Does your benchtop power supply have a green ground terminal such as the B&K Precision 1550 supply shown in Figure 1?
Assuming it does, how and when are we to use the connections? For example, Figure 2 shows three possible connections with the jumper bar that is often sold with the power supply. Will the power supply be damaged if the green ground terminal is connected to the red output terminal? Is there a safety issue if the ground is left unconnected?
To better understand these questions, we will open the power supply and inspect the internal connections associated with the ground lines. The answers will help us understand all line driven power supplies and leave us with an improved understanding of safety.
Figure 1: Picture of the B&K Precision 1550 single-output power supply with emphasis on the front panel connections.
Figure 2: Picture showing three possible connections for the power supply ground connection.
Let’s start with the obvious: a full discussion about grounding and bonding is beyond the scope of this article. Consult authoritative bodies such as UL and NEC for the latest standards.
With that said, we can discover a few guidelines by examining the B&K 1550. The first and perhaps the most important is that the exposed metal chassis is grounded. This is clearly shown in the left-hand panel of Figure 3. Here we see the back of the IEC power connector. The ground connection (yellow wire) is bolted to the chassis. The terminal lug includes an external-tooth lock washer to ensure a low resistance connection between the wire lug and the chassis.
Figure 3: Picture of the input ground bolted to the chassis (left) and front panel ground spot welded to the chassis (right).
Tech Tip: Under ideal operating conditions there is zero current flowing in the safety ground. However, should the equipment or in-the-wall wiring fault, the safety ground must be able to conduct the full AC line current. This low resistance to things like the exposed metal chassis protects the user. The low resistance is also necessary to provide a high current that will trip the main circuit breaker. High wire resistance is especially dangerous as it will result in continued fault current with potential for fire.
The connection for the front panel ground terminal is shown on the right-hand picture of Figure 3. Here we see a yellow wire with green stripe leading from the chassis to a small circuit board the connects to the front panel banana jacks. Observe the wire’s terminal lug is spot welded to the metal chassis. The other side is soldered to the PCB.
Before we answer our initial questions, we should look at Figure 4. This is the primary circuit board for the power supply. This is a switch-mode power supply (SMPS). Instead of a large 60 Hz transformer it features a high frequency transformer. The result is small and lighter than the traditional transformer solutions.
The important part of this picture is the red line of demarcation. This shows the boundary between the AC line input of the circuit (upper) and the regulated DC section (lower). There are only a few items that span the line including transformers, optocouplers and safety rated capacitors. This provides a good measure of isolation as there are no direct connections.
This isolation between the input and output sections is central to our opening questions.
Figure 4: Primary circuit board for the B&K Precision 1550 power supply with the highlighted demarcation line between input and output sections.
With proper isolation, the DC section of the power supply floats. In an ideal world, it acts like a battery with no inherent connection between the positive and negative terminals. Just like a battery, we are free to connect the floating power supply to other power supplies. For example, we can stack (series connect) two of the 36 VDC B&K 1550 power supply together to obtain 72 VDC.
SAFETY: A floating power supply does not eliminate all current paths. We can expect high-impedance leakage currents. As an illustrative story, consider my old laptop computer. It has very high leakage current. You couldn’t use the computer when you were barefoot and when it was charging. You could feel the leakage current in your toes! If you can feel the leakage current, it is certainly high enough to punch through and destroy the gate of any unprotected Metal Oxide Semiconductor (MOS) devices such as MOSFETS or microcontroller. Of course, if you can feel the current, something has gone seriously wrong and needs immediate attention.
Tech Tip: While the output of the power supply floats, there are limitations. For example, suppose we had 4 of the 1550 power supplies. In theory we could earth ground the lowest supply and then series connect the power supplies together. This is like stacking 4 each 36 VDC batteries providing a total of 144 VDC. In practice we should be very careful as there are limitations to the electrical isolation. Be sure to consult the datasheet or manufacturer to determine the maximum continuous DC offset for any given power supply. In our example, in our 4-series connected example, the upper power supply has a 108 VDC offset.
SAFETY: There is a safety hazard that increases with the number of power supplies connected in series. First, we pass the OSHA threshold of high voltage with a real danger of electrocution. Second, human factor limitations come to play as we may become complacent and forget to turn off a portion of the power supplies. Again, this is an electrocution hazard. You and those who work around you may be safer using a single larger power supply instead of a swarm of smaller units.
All things considered, it is best practice to keep the jumper installed with good connection to safety ground e.g., connect the green terminal to the black terminal. This may be more important as we connect our project to external test equipment such as function generators and oscilloscopes. Note that the oscilloscope probe’s ground lug is connected to earth ground. That caution about high-impedance leakage currents may be very important when working around MOS. The result of you operating an ungrounded power supply may be the same as working on your circuit without a properly grounded workbench or you without a ground strap. The only downside is ground loops, but that is a discussion for another day.
P.S. Please see this new article for an application of the floating power supply: