32 Rockwell Automation Publication 2080-UM002N-EN-E - November 2022
Chapter 2 About Your Controller
Safety Circuits
Circuits installed on the machine for safety reasons, like overtravel limit switches, stop push
buttons, and interlocks, should always be hard-wired directly to the master control relay.
These devices must be wired in series so that when any one device opens, the master control
relay is de-energized, thereby removing power to the machine. Never alter these circuits to
defeat their function. Serious injury or machine damage could result.
Power Distribution
There are some points about power distribution that you should know:
• The master control relay must be able to inhibit all machine motion by removing power
to the machine I/O devices when the relay is de-energized. It is recommended that the
controller remain powered even when the master control relay is de-energized.
• If you are using a DC power supply, interrupt the load side rather than the AC line power.
This avoids the additional delay of power supply turn-off. The DC power supply should
be powered directly from the fused secondary of the transformer. Power to the DC input
and output circuits should be connected through a set of master control relay contacts.
Periodic Tests of Master Control Relay Circuit
Any part can fail, including the switches in a master control relay circuit. The failure of one of
these switches would most likely cause an open circuit, which would be a safe power-off
failure. However, if one of these switches shorts out, it no longer provides any safety
protection. These switches should be tested periodically to assure they will stop machine
motion when needed.
Power Considerations The following explains power considerations for the micro controllers.
Isolation Transformers
You may want to use an isolation transformer in the AC line to the controller. This type of
transformer provides isolation from your power distribution system to reduce the electrical
noise that enters the controller and is often used as a step-down transformer to reduce line
voltage. Any transformer used with the controller must have a sufficient power rating for its
load. The power rating is expressed in volt-amperes (VA).
Power Supply Inrush
During power-up, the Micro800 power supply allows a brief inrush current to charge internal
capacitors. Many power lines and control transformers can supply inrush current for a brief
time. If the power source cannot supply this inrush current, the source voltage may sag
momentarily.
The only effect of limited inrush current and voltage sag on the Micro800 is that the power
supply capacitors charge more slowly. However, the effect of a voltage sag on other
equipment should be considered. For example, a deep voltage sag may reset a computer
connected to the same power source.
WARNING: Explosion Hazard
Do not connect or disconnect connectors while circuit is live.
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