This short post will describe a small side-project that I’ve had in the back of my head for some time now, and was finally able to complete. It is meant to enhance the usability of my BK Precision 8500 electronic load by adding banana jacks as well as remote sense terminals to the front panel. Since many other electronic loads out there share the same physical structure as this series from BK Precision, the final design can be useful for many other people with a variety of different instruments.
To add the banana jacks to the front panel I’ve decided to go for a small PCB that will be placed and connected to the front connectors, and have banana jacks soldered to it. After looking for different options, I’ve found the CT3151V1 series of banana jacks, which seems to be well made, and had the desired orientation relative to the PCB. This is therefore easy enough to implement. To add front panels remote sensing terminals, I’ve used an additional pair of banana jacks, and ran a wire from this small PCB to the remote-sense terminals on the back of the instrument. The resulting PCB can be seen below in figure 1.
The board can be mounted to the unit with 2 different orientations, so that the banana jacks are either located above, or below the main input terminals. On one of the sides there is a footprint for optional “sense” resistors. These are meant (as an option) to have some path from the main input terminal to the remote-sense terminals even if the remote sense terminal weren’t connected with an explicit cable. This will allow users to leave the unit on remote-sense constantly, with the resistors offering local sensing when the sense terminals aren’t connected with a cable.
I want to add a couple of notes about these (optional) resistors, assuming the instrument is set to remote-sense. Their value is a compromise between accuracy when the S+/S- banana jacks aren’t connected, and when they are connected. If a higher value of resistance is used for these sense-resistors, they will form a voltage divider with the input resistance of the rear-panel remote-sense circuitry. Therefore a lower value of resistance is desired. However, if a low value of resistor is used, this will result in error when remote sense cables are used, as it will offer a non-negligible path in parallel to these remote sense cables. Measuring input resistance of the BK Precision 8500 showed it is ~616Kohm on the low voltage range, and ~551Kohm on the high voltage range. Therefore, I think resistors with a value of 270R are a good compromise. This will offer a total resistance of 540R, which is <0.1% error when no remote cable is used, and low enough conductance when proper remote sensing is used (<0.1% error for wire resistance of 0.5R). I chose to leave these unpopulated on my PCB, and switch to remote-sense in the options menu of the instrument when needed.
Figure 2 shows the final assembled PCB placed at the front panel of the instrument. Just below the edge of the PCB you can see a glimpse of the wires running to the remote sense terminals on the back of the unit.
The Gerbers for this PCB can be downloaded in the link below if you wish to use such a PCB on your own instrument. The part numbers for the connectors are CT3151V1-0/CT3151V1-2 for the black/red connectors.