#### Issue

Need to control six fans at same speed.

#### Product Line

Andover Continuum

#### Environment

b3920, ABB VFD

#### Cause

How to properly calculate the load on the analog output.

#### Resolution

The b3920 has a 0-10V output and the VFD has a 0-10V input.

The VFD data sheet indicates the input presents a resistance of 2 Megohms. The b3920 data sheet indicates the output load resistance must be greater than 2Kohms.

When resistors are wired in parallel (like when sending the same signal to multiple VFDs) the total resistance is always less than the smallest resistance. When the resistors are the same size the calculation of the total resistance is simply the resistance divided by the number of resistors.

In this case we have 2,000,000/6 = 333,333 ohms - way more than the 2000 ohm minimum. So, the b3920 output should be able to send a proper signal to the six VFDs.

For more info on the math see http://en.wikipedia.org/wiki/Series_and_parallel_circuits#Parallel_circuits