EMI Power Line Filters are required due to the use of high frequency switching techniques used in today&#;s power conversion circuits. Harmonic noise created from fast changes in voltage and current from Switch Mode Power Supplies (SMPS), inverters, rectifiers, etc. require EMI filters to meet the conducted emission requirements as set forth in the various international and military EMI standards.
In addition to reducing the harmonic energy propagating out onto the power distribution system, an EMI power line filter serves the dual function of reducing noise already on the power distribution system from entering an equipment.
The design of an EMI power line filter involves knowledge of filter impedances and how they interact with circuit impedances across a broad frequency range. However, there are installation and mounting details that can equally determine the effectiveness of an EMI power line filter.
Three common issues are presented:
1. Filter case bonding to the host equipment: It is imperative that the metal case of the EMI filter is &#;metal to metal&#; bonded to the equipment metal case/chassis. Painted or nonconductive surfaces must be eliminated as they will reduce the effectiveness of the line to ground capacitors in the filter.
2. Filter placement at the entry point of the incoming power lines: Placement of the EMI power line filter at the entry point of the incoming power lines is critical to filter performance and maintaining overall shield integrity of the equipment&#;s metal case/chassis. The further away a filter is mounted from the power entry point, the greater the possibility noise from the power supply or logic circuits can radiate and couple onto the power lines bypassing the filter.
3. System cable routing: In conjunction with item 2 above, it is important to physically isolate the incoming power lines to a filter from other system cables and wires. If system cables/wires are routed too close to a filter&#;s input wires, noise can couple onto the filter wires bypassing the filter.
At Premier Filters, our extensive line of standard and custom EMI power line filters are coupled with our unparalleled knowledge of EMI to deliver the optimum filter solution. Give us a call to see how Premier can deliver your project on time and on budget.
Theoretically, in ideal conditions, it makes no difference where you put your filter in a gravity water filter system. However, placing the filter at least a few feet below your water source has practical advantages.
The difference in pressure between the filter inlet and outlet is what forces water through it.
In a &#;bottom filter&#; setup, each foot of elevation between the water source and filter inlet creates .434psi of hydrostatic head at the filter. Ignoring atmospheric pressure, the filter inlet pressure is .434psi for each foot of head and the filter outlet pressure is 0.
Link to Daji
In a &#;top filter&#; setup, for each foot of elevation between the filter outlet and the lower end of the tube, there is also .434psi of hydrostatic head. The head creates a &#;vacuum&#; effect at the top of the tube equal to .434psi. Ignoring atmospheric pressure, the filter inlet pressure is 0 and the filter outlet pressure is negative 0.434psi for each foot of head.
In a top filter system, head between the water surface of the source vessel and the inlet of the filter may be sufficient pressure alone to force water through the filter.
However, for the &#;vacuum&#; effect of the top filter system to work effectively, the tube must be primed and handled carefully so that prime is not lost. This is where the bottom filter setup has an advantage.
In the following pictures, I try to demonstrate the theoretical pressure differences.
In this picture, a tube is filled with water, both ends leveled and a pressure gauge attached. The gauge reads zero.
In this picture, the gauge is at the bottom with approximately 34&#; of head. (34&#; of water is approximately 63mm of mercury). Gauge shows about 60mm hg.
In this picture, the gauge is at the top with approximately 38&#; of head below it. (38&#; of water is approximately 71mm of mercury) The gauge moved from 0/320 down to 250; a decrease of 70mm hg.
These two pictures show the overall setups.
My observations and calculations confirm that the pressure differences across a filter should be the same in theoretical top filter and bottom filter setups.
My personal experience with Sawyer, Seychelle and Frontier Pro filters is that several feet of head between water source and filter work best because of the priming issues mentioned above.
Conversion factors used:
1 inch = 25.4 mm
Specific gravity of mercury = 13.6
If you want to learn more, please visit our website Soft Housing Filters for In-Line Use.