency would be the solution.

 

Agre

The challenge is that the small number of solar cells only produce a small amount of power, and sharing that power between battery charging and motor spinning involves compromises. Making it more challenging is that any additional circuits are less than 100% efficient. Better motor efficiency would be the solution.

Agreed circuit efficiency is the key. The motor is already quite good (starts running at 144mv 11mA)

 

Ground Q1s emitter and take the voltage from the collector.
Below is the LTspice sim of an example circuit using a P-MOSFET is place of a diode across the motor to carry the current when the PWM signal is off:
Note that the voltage across the MOSFET is <10mV (yellow trace notation) when carrying the motor current (red trace) , much less than the 0.3V-0.4V a Schottky diode would have.

The MOSFET must be a logic-level type with a Vgs threshold of ≤1V maximum.

View attachment 332418

Hi,

thanks for the detailed help. Just a question for selecting the P channel mosfet: I looked at the datasheet of the one in your schematics (si155dl) and it gives a 1.4v vgs with low rdson, am i missing something, not sure I really understand it? On mouser, I've found ZVP4424A , but it's more difficult to get and rdson is 9ohms. what would be the best choice? Can't seem to find low vgs low rdson?

 

Is there anything else that you can do to reduce the motor friction??

 

Is there anything else that you can do to reduce the motor friction??

not that I can think of. The motor was designed to be as efficient as possible and custom built by a friend.