Hello,

I have a circuit that I am trying to add more protection too. However, I really only know the basics of circuit design and this is a little over my head.

I have an ATMEL microcontroller operating my board. I also have 12 inputs to this controller. These inputs are origonally 24VDC coming from the output card of a PLC rack. The 24VDC is then regulated down to 5VDC through a resistor divider circuit.

There is a chance that the input voltage to the resistor divider is over 24VDC. Once it gets to 26.5VDC, there goes the microcontroller... so i'm looking for someway to keep a stable 5VDC to the microcontroller no matter what the input voltage is (say 23 to 30?). I appreciate the help!

 

if the divider is just to give you 5 v then maybe u shud remove it and replace with a voltage regulator ic like lm 317 (other members will help u out better with this as i do not much experience in designing small psu)

 

That was one idea I was tossing around was just to use a voltage regulator for each input (12 in total) as you said. I have enough real-estate on the board that they would fit, and a 7805 would be cheap, which would also give the constant 5V that I would need. My only concern is the heat. This board has the potential to be installed in very warm climates. Also, is the voltage regulator enough protection?

My other thought was to use an optoisolator. However, my knowledge of them is limited. I looked at some datasheets and they say I need to use two separate grounds. However, the ground for the DC power of the board and the inputs to the PCB are the same. Does that mean I need to create a separate ground? (So the input to the board and its ground and the 2nd would be the 24V and ground used for powering the board?)

 

My other thought was to use an optoisolator. However, my knowledge of them is limited. I looked at some datasheets and they say I need to use two separate grounds.

You can use the same ground if you want.

You can put series resistors on the input line to the microcontroller to limit the current in an overvoltage condition. The tradeoff is speed.

You can also divide down the voltage more... use 4V instead of 5V. You need 0.7Vcc for input to be read as High for most of the pins ... check the datasheet to be sure.

You can put zeners or 'rail-clamp' ckts on each input line. A schottky diode from the input line to the 5V VCC would work for this.

 

I would agree with Nanovate. I think a zener clamp or Schottky would be best. It's not a question of offering a solution, for me it is a question of why would you want to operate in such a condition?
Over current or over voltage can easily happen in many applications, but I usually encounter fast transient issues. You can protect against sustained overvoltage, but at what cost, and real estate of your board?