Need Help Finding A Resistor Size... 3

[sDotOne]

I usually work on high voltage equipment, so this question has me a little stumped. I am looking to monitor battery/solar panel voltage in a remote solar panel. I have a wireless node out in the panel with a few discrete I/O as well as 0-10vdc analog input. Based on the amount of sunlight the voltage fluctuates between 12 to 25vdc. The analog input has an input rating of 24mA. What size resistor would I need to put in between the battery and the analog input as to keep the voltage below 10vdc. Thanks!

 

[VEBill]

The analog input has an input rating of 24mA.

The above will need clarifying. To correctly calculate the resistor divider values, the input impedance (resistance) needs to be known.

The "12 to 25vdc" and "10vdc" input is clear enough.


Also, how many watts total is the installation? One needs to calculate the impact of the measurement on the installation.

 

[ScottyUK]

Is it possible that the analog input is also configurable as a current input for a 0-20mA or 4-20mA nominal input? These are quite often capable of taking 125% of nominal input current, i.e. 24mA.


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If we learn from our mistakes I'm getting a great education!

 

[StrykerTECH]

sDotOne,

Make the following hook ups:
Connect a 15k ohm resistor from your solar panel positive to your analog input.
Connect a 10k ohm resistor from your analog input to common/gnd.
Connect your solar panel negative to common/gnd.

With this setup, your analog input will be 10V when the solar panel is at its maximum of 25V. The analog input will be 4.8V when the solar panel is at its minimum of 12V.

This circuit will draw at most 1mA from your solar panel. The current going into the analog input on your controller will probably be less than 0.1mA as generally all analog inputs have very high input resistance (load load impedance). So you should be set to go.


StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[VEBill]

Agree that a voltage divider is the correct approach.

But the OP had stated, "...0-10vdc analog input..." [good] and "...analog input has an input rating of 24mA" [???]. "...generally all analog inputs have very high input resistance..." is true, but I think it'd be worth getting more details on the "24mA" just to confirm that it's a red herring and the input really is high Z.

I would recommend it be scaled a bit further from the max (also depends on Z in). There may be a trade-off to be made of resolution and making the ratio a whole number (depending on the display scheme).

Also there may be over-voltage failure modes; one should *always* leave some headroom to accurately display the failure modes. Having a reading max-out (or fail) during a system crisis can make things worse.

Apologies for being so cautious.

 

[sDotOne]

Thanks for the quick replies!! Sorry about the 24mA input rating, after reading what ScottyUK said, that would be the upper limit. It can be used for both 0-20mA/4-20mA and 0-10vdc. I was reading the spec sheet of the I/O and when I saw input rating I thought of resistance for some reason, that was just me not thinking correctly. Thanks StrykerTECH, I will try what you posted.

 

[sDotOne]

I hooked it up to the analog input the way StrykerTECH recommended and everything seems to work great. I used 15.1k Ohms on the positive side and 10.3k Ohms back to common (those were what I could make with the combination of resistors I had). Thanks for the help!!

 

[DRWeig]

Thanks for telling us how it worked out, sDotOne. Nice of you to take the time!

Good on ya,

Goober Dave

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