Help setting up a sensor node

[xsr_Gx]

We are considering using a 12W solar panel, with the following specs:

-Max Power Voltage = 17.2V
-Open circuit voltage = 21.6V
-Max Power Current = .7A
-Short circuit current = .8A

This sensor node consists of powering up a camera, a microphone and a microcontroller that draws an averate total current of 300mA-hrs /day. We have considered using (3) 1.2V NiMH batteries with a capacity of 10AH, therefore our bank size is 3.6 V(nominal), 10AH. This is our current set up:

SOLAR PANEL ---> NiMH Charge Controller (DS2715)--->Battery Bank --->Switch Regulator (LM2852)--->Linear Regulator (LP3874)---->3.3V regulated, to system.

The DS2715 requires a voltage of 4.5V - 16.5V to operate. What can i do/use to prevent a higher voltage going through the DS2715 from the solar panel? I know that it depends on my location, but either way we need to prevent this from happening. ANy suggestions will be appreciated. ALso do any of you have any experience with this charge controller.

Thanks in advance

 

[crutschow]

You could use a low dropout regulator in series with the solar panel output such as **broken link removed** from National Semi. You set the voltage to the maximum you want. An any voltage below this, its output will follow the solar panel voltage with less than a 0.5V drop, depending upon current.

Are you configuring the DS2715 for switch-mode operation to maximize its efficiency?

Why do you have a linear regulator following the switching regulator? That will reduce your efficiency. Are you concerned about its noise getting into your electronics?

 

[xsr_Gx]

Hello Carl,

Thanks a lot for responding. Yes, i am configuring the DS2715 for swith-mode, if you look at this ap note, Figure 3 shows a more improved application, this is more useful if the DS2715 is connected to non continuous loads such as my application.

For what i read (correct me if i am wrong), under this configuration, the solar panel will provide current to teh load directly as well as charge the batteries at the same time. When there is sun, the chip will be powered from the batteries and will provide current to the load. For our purposes, we will have a camera taking one picture every 1hr and the mic will be recording audion 1 min every hour.

I am having a hard time understanding how part of this circuit works. For instance, R7, Q3, Q5 and diodes D2 and D4. Also, can you guide me on how to select the right value for Q5 considering the solar panel described and considering that our loads will draw an average of 300mA-hrs per day.

The reason we have a linear regulator following a switching regulator is because we want to have a clear signal. Linear regulators dont have noise compared to switch regulators. Do you recommend any better idea?

Thanks

You could use a low dropout regulator in series with the solar panel output such as **broken link removed** from National Semi. You set the voltage to the maximum you want. An any voltage below this, its output will follow the solar panel voltage with less than a 0.5V drop, depending upon current.

Are you configuring the DS2715 for switch-mode operation to maximize its efficiency?

Why do you have a linear regulator following the switching regulator? That will reduce your efficiency. Are you concerned about its noise getting into your electronics?

 

[Mr RB]

You shouldn't need a linear regulator as well. Just run the switching regulator direct to the battery bank, and power your load from the battery bank.

Noise should be minimal, if you think noise may still be an issue then the DS2715 datasheet will probably have a schematic for a very simple output filter you can add after the switching regulator. There's no need for 3 regulators, you just need 1 and the battery!

 

[xsr_Gx]

Thanks for your answer. I also wanted to implement a fuse and LED to let the user know if they misktakenly place one battery backwards, how can i implement this? Im a newbie. Steps would be appreciated. Thanks

 

[crutschow]

There's no need for 3 regulators, you just need 1 and the battery!

The linear regulator (or some similar device) from the solar cell is needed to prevent the voltage from exceeding the 16.5V input rating of the DS2715.

 

[Mr RB]

Oops sorry I didn't see the Vin specs of the DS2715.

At this point I would be looking for a different IC, or use a 2-transistor SMPS regulator.