I recently moved house and found that my PDP-8/e triggered the RCD (Residual Current Device) in the switchboard of my “new” (15 year old) home.
The problem was quickly identified to be the Sprague 0.1uF “line-to-earth” capacitors in the 8/e’s H724 Power Supply.
In summary: the line-to-earth capacitors cause an earth leakage current that exceeds the trip threshold for (now compulsory) RCDs in Australian homes.
The fix was to simply remove the Sprague capacitors. However, this leaves 11 mains-voltage spade connectors floating inside the H724 power supply. Not ideal.
So I’ve drawn up a simple PCB that replaces the Sprague capacitors with a suitable array of terminals for the spade connectors. It also optionally provides for a small array of 4.7nF Class Y line-to-earth safety capacitors that may offer some RFI suppression benefits (albeit much reduced from the larger original Sprague capacitors).
A more detailed commentary, and PCB artwork, are available here -> http://avitech.com.au/?page_id=3585
This small modification reduced the earth leakage current from 13-21mA, down to 0.7mA. Earth leakage problem solved, though admittedly I really don’t yet understand whether I have introduced a significant RFI problem in doing so (by reducing the line-to-earth capacitors from 4x100nF to 4x4.7nF).
Malcolm.
The problem was quickly identified to be the Sprague 0.1uF “line-to-earth” capacitors in the 8/e’s H724 Power Supply.
In summary: the line-to-earth capacitors cause an earth leakage current that exceeds the trip threshold for (now compulsory) RCDs in Australian homes.
The fix was to simply remove the Sprague capacitors. However, this leaves 11 mains-voltage spade connectors floating inside the H724 power supply. Not ideal.
So I’ve drawn up a simple PCB that replaces the Sprague capacitors with a suitable array of terminals for the spade connectors. It also optionally provides for a small array of 4.7nF Class Y line-to-earth safety capacitors that may offer some RFI suppression benefits (albeit much reduced from the larger original Sprague capacitors).
A more detailed commentary, and PCB artwork, are available here -> http://avitech.com.au/?page_id=3585
This small modification reduced the earth leakage current from 13-21mA, down to 0.7mA. Earth leakage problem solved, though admittedly I really don’t yet understand whether I have introduced a significant RFI problem in doing so (by reducing the line-to-earth capacitors from 4x100nF to 4x4.7nF).
Malcolm.