This is an introduction to the 24 VDC DIN rail mounted industrial power supply. It presents system-level information for setting voltage levels as well as input and output protection. We focus on boundaries to identify incorrect assumptions:
- Internal fuses and current limiting protect the power supply, not the load.
- The external circuit break protects the branch circuit, not the power supply.
- Output protection limits power supply damage during shorts but does not prevent system brownouts or PLC resets.
- System reliability requires external measures (UPS, load isolation, monitoring).
Is the power supply protected against short circuits?
Shorts happen!
Industrial automation electronics systems operate in harsh environments. For example, sensors are especially susceptible as cables are snagged or crushed. Also, people make mistakes such as accidentally crossing wires during the repair / modification process or a slip of the meter.
Always consult the datasheet to determine if the power supply can gracefully handle a short circuit.
As an example, consider the Phoenix Contact 1399932. The datasheet states that the DC output is short-circuit-proof. This was tested by placing an ammeter directly across the output terminal. The power supply dropped into a low current mode (approximately 0.7 A):
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This is not a recommend practice as repeated or long duration short circuit could damage the power supply.
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The meter’s probes did spark when connected. Presumably this is the energy being dissipated from the output filter capacitor.
It is in the OEM’s best interest to make a robust power supply that handles a short circuit gracefully. They would suffer reputation damage if the DC power supply required attention after every accidental short.
Figure 1: Image of the 30 W Phoenix Contact 1399932 power supply.
System Impacts of Overcurrent Protection
The power supply automatically entered and then exited the overcurrent condition. It did so silently without providing alarm other than the loss of the “DC OK” LED on the face of the unit.
The supply switches operating modes from stable 24 VDC source to constant current mode similar to a bench-type DC supply in a constant current mode.
Normally we don’t need to worry about this behavior. However, this could be a problem if a large load is activated. The problem could manifest as a random PLC resets or release of control relay latches.
Figure 2: Interior of the power supply; the 35 V 560 uF output capacitor is on the right.
Mitigation for System Level Protection
The power supply’s output protection protects the power supply, but not the system. There are several mitigating system-level options:
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Isolation: Use an independent DC power supply for high-inrush loads.
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Buffering: Provide the PLC with an Uninterruptible Power Supply (UPS) as shown in Figure 3.
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Observe: Select a supply that featured a dry contact output for the “DC OK” signal (Figure 4). This contact can be captured and reported by the PLC.
A UPS driven PLC that monitors DC bus status is the ideal, no cost spared, option. This system can respond to power failures with an orderly shutdown of the plant.
Figure 3: Schneider Electric Industrial (DIN rail) BVS240XDPDR uninterruptible power supply.
Figure 4: Image of a MEAN WELL MDR-100-24 supply featuring a DC OK contact.
Is the power supply input protected?
As the surface level, we recognize that a 4 A slow-blow fuse is used to protect the featured power supply. This fuse is located on a daughter board (Figure 2) just behind the large blue varistor. A more detailed answer requires that we explore the boundary conditions surrounding the term “input.”
Input Protection Boundaries
Carefully define input protection:
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The 4 A fuse provides protection against catastrophic failure within the power supply. An example is a short circuit of the line side power semiconductor.
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The input fuse is not designed for output short circuit as previously described.
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The OEM recommends a “6A to 16 A external circuit breaker. This is not protection of the power supply per se, but protection of the branch circuit for conditions such as a short to the chassis.
Do not conflate input fuse protection with output protection. We can visualize this by comparing the 4 A fuse to the full 0.25 A current for an ideal 30 W supply (calculated as 30 W / 120 V). The fuse is an order of magnitude too large to provide output protection. Also, we don’t want the fuse too small, or it will open e.g., on start up as the input capacitor is charged.
Can the output voltage be adjusted?
This class of power supply has a readily accessible potentiometer located on its face. This provides limited adjustment between 24 VDC and 29 VDC.
The 24 VDC rating is the nominal voltage for a class of industrial control equipment including the PLC and field devices. The ability to set the power supply higher than 24 VDC is mitigation for voltage drops in the wires. For example, suppose the power supply is used to drive a 10 W load in a auxiliary cabinet separated 30 feet from the main enclosure. The adjustment allows the technician to set the voltage at 24 VDC as measured at the load as opposed to the power supply.
Related Articles by this Author
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians.
Dahlen has returned to his Northern Minnesota home, completing a decades-long journey that began as a search for capacitors. Read his story here.