Placing a current sensor on the motor conductor of a saw, sander, or CNC machine lets the dust or mist collector serving it start and stop when the machine itself starts and stops. In a dust collector auto-start circuit, the sensor contact closes when the motor draws current above the sensor’s threshold, which energizes the collector’s starter coil directly or through an interposing relay. When the motor stops, the contact opens and the collector shuts off. The sensor clamps around the conductor, so no connection to the machine’s controls is required.
Functional Devices, Inc. manufactures RIB® (Relay-In-a-Box®) current switches and relays used for this interlock, including the RIBXGH series, whose sensor contact switches 120 Vac loads directly. The selection decisions are the sensor’s threshold, set by how much current the machine draws when idle, and the wiring method, set by the voltage and current of the collector’s starter coil.
How Current-Sensing Auto-Start Works
A current switch is a current sensor with an on/off contact output. It is built around a current transformer (CT): the monitored conductor passes through the CT’s opening, and current in that conductor induces a signal the switch compares against its threshold. Above the threshold, the sensor contact closes; below it, the contact opens.
The RIBXG and RIBXGH series are split-core switches, so the sensor opens and closes around the conductor and is installed without disconnecting it. Both sense up to 150 A on conductors up to 600 Vac, through a 0.52” x 0.52” opening.
- RIBXG Series: The sensor contact switches 30 Vac/dc at 0.4 A maximum. It drives a relay input or a controller input.
- RIBXGH Series: The sensor contact switches 120 Vac loads only, at 1 A maximum up to 50° C and 0.5 A up to 60° C.
Choosing a Current Switch Threshold
The threshold must sit above the current the machine draws when idle and below the current it draws when running. A fixed-threshold switch works when the idle current is near zero, as on a saw or sander whose motor is either running or off. The RIBXGTF has a fixed 0.35 A threshold, and the RIBXGHTF has a fixed 0.50 A threshold.
Machines that draw standby current when not cutting, such as CNC equipment with control power and drives on the same feed, can hold a fixed-threshold switch closed and keep the collector running. Functional Devices recommends adjustable models for loads with standby current. The RIBXGTA adjusts from 0.75 to 150 A, and the RIBXGHTA adjusts across the same range with a 120 Vac contact. Setting the threshold above the measured idle current separates running from idle.
The self-calibrating models, such as the RIBXGTA-SCAL, close their contact only while current is within a set range and open it when current rises above that range. A cutting load that spikes above the calibrated range would open the contact and stop the collector, so these models are suited to status monitoring rather than auto-start.
Switching a 120 Vac Starter Coil With the RIBXGH Series
When the collector’s motor starter or contactor has a 120 Vac coil, the RIBXGHTF or RIBXGHTA sensor contact is wired in series with the coil. The machine starts, the contact closes, and the coil pulls in. No interposing relay or separate control transformer is needed.
The coil must stay within the 1 A contact rating, derated to 0.5 A where the sensor is mounted in ambient temperatures between 50° C and 60° C. Starter coils draw more current at pickup than when sealed in, so the coil’s inrush rating is checked against the contact rating, not only its sealed current.
Wiring Larger or Non-120 Vac Coils With a Dry Contact Input Relay
Where the coil exceeds 1 A or runs at 208–277 Vac, a RIBXG series switch drives a relay that switches the coil. The RIB21CDC is a dry contact input relay powered from 120–277 Vac: it supplies its own Class 2 input voltage, so the current switch’s sensor contact closes the relay’s input directly without a separate low-voltage power supply. The relay’s 10 A SPDT contact is rated 470 VA pilot duty at 125 Vac and 770 VA pilot duty at 250 Vac.
The RIB21CDC has a 1.8-second operate time, so the collector starts shortly after the machine rather than at the same instant.
Current Sensing for Mist Collector OEMs
A mist collector on a CNC machine is commonly interlocked with the spindle or coolant pump motor. An equipment builder can ship the collector with a current switch prewired to its control circuit, so installation at the customer’s site is limited to closing the split core around one conductor of the machine’s spindle or coolant pump motor. Because the RIBXGH series switches 120 Vac directly, a collector with a 120 Vac control circuit and a coil within the 1 A contact rating needs no added relay or transformer for the interlock.
Frequently Asked Questions About Current-Sensing Auto-Start
Why does the collector keep running when the machine is idle?
The machine is drawing standby current above the switch’s threshold. An adjustable model such as the RIBXGTA or RIBXGHTA, set above the measured idle current, corrects this. Moving the sensor from the machine feed to the spindle motor conductor also excludes control power and other standby loads.
Can several machines share one collector?
Yes. Each machine gets its own current switch, and the sensor contacts are wired in parallel to the starter coil or relay input, so any machine running starts the collector.
Can a self-calibrating current switch be used for auto-start?
The -SCAL models open their contact when current rises above the calibrated range as well as below it. A load spike during cutting can stop the collector, so a fixed or adjustable model is used for auto-start.
Does the collector keep running after the machine stops?
Not without an added timer. The current switch opens when the motor current drops below the threshold, and the collector stops with it. A separate off-delay timer provides run-on to clear the ductwork.
Sourcing RIB® Current Switches for Collector Auto-Start
To source RIBXG and RIBXGH series current switches, find an authorized Functional Devices distributor near you. Equipment builders designing current-sensing auto-start into collectors can contact Functional Devices through OEM Solutions.