resistor across analog input for voltage monitoring

[diat150]

Hello,

I have a 24 volt dc system. I want to monitor the system voltage by using a resistor. Ohms law says it should be 1.5k for a range of 6 = 4ma and 30 = 20 ma. When I go directly to ground it correctly measures the 16 ma at 24vdc. If I go to the analog input card it drops to 13.87 ma. Is this due to the internal resistor on the analog input card?

 

[OkiePC]

I am going to say yes, off the top of my head...wouldn't the current input have another 250 ohm precision resistor internally shunting it to common?

 

[kamenges]

A lot depends on the circuit construction of the input you are dealing with and how you have the resistor wired into the circuit. However, it sounds to me like you are creating a parallel circuit and splitting the current from the sensor between the input point and the burden resistor.

If I were doing this I would probably add a 100 Ohm resistor in series with the input wire and measure across that. This is less intrusive as it still maintains the correct current value entering the input point.

Keith

 

[keithkyll]

If analog card has 250 Ohm impedance, then use a 1.25K resistor in series. At 30 Volts, you will have 25V across the 1.25K, and 5V across the 250 Ohm inside the card.

 

[diat150]

It was the internal 250. I did ohms law right after I posted and it matched. Changed scaling to 7-35

 

[joelolson]

Ohms law is just a good starting point. I use this same set up but always use a voltage simulator to set my SCP parameters. Never know how accurate you resister is or the variance between your analog input module.