We run into situations where a system is so critical that no down time, whatsoever, is tolerable. These are often hospitals, nuclear plants, chemical processes and the like. We had a nuclear plant call B&D Technologies’ Applications Engineering Group with the need to create a redundant 24VDC supply unit as a standalone source for several sensors going into their DCS system.


For a device with a rated current of 4 A, this means, for example, that two power supplies, each with 4 A on the output side, are connected in parallel. If one of the power supply units develop an internal fault or if the primary power supply fails, the second unit automatically takes over and supplies the load. This means that the power supply units must be programmed so that one power supply unit can cover the complete power demand of the connected devices, in all operating states.

The challenge is always the summing/redundancy module. If the primary fails, based on specific failure modes, it needs to be blocked so it does not prohibit the secondary unit from taking over. Different manufacturers have different designs for this purpose. The PULS SLR02 is a favorite design by B&D.


The redundancy module SLR02 is designed to create an N+1 redundancy in combination with the PULS 24…28V power supplies units SL10, SL20 or SL30 or other 24…28V power supplies with an output current of up to 30A (max. 35A). One SLR02 is required for two power supply units. The module decouples the power supply outputs from each other, so that in case of failure, one power supply unit cannot overload the other power supply units. A relay-changeover contact, picked-up under normal conditions and dropped in case of failure, indicates the status of each connected power supply unit.


It is so much easier to buy these things than create your own.  In years past, we would create our own diode network and change-over relays for critical systems. The factory-made unit has all of the R&D time behind the design, as well as UL and other certifications for reliability. Home grown systems do not.

Last Words

B&D Technologies has added redundancy to many different types of systems. We have added UPS and redundancy modules to critical VFD systems, PLCs, HMIs, sensors, industrial computers, etc.

Have an idea?  Please email us at

C. Tolbert DOE