Internet PLC Forum

General => Technical support => Topic started by: karen on January 06, 2003, 11:58:06 AM

Title: An Electrical Question
Post by: karen on January 06, 2003, 11:58:06 AM

i have a 0-250ma scale that i would like to read with the ADC what resistor network would i need to use

thanks Karen

Title: Re: An Electrical Question
Post by: support on January 06, 2003, 03:48:49 PM

Do you mean your analog data can output 250mA full scale (or do you mean 250 mV??).  If it is really 0-250mA current output, then  you can convert it into 0-5V by using a 20 Ohm resistor. However, take note that the heating:

  P = I^2 * R = 0.25*0.25*20 = 1.25 W

That means that you need a resistor that is quite stable when heated up.

A better way is to use a smaller resistor to convert into lower voltage and use an opAmp to amplify the signal to 0-5V.

If your circuit is 0-250mV output, then construct a non-inverting OpAmp with 20 time gain to amplify the output to 0-5V.