# [solved]High voltage 1.5-50V shunt resistor analogRead

I know how to measure 1.5-40V voltage safely by 2 resistors, but I am not sure if my shunt resistor reading is correct.

Basically, I have 1.5-40V input going in some device[Load] , then going out to a 1 ohm 5 watts resistor, then to the ground. I am reading my shunt resistor voltage drop between the load and the 1 ohm resistor. my concern is what happen if my load got shorted, and 40V goes in directly to my shunt resistor reading input. Will this damage my chip? The reading point is basically near the ground. I tried to test this with my multimeter, but its speed is not fast enough to show the voltage when I shorted the load.

any tips will be great, thanks

----1.5-40v | | [load] | |----{reading here} [ 1 ohm resistor] | gnd

The input to the Arduino is quite high impedence i.e 10's of k

So probably one way to protect the input is to feed it via a resistor e.g 1k and have a 5V zener to GND on the Arduino side of the resistor.

So if the input voltage exceeds the threshold of the zener, the zener will breakdown and draw current 35v / 1k = 35mA through the zener.

You'd need to set your resistor value so that it didn't have an effect on the input, but I suspect 1k in series would be OK, also check the zener will handle the max current it needs to handle in case of your worst case voltage

rogerClark: The input to the Arduino is quite high impedence i.e 10's of k

So probably one way to protect the input is to feed it via a resistor e.g 1k and have a 5V zener to GND on the Arduino side of the resistor.

So if the input voltage exceeds the threshold of the zener, the zener will breakdown and draw current 35v / 1k = 35mA through the zener.

You'd need to set your resistor value so that it didn't have an effect on the input, but I suspect 1k in series would be OK, also check the zener will handle the max current it needs to handle in case of your worst case voltage

thanks. I got the idea. I don't think I have a 5 v zener, but I got few 3.3v, and since my shunt voltage reading will never go up to 3.3v. 3.3v= 3300 ma. Since I am planning to use about 1 amp. I can use 3.3v zener diode here, correct?

I don't think I have a 5 v zener, but I got few 3.3v

If you have some standard diodes e.g 1N4148 (not power diodes like 1N4001), if you put them in series with the 3.3V zener, but forward biased i.e pointing down towards ground, each 1N4148 will give you about an additional 0.5V, so if you use 2 or 3 of them you can get 4.3V or 4.8V if necessary.

(because there is a forward threshold on most diodes of 0.5V before they will conduct at all).

rogerClark:

I don't think I have a 5 v zener, but I got few 3.3v

If you have some standard diodes e.g 1N4148 (not power diodes like 1N4001), if you put them in series with the 3.3V zener, but forward biased i.e pointing down towards ground, each 1N4148 will give you about an additional 0.5V, so if you use 2 or 3 of them you can get 4.3V or 4.8V if necessary.

(because there is a forward threshold on most diodes of 0.5V before they will conduct at all).

why power diodes won't work? I think I got many of 1n4001.

I have another question. I think I just burn 2 of my MCP41100 digital potentiometer chips; It's used to control the voltage of my booster circuit. one end is connected to 50V output, and one end is connected to reference point (4.7k to ground). what can I do at this point? I still got one 41100 left. exactly what I did wrong? I simple removed the original 10K VR, and replace it with my 10K DP.

1N4001's are a bit slow, the may not conduct before you fry the input to the Arduino.

Re: Digital pots

I suspect you either over-volted them (including transient spikes etc) or tried to get them to pass too much current. (check the spec) (But.... I've never used them myself)

rogerClark: 1N4001's are a bit slow, the may not conduct before you fry the input to the Arduino.

Re: Digital pots

I suspect you either over-volted them (including transient spikes etc) or tried to get them to pass too much current. (check the spec) (But.... I've never used them myself)

I got the whole point. thanks for the help