i need someone to look at this circuit before my teacher will get it

Thread Starter


Joined Sep 24, 2018
my teacher doesn't know much about electronics but is willing to get this circuit for a class project i am doing.

I have been building a constant power circuit to drive a motor on a solar powered car, and was after someone to have a look at it to see if there is any problems with its design.

The circuit is powered by a 6ish watt solar panel being fed into an MPPT that outputs 12 volts. The circuit uses an arduino uno to control a buck converter that feeds the motor a constant amount of electrical energy. The output of the buck converter is shunted, the voltage drop from the shunt is amplified by the LM158 which in turn feeds into the arduino. Also the output voltage is measured by the arduino after being divided by 5.

Note- R6 should be 10k, had editing problems



Joined Mar 31, 2012
Is this a toy car of some kind?

What is the motivation for feeding the motor a constant amount of electrical energy? What do you even mean by this? Do you really mean to supply it with constant power, as opposed to constant voltage or constant current?

Why do you want to supply the motor with constant anything? Does it make any sense that you want the motor to be receiving the same amount of (you name it) if it is sitting still, going downhill, or going uphill?

Thread Starter


Joined Sep 24, 2018
the solar car is meant for a race around a fixed track, and will become powered at the start of the race and at the end it will be stopped by placing a towel over the solar panel, so the motor is running the entire time so therefore should be using all the energy the panel provides in order to make the car go as fast as it can.


Joined Jun 4, 2014
the motor is running the entire time so therefore should be using all the energy the panel provides in order to make the car go as fast as it can.
That's a very different thing to 'constant amount of electrical power'.
An MPPT extracts maximum power from the solar panel so then feed that to the motor - job done.


Joined Feb 8, 2018
An MPPT cannot deliver a constant voltage unless the load is less than the power available at the MPP of the module under actual operating conditions.

Suppose the MPP of the PV module is at 10 volts and 1 ampere under standard test conditions (usually 1000 watts per square metre "insolation" at 25 °C module temperature). That is 10 watts. Lets suppose we have a load of 10 ohms connected to the output. That just happens to be a perfect match so the module can operate at MPP. Now consider what happens if the illumination on the panel drops to 500 watts per square metre. The nature of PV modules is such that the voltage of the MPP doesn't change very much with illumination until you get down to very low light level. It does change with temperature. In order to keep the module at MPP, the "tracker" must make the 10 ohm load appear to the module as if were a 20 ohm load - keep the module operating at 10 volts but at only 500 mA. You can't get 10 volts across a 10 ohm resistor with 500 mA through it. We have 5 watts available and we want to put it into a 10 ohm load.
P = V^2/R - the power in watts dissipated in a resistance of R ohms is equal to the square of the applied voltage divided by the resistance
V = √(P * R) = √(5 * 10) = 7.07
To keep the module operating at its MPP of about 10 volts and 500 mA with a 10 ohm load at the output, the tracker must reduce the output voltage to 7.07 volts, assuming 100% efficiency. If we had a 1 ohm load, the output would have to 2.24 V and the output current would be 2.24 A. If we had a 0.5 ohm load the output would be √2.5 = 1.58 volts and the the current would be 3.16 A - the power delivered into the load must be equal to the power available from the PV module.

Most MPP trackers use a "buck" switch mode converter, usually because the expectation is that the load will be a rechargable battery with a relatively narrow voltage range and the PV module's MPP voltage will be higher than the battery voltage at all times except when light levels are very low. With a buck converter the output voltage is always less than or equal to the input voltage. Other topologies can be used - boost (output greater than or equal to input voltage), buck-boost (output voltage can be higher or lower than input), or flyback (output can be higher or lower than input). True buck-boost converters are fairly complex. There is something often called a buck-boost converter that is actually neither and operates in a manner identical to (actually is) a flyback converter but with just a simple inductor. There are derivatives of these converters.

With a motor, especially if it can be "unloaded" or even potentially make to operate as a generator when going down hill, an MPP tracker that can produce an output voltage both higher and lower than the input voltage is advantageous because it can always put the MPP power into the motor. If the car is going down hill and the motor input power drops, the MPP tracker would turn up the output voltage and make the car go down hill even faster - which is a little perilous if there is a curve on or at the bottom of the hill, but that's another matter. For a low power system, I'd probably use a flyback converter because of the comparative simplicity. A SEPIC converter would also be suitable.

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