I've been working with Arduino and other circuits for over a year now, and while I've gained a tremendous amount of knowledge, I'm still struggling to understand basic concepts around voltage, current, and power.
Here's what I'm struggling with at the moment:
I'm trying to build protection in a circuit that I'm placing in a vehicle. From all of my reading, from time to time there can be transient voltage spikes in a vehicle's electrical system, sometimes in the neighborhood of 1,000 volts for a few milliseconds. I found a company that makes a TVS diode specifically for use in vehicles, called LittleFuse.
I settled on this model after talking to one of their engineers:
My circuit will be drawing less than 50ma during normal usage, and I don't need any more than 5V for the normal operation of my circuit. I need to protect the power input coming from the vehicle battery. The power input is used in two places, basically branching off. First, it goes into a DC/DC 5V buck converter to give me the "clean" 5V for my circuit. The second goes to a 22k/10k voltage divider so I can measure the vehicle's voltage so I can send an alert if the voltage drops too low.
So here's my question: the engineer at LittleFuse told me that I would need to put a series resistor in between the power input from the vehicle battery and the diode/my circuit (the diode will obviously go across power/ground). He said 75ohms will work fine.
What exactly is that resistor doing? The reason for the resistor is that I didn't want to use a larger diode that they make, because it's leads were too big to fit into a breadboard, so he said I could use the smaller 1.5KE, but I would need the series resistor.
He also said that the resistor would be dropping the voltage, so I would need to take that into account when measuring the battery voltage of the vehicle using my voltage divider going into my analog pin.
How do I calculate how much voltage the 75ohm resistor is going to drop when taking that into account with my already-in-place voltage divider? So now it's essentially a voltage divider with 3 resistors instead of two?
If I only put the 75ohm resistor in series with the "branch" that's going into my DC/DC buck converter, a) will the TVS diode still give me "protection" in the circuit, and b) given the 22k/10k resistors (on "branch" b), will I be "missing" anything critical by not also having the 75ohm resistor?
If anyone has time to speak for a few minutes by phone (would be awesome), please PM me.