Measure voltage on 12v SLA battery with arduino – All About Circuits


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Thank you for your input. I’ll have to study that diagram more to understand what’s going on. I think I saw something similar but I didn’t understand connecting to the 5v on the microcontroller. My thought now looking at your diagram, is when the input voltage to the analog pin is less than 5v, the top diode (connected to +5v) would normally allow voltage/current to flow (forward bias) but since it’s connected to the +5v at the cathode, the anode side is a lower potential so it does not flow into the +5v arduino pin. So the diode is reverse bias when less than 5v is on the pin. As long as breakdown voltage is higher than 5v then that acts like an open circuit, correct? So this protects the input pin when higher than 5v is at the input pin? Assuming the input pin sees a voltage in the range of 0 to 5 volts, this diode will always be reverse bias, and always an open circuit? But that’s the “what if” protection if it somehow gets higher than 5v, then it (the arduino input pin) is protected?

Something not obvious to a noob, what happens if you have say 20v there, is it okay to put that on the 5v input of the arduino? I thought that had to be a well regulated voltage that didn’t stray much from 5v. I am powering the arduino with the 5v output from the 7805 into the 5v pin on the arduino.

Going back to the zener circuit I referenced…If I add a resistor to the zener circuit is there any drawback to using that circuit? Just uses less components and at this time I can follow the zener circuit and understand it better (I think.) I also came across this video (at 22: 25) which shows the zener circuit to limit voltage into the input pin, but does not use the capacitor so even less components.

I need to search clamping diodes I think to learn more about your diagram. It’s hard for a noob to wrap mind around the diode and how it limits the output voltage. I’ve read and seen more videos so it’s starting to make a little more sense. The lower diode (between input pin and GND/COM) I’m not sure I understand its purpose.

My design assumption is the input voltage won’t be drastically high. Meaning I always use a 12v battery, not a 24v or 6v, and charge it when it gets around 11v. And if using an AC to DC wall adapter, it would be rated at 12v, but may stray depending on the quality. It won’t ever be AC voltage input to my circuit. But with the adapter, maybe you get some variation depending on the quality of the adapter? Not sure if that changes any thinking on the circuit. Does your diagram with clamping diodes provide “more” or “better” protection for higher (or varying) voltages than just a zener diode? I’m thinking a voltage divider to bring down to 5v (and I have to assume an upper limit input voltage to design the divider) and then use a 5v zener incase the voltage is over my design and the divider creates more than 5v would work. What am I missing? Or do both circuits work and there are advantages/disadvantages to either of them?

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