limiting voltage in solar application

[fred_b]

My project is a solar powered time lapse camera that will be in a very warm environment (desert- lots of sun).

I plan to have a cooling fan that is turned on /off by the picaxe (based on time of day read from real time clock).

When the fan is on, it assumed there will be lots of sun and power generated.

The output of the solar panel can be as much as 17 volts. The fan has a allowable range of 7-13.8 v.

My schematic shows a 12 volt battery that is intended to serve the camera and picaxe circuit board. A voltage regulator is planned to reduce this to 7.4 v. (The picaxe circuit board has its own voltage regulator that will further reduce its voltage to 5v).

I would like to power the fan directly from the solar panel to prevent it from running off the battery power.

What is the simplest way to prevent the fan motor from getting too much voltage. Also, would low voltage damage the fan motor?

Thank you.

 

[westaust55]

More information on the specific components you are intending to use would be helpful.

Many solar panels are around 17 Volts at Peak Power but if the battery is fully charged so the solar panel is nominally isolated from the battery/load, then the open circuit voltage of the solar panel may be around 21 volts.

What type of motor is within the fan are you using?

For many types of solar (PV) panels, the output also drops as the PV panel temperature increases.

Since you are trying to be a little “rough” with not including voltage regulation for the fan motor, you could extend on your concept . If your fans can operate from as low as 7 Volts, then you could put two fans in series. One fan to cool the camera and another blowing some air across the rear of the solar panel. Nominally, no added current used with the two fans in series.

Keep the panel cooler and you get some more power/current which at least partially offsets the power drawing by the fan motor.

If a motor is supplied at full voltage and in a stall condition, it will draw many times the normal current and if this condition continues it will eventually burn out. In the case of the fans across a smallish PV panel, if the voltage is not high enough to turn the motor, then generally, the PV panel cannot provide sufficient current to cause damage either.

Some years ago I did a similar thing myself with a camper trailer for camping in bush/desert country. I had a PC type fan (there are some fans that will run at lower voltage and current but the PCD fans were free) to draw hot air away from the fridge to outside the camper when the sun was directly onto the side of the camper where the fridge was mounted. The small PV panel needed to be in direct sun and facing directly towards the sun to generate enough power to start the PC type fan motor turning. But at other times of the day, the sunlight was at an oblique so the fan did not turn but there was still current flow through the fan. Rough but it worked and never burnt out the fan motor. Some fans however, may not be as robust and tolerate this type of abuse.

 

[moxhamj]

If you have a standard solar regulator (not a switching or MPPT one), then even though the open circuit volts on the panel might be 21V, when it is connected to the battery you will have only about 14V on the panel. (measure this if you like). So there is no real advantage running the fan off the panel. Just run it off the battery and if it is picaxe controlled you turn it off when needed.

 

[Haku]

Running the fan off the battery would be the easiest way, the picaxe could also measure the voltage of the battery and not switch on the fan if the voltage is too low.

 

[Andrew Cowan]

I'd agree with Haku - power everything from the battery, keep the battery charged, and disconnect the solar pannel if the voltage at the battery is getting too high.

A

 

[premelec]

If I understand correctly you don't need the fan when the PV panel is not generating power - then power the fan directly through a series diode to a 12V three terminal regulator which will produce 12 v when the sun shines and not draw any power when it's dark - and to save power only turn the fan on when needed rather than by time... Though perhaps I've misjudged your climate suggesting you don't need it when the PV is not producing power.

 

[LizzieB]

Does the camera generate a lot of heat that would take it above that very warm ambient? If not how is circulating ambient air with a fan going to cool it? So long as it is shaded but well ventilated it will be pretty much at ambient air temp.

 

[gengis]

Not all brushless fans will start turning if you creep up on the voltage - you might want to check yours for that. I'd also go along with a thermostat since the axe can check that easily enough.

 

[westaust55]

camera with solar applications

Does the camera generate a lot of heat that would take it above that very warm ambient? If not how is circulating ambient air with a fan going to cool it? So long as it is shaded but well ventilated it will be pretty much at ambient air temp.

A lot of truth in that Lizzie.

We really need to know more about the shading “device”.

In say 45 degC, a relatively sealed box in the sun can reach 80deg C inside. Obviously at least one side is open for the lens to have a view through. But how restricted is the air flow?

If using the solar panel for shade, the solar panel gets hot and radiates that heat (= more heat towards camera).

It does not take a lot of air flow to cool a device. Only have to look at overhead power lines. In still air, their rating might be around half that compared to in 0.5m/s wind.

If ambient temp is the root problem as Lizzie has flagged one option is to use Peltier effect devices as coolers. They do consume a lot of power so you would need added solar panels. Then that PICAXE can come into its own with a temp monitor to turn the cooler on and off.

 

[fred_b]

Thank you for the replies.

then power the fan directly through a series diode to a 12V three terminal regulator

This was one option I was considering. Any suggested part#?

Does the camera generate a lot of heat that would take it above that very warm ambient? If not how is circulating ambient air with a fan going to cool it? So long as it is shaded but well ventilated it will be pretty much at ambient air temp.

I thought about that too but figured the inside of the enclosure would probably warm up some relative to the outside. Every little bit will help as far as cooling.

Right now I am leaning toward having two smaller batteries- see revised schematic.

My timelapse controller board has one unused output pin that I will use to switch the fan on and off. To keep things simple, running the fan at certain times of the day or with a thermostat is planned. Any suggested thermostat?

If you have a standard solar regulator (not a switching or MPPT one)

I haven't bought the parts yet. I will need to research and see what the difference is.

As far as switching the fan, (computer fan 12v .5 a) , I was considering the IRF 530 MOSFET. Can this control a device powered at 12v when the picaxe is at 5v? Also, it appears that this device generates a lot of heat which is bad for two reasons: internal temperature and wasted energy. Is there a more efficient way? I am using a TLP621 optocoupler for the camera control.

Thank you.

 

[westaust55]

My timelapse controller board has one unused output pin that I will use to switch the fan on and off. To keep things simple, running the fan at certain times of the day or with a thermostat is planned. Any suggested thermostat?

If you are using a PICAXE, then consider using a DS18B20 as the temperature sensor. The PICAXE has the READTEMP command specifically to work with the DS18B20. Then the PICAXE via a transistor/FET can control the fan.

As far as switching the fan, (computer fan 12v .5 a) , I was considering the IRF 530 MOSFET. Can this control a device powered at 12v when the picaxe is at 5v? Also, it appears that this device generates a lot of heat which is bad for two reasons: internal temperature and wasted energy. Is there a more efficient way? I am using a TLP621 optocoupler for the camera control.

Yes. You can run the PICAXE at 5V and control the fan using a BJT or FET type transistor.
If using a single switching transistor, you will need to use low side switching and for the FET make use it has logic level gate control (have not checked datsheets for the suggested/intended IRF 530).

 

[Andrew Cowan]

The IRF530 is not logic level - Vgs of 5v gives an on Rsd of about 4 ohms.

A

 

[Colinpc]

The output of the panel also depends on the load on it. 17+ volts is the open circuit voltage. If you have a charging circuit and fan across the panel, it is unlikely to get up to anywhere near 17v.

I use a 5 watt panel on a similar voltage rated fan to move air through a bush toilet and have the fan directly connected to the panel. As the sun rises, the voltage increases until there is sufficient voltage and current available to start the fan.

I have also just connected and disconnected the fan in full sun and had no problems.

fred