Mapping outputs

I have a project that takes a voltage reading out of a box, used for remote telemetry, and I want to convert that to an appropriate meaning value. For example,

The output of the box is .8 volts and that should relate to a reading of 75 watts, then on the same output it will have a value of 1.6 volts and I want that to correspond to 500 watts.

I cannot get my head around the ratio and the equation needed to interpolate the wanted values.

I've tried mapping and when I set the variable to map(Vin, 0, 1023, 0, 450) I get the desired 75 out.
When I increase the Vin to 1.6 the output reading on goes up to 150. Which all makes sense since it is double the .8, but I need to change the ratio.

Can anyone enlighten me? Thanks.

The power in a resistive load is proportional to the square of the applied voltage. That is why you "want" to see 500, not 150. But it should be 300, not 500. 75*(1.6/0.8)^2

usdharris:
, but I need to change the ratio.

Can anyone enlighten me? Thanks.

Don't use the map function. Just do some simple maths.

I presume yo know from the specifications of the box (which you have not told us) whether doubling the voltage represents a doubled or squared change in the power?

...R

Don't bother looking for the other version of this question, I've deleted it.

DO NOT CROSS-POST, CROSS-POSTING WASTES TIME.

Sorry for the cross post. I couldn't remember which category I had posted under and never received any notice that the post had been answered, so I posted again. My bad.

So here is my setup and readings that I want to get my head around. It is very possible that the values I get from the transmitter are not what they should be, but there is nothing in the manual that indicates what they should be, so I'm working with what I know from my measurements.

This is an AM broadcast transmitter that has 3 different power settings and I'm trying to convert the telemetry output (meter readings) so I can use them in a IOT arduino setup. The reading I get are as such:

Low Mid High
#1 PA volts meter 35 50 68
Telem out .69 .95 1.5

#2 PA current meter 4.5 6.25 8
Telem out .22 .3 .6

#3 Power out meter 70 250 500
Telem out .36 .5 .84

I'm sure that the values on the Telem out must be too low on the #3, but as I said, it is what it is.
I'm still thinking the meter scale must have a logarithmic scale to it but it's not my area of expertise.
I look forward to learning though.

Thanks.

usdharris:
Sorry for the cross post. I couldn't remember which category I had posted under

Bookmark this for future reference :slight_smile:

Hi,
What does the transmitter documentation say about the telemetry, its certainly isn’t linear if you measure telemetry output volts.
Are you assuming it is in voltage format?

Is the output you are measuring supposed to go to calibrated meters, hence the non linearity?

Also PA current * PA Volts does not equal PA OUT, it equals PA INPUT Power to the PA.

Tom… :slight_smile: