Author: Industrial Control Lao Zhou PLC
Power is often considered one of the most important considerations in a DC control circuit – but we can’t ignore other key factors: power filters, converters, and backup power modules.
Input power is the basic configuration of any electrical control system. Without power, there is no sensor, controller, load device, nothing. So it’s no surprise that the power industry has received so much attention in modern manufacturing. Almost every supplier and supplier offers a variety of panel mount or DIN rail mount options for supplying DC and AC power to control circuits.
We’re probably all familiar with the normal power lineup. 220VAC to 24VAC transformer, or 24VDC DC power supply. But in addition to these devices, there are many other power-related devices, so let’s take a moment to discuss some of the other components that power the control electronics.
Figure 1. Power and regulation are critical to any modern control system.Image courtesy of Canva
Power converters are a very common method of distributing DC voltage from one level to multiple other levels. This device is common when a voltage level is introduced into the system, possibly a 24 volt branch, which must then be supplied as an output to various 24, 12 and 5 volt devices. Requiring separate power supplies for each voltage would be expensive and would require 220 volts AC to be brought into the sub-panel.
Instead, these devices, called DC-DC converters, can be used to output a single DC voltage when provided with different input levels. Two such converter families exist. First, the most common is the DC buck converter, which steps down the input voltage to a lower regulated output. The other is called a boost converter, which boosts the input voltage to the output.
For industrial applications, an input feed of 24 volts may appear and further down to 12 and 5 volts, so 24 volts is a common DC-DC converter input. However, when the input power is for a motor or other load device, you may find these converters with an input range of 10-30 volts (similar to many discrete sensors) or up to 90 volts DC.
These converters exist in many form factors, including panel mount and DIN rail versions. One of the most interesting examples I’ve seen is the terminal block style, such as the Figure 2 example from WAGO, which has a low current output but is designed to be very efficient and ideal for small cabinet spaces.
Figure 2. Terminal block DC-DC converter dimensions.Image courtesy of WAGO
Compared to discrete PCB designs, DC filters in control circuits are less common for two reasons. First, the load device in the control circuit needs more current, and small stray interference signals are usually not enough to affect the load. Second, for those circuits that are sensitive enough, many solutions already exist, including differential signaling, twisted pair, shielding, etc.
But for extreme or very sensitive DC circuits, filters can be used to subtract high frequency “noise” signals from otherwise normal circuits. For a DC control voltage, the level changes abruptly, but it doesn’t switch back and forth as quickly as in an AC circuit. This means that any high frequency signal is not needed in the DC case, but low frequency (slower) changes may be allowed.
These devices are most commonly panel mounted, connected directly to the inside of the control cabinet or to machines close to the point of power demand to minimise further interference after the filter. These can also be purchased as PCB-scale chips or as a set of components for a single use case (usually a design engineer). Vicor Power Products has created a series of such designs with voltage inputs in the low DC range and stable outputs without high frequency noise.
Figure 3. Input DC Line Filter.Image courtesy of Vicor
This section may still be considered a simple power supply scenario, but goes a step further with a solution where power is supplied directly at the location of the final equipment, rather than a large power supply placed in a central cabinet and distributed from the cabinet for safety’s sake . Power on each machine is a good concept, as long DC lines cause voltage loss (and cost), but placing open chassis components on machines in the field is risky. Moisture, vibration and temperature do not mix well with sensitive Electronic components.
IP67-rated sealed power supply
Allows power (input and output) to be connected to a module that is completely sealed from external dust and moisture, a good solution for some of the dirtier field situations. If water or oil splashes or flushing are an issue, it is wise to find equipment that can handle such environments. Otherwise, be prepared for power failures and regular replacements. Screw terminals are not found on these devices, instead relying on threaded connections such as the M12 variety where signal and network cables are already common. Murr Elektronik has such equipment, the Emparro67 series, for this purpose.
Figure 4. EMPARRO67 power supply with sealed inputs and outputs.Image courtesy of Murr Elektronik
Power over Ethernet (PoE)
If devices are to be networked to a central controller, then why do we need a network and power cables? Why not use the same bundle? That’s the point of PoE. With dedicated PoE “injectors” that supply voltage to field devices, cabling can be reduced, and due to the nature of twisted pair cabling, the system already has greater immunity to interference built into the system.