Internet PLC Forum

General => Technical support => Topic started by: Bruceh on September 27, 2005, 03:55:45 PM

Title: Increasing current avaliable from analogue power supply
Post by: Bruceh on September 27, 2005, 03:55:45 PM

I am looking to power a sensor from the 5V reference voltage output of my MD888+.
Problem is that the sensor requires 20mA and the analogue reference voltage is limited to 10mA.

The installation manual for the PLC mentions that the 10 ohm resistor RA3 can replaced with a lower value to increase current ....what value should it be to deliver 20mA?

Thanks

Title: Re:Increasing current avaliable from analogue power supply
Post by: support on October 04, 2005, 12:36:20 AM

The 10 Ohm resistor is only a current limiting resistor to handle brief short-circuit of the 5V supply.

It is a matter of how much voltage drop you can tolerate. At 10mA the 10 ohm resistor cause a voltage drop of 0.1V.  At 20mA the voltage drop is 0.2V, which means that the voltage available to the sensor is only 4.8V out of a 5V supply. The current cost an I2R heat dissipation of  0.02x0.02x10 = 0.004W so the existing resistor rating is OK.