Protect small DC motor from too much solar power

I'm setting up a hobby project where I have a DC motor running from a small solar panel.

I've tested connecting the panel directly too the motor and it works great.

The problem is that in bright light the panel could theoretically generate too much power.

I'm wondering if there is a simple way to clamp the power to some maximum value.

I know you can buy circuits that regulate solar power for charging batteries.

Is there anything similar that is appropriate to use on a motor?

I've read that adding a battery and a charging circuit could solve my problem but since I'm only using the motor during the day I'm curious what other options might exist.

I don't have an exact specification for the motor but it was originally running off 8 AA batteries in series so I think that's around 12V 50mA. The solar panel's maximum output is 15V 100mA. I know I won't get that in practice very often, but if possible I'd still like to have something in place.

The motor doesn't need to spin at a constant rate so if the voltage varies with the amount of sun that's ok as long as it's within a safe operating range.

• You can use a low dropout voltage regulator to bring down the voltage to 12V. If the motor can run at a lower voltage, you can use a buck regulator to bring down the voltage more efficiently. You can also use PWM to efficiently regulate the motor speed, but you will need some kind of sensor to check the motor speed for the negative feedback. Commented Apr 23, 2021 at 11:29
• Thanks for this answer. It's surprisingly hard too research this online if you don't have a strong electrical background. For clarification, is a buck regulator the same as a buck converter? Based on what I've read It's not entirely obvious to me. It looks like some buck converters are based on pwm. In that case would I still need to monitor for negative feedback if I'm using a buck module or would that be built in? Commented Apr 24, 2021 at 16:38
• Yes, it's a buck converter (u r right, it converts higher voltage to lower voltage). It is based on PWM, but the feedback is based on output voltage, which is kept constant, irrespective of input voltage (so u don't need any sensor). PWM for motor control does not have voltage feedback and uses the motor coil as the inductor (so less parts required, but you do need speed sensor for feedback if input voltage is variable). I am not an engineer, so maybe it is possible to do PWM without a speed sensor also (maybe by monitoring current via sense resistor), but I don't know how. Commented Apr 24, 2021 at 21:27

For this very specific situation - slight oversupply of power from an inherently current-limited supply - you can use a Zener diode.

Pick a 12V one. If you pick one with a power rating above 1.5W, it should be capable of surviving the entire power output of the solar panel, although it would only need to do that if the motor fails.

(For some reason I can't find a "motor" symbol, so have a lamp in the schematic instead)

simulate this circuit – Schematic created using CircuitLab

Note that solar panels usually can't put out both their maximum voltage and maximum current ratings at the same time. So in the normal case you'll probably see a lower voltage across the motor terminals than the peak 15V.

• if he adds the 3 v zener diode (with atleast 1.5w)in series with the load, won't that be a good idea? Commented Apr 23, 2021 at 13:21
• In series? Absolutely not. That would prevent the motor running normally. Commented Apr 23, 2021 at 13:43
• but you didn't add source resistor Commented Apr 23, 2021 at 13:53
• In this case you don't need one, because as I pointed out, the solar panels are inherently current-limited. Commented Apr 23, 2021 at 14:17
• when the source is at full 15v, 500ma , if you want to satisfy KVL you must have 15v across zener, will the zener work if you apply 15 v directly? Even if the zener doesn't fail, won't the 15v will appear across the bulb? Commented Apr 23, 2021 at 14:28