An industrial detergent dispensing pump controller.
The product as tested failed to meet the EMC standards. The objective of the
investigation by ETS was to identify the cause of the excessive emissions and
implement a fix. It was very important to the customer that the cost of
introducing the fix would not effect the production cost of the unit. This
constraint is common in massed produced items and dictates a solution based on
suppressing noise at source rather than more expensive shielding and filtering
further from source.
The pump controller consists of two PCBs in a screened steel enclosure. Ribbon
cables connect the PCBs to each other and power and control cables link the
controller to external devices. The first stage of the investigation was to
identify which PCB was the source. The board loop antenna of the ETSi 4334 was
placed over each PCB in turn and a simple level measurement made of the
emissions at the problem frequency. This quickly identified the main CPU board
as the culprit.
The next stage of the investigation was to pin point the source of emissions at
this frequency to a track and component level. To do this, the PCB was placed
on the ETSi4334 and a 1mm X-Y plot carried out at the offending frequency. This
took less than 20 minuets. This finished plot contained sufficient detail to
enable the PCB designer to immediately identify two problem areas; A track from
the CPU to a peripheral IC and a hot-spot beneath a driver chip. If necessary,
the gerber file for the component layout or track layers could have been
overlaid on top of the X-Y plot to assist with identification.
Plot of emissions
from original board
Armed with this level of information on the
problem it was a simple matter to implement suitable suppression. Components
costing only a few pennies/cents were applied to suppress the driver IC and
filter the signal track.
The suppressed PCB was re-scanned and compared with the original scan to
confirm the effectiveness of the fix. This showed a significant reduction in
the noise level as shown. The system could then be re-assembled and tests on
the OATS completed.
Plot of emissions
from modified board
The total time to identify and suppress the fault including dismantling and
re-assembly was less than two hours. The system went on to successfully meet
the applicable standards, and the cost of the extra components was acceptable