APPLICATION NOTE Guidelines For Extending Power Supplies
In a typical integrated operating room surgical displays are suspended from the ceiling using surgical boom arms so they can be easily positioned as needed around the surgical table. It is not possible to safely route a high voltage AC power line through most surgical boom systems.
For this reason, the low voltage DC output of the display’s power supply is typically extended and routed through the boom arm and the power supply is installed either in the ceiling or in a equipment rack nearby (see figure 1 below). In order to ensure both electrical safety compliance* and proper operation of the display it is important to follow certain guidelines when extending the power supply’s DC output cable. This application note is intended to provide guidelines that may be referred to when fabricating a DC extension cable for an NDS surgical display. Installation, electrical connections, maintenance, and commissioning should be performed by qualified personnel only. NDS shall assume no liability for damages resulting from faulty installation, operation, improper use or maintenance.
Construction of DC Extension Cable:
A DC extension cable for an NDS display should have certain basic construction characteristics. It should be constructed using connectors of the same type and rating as the display and power supply it is intended to be used with. Both the positive DC power lines and the return lines should have equivalent resistance. Permitted that the display’s earth ground electrical path is no greater than 0.1 ohms (typically achieved by grounding the display’s earth ground post), then the DC extension cable can consist of only the positive DC power and the return lines (no earth ground required).
Example of Extension Cable Construction
Overview of Connectors:
Below is an overview of the two different types of connectors used for extending power to NDS displays., including their corresponding pin assignments. Please refer to the table on page 4 for the connectors to be used for specific displays.
Calculating Maximum Extension Cable Length:
The maximum length of an extension cable depends upon both the minimum input voltage of the display and the electrical current capacity of the power supply. The cable cannot be so long that the voltage drops below the display’s minimum operating voltage. The cable must also not dissipate power to the point where the power supply’s electrical current capacity is exceeded. The maximum allowable length needs to be calculated for each of these two requirements. The shorter length should not be exceeded to ensure both requirements are met.
For the NDS display being installed, determine the values for Vps , Pps , and Pdisplay using the information contained in the table provided on the following page. Determine the value of rcable based upon the type of wire being used. Plug in all values into the equation above to calculate the maximum allowable length based upon power dissipation.
Power extension cables can be designed and fabricated based upon calculated theoretical values. However, it is highly recommended that measurements be taken after installation is complete to verify that the minimum voltage specifications are being met. The voltage delivered to the display after extending the power supply must be measured while the power supply is under its maximum load which occurs at the moment the startup screen is displayed during initial power up.
The following procedural guidelines are recommended for correctly measuring the input voltage under load:
- Without disconnecting the power to the display, gain access to the power electrical connections within close proximity to the monitor’s power input connector. This can be done by sliding back the connector shell of the power connector that plugs into the monitor, or by using a special “Y-cable”. Refer to illustrations below and page 2 for Y-cable part numbers.
- Connect the leads of a volt meter to the +24VDC and ground connections of the monitor’s power input (refer to pin assignments on page 2 or measurement point on Y-Cable below).
- With the volt meter connected, power on the monitor using the power switch.
- Observe the voltage reading as the monitor powers on. Immediately after the startup screen is displayed, the reading should fall to a lower value. Please note, in some circumstances the lowest value may only occur for a fraction of a second.
- Record the lowest reading measured after the moment the startup screen is displayed as the monitor powers on. Compare it to the minimum voltage specification in the reference table provided on page 5 for the NDS display being used. If below the minimum value, adjust the extension cable length or construction as necessary.
Reference Table :
The table provided below specifies the minimum operating voltage (under load), maximum power consumption, and power supply capacity for various NDS displays. Also provided are example maximum extension cable lengths that will meet the requirements of each display when constructed using the standard wire types specified in the table. This table is provided for reference only. Note: these are theoretical values, adjustments may be required based upon environmental conditions and other factors.