Percent math help

TC

I am not that great with math, and I have ran into a problem that I need some help with.

I am working on a voltage/current meter for a PC power supply. I have the current part figured out and working, but I cant figure out how to do the voltage. What I have is a Microchip PAC1710, it has a 11-bit ADC that has a input range from 0 to 39.98V. I have it figured out to convert the ADC value to a voltage. But now I need to figure out how to change it into a percentage from a set value.

For example:
If I have a set voltage of 3.3V ($A9), and what is read from the ADC is 3.457V ($B1). I need it to output 47 (4.7%). I have this part figured out.

But also, if I had the same set voltage, and the value from the ADC is 3.124V ($A0). I need it to output -56 (-5.6%)

I can change the way my program takes the value if it would be to hard to do a positive and negative value. What I would like to be able to do is just set the set voltage, and the prop output what percentage is the ADC value from the set voltage, with the set voltage being 0%

There will only be 3 set voltages, “3.3V”, “5V”, and “12V”

Thanks
TC

lonesock

Looks like you want "per-mil" error [8^)

pme := 1000 * (value - target) / target

Note that this does not have to be in volts...you can use native ADC counts, if you wish. Also, you can pre-round to the nearest integer value like this:

pme := (1000 * (value - target) + (target / 2)) / target

You shouldn't have to worry about overflow.

Jonathan

Edit: I think 3.124V is 5.3% below the target value of 3.3 (not 5.6%), assuming you want a percentage of the target value, not just the lower value.

TC

lonesock wrote: »

Looks like you want "per-mil" error [8^)

pme := 1000 * (value - target) / target

Note that this does not have to be in volts...you can use native ADC counts, if you wish. Also, you can pre-round to the nearest integer value like this:

pme := (1000 * (value - target) + (target / 2)) / target

You shouldn't have to worry about overflow.

Jonathan

Edit: I think 3.124V is 5.3% below the target value of 3.3 (not 5.6%), assuming you want a percentage of the target value, not just the lower value.

Wonderful! That works, Thank you.

And the way I got the 5.6% is from (( 3.3 / 3.124 ) -1 ) *100 = 5.6%
It is the only way I know how to do percentages.

lonesock

Ah, got it! Your equation works, but you want to know 3.124 as a percentage of the reference, meaning 3.3 is on the bottom of the fraction. So, same equation: (3.124 / 3.3) - 1 = -5.333.

Jonathan

Duane Degn

I'd do the percentage math (when not worrying about integers) one of these two ways.

((3.3 - 3.124) / 3.3) * 100% = 5.333%

or

(1 - (3.124 / 3.3)) * 100% = 5.333%

Edit: I hadn't seen Jonathan's last post

TC

lonesock wrote: »

Ah, got it! Your equation works, but you want to know 3.124 as a percentage of the reference, meaning 3.3 is on the bottom of the fraction. So, same equation: (3.124 / 3.3) - 1 = -5.333.

Jonathan

Ah, now I get it. Thanks

TC

Duane Degn wrote: »

I'd do the percentage math (when not worrying about integers) one of these two ways.

((3.3 - 3.124) / 3.3) * 100% = 5.333%

or

(1 - (3.124 / 3.3)) * 100% = 5.333%

Edit: I hadn't seen Jonathan's last post

I never learned much about percetages, But now I can say "I know 2 ways to do it. And I understand what is going on"
Thank You