If you get a short in your wiring between your heater or fan wiring and the VSSA side of a thermistor connection, and you have a Duet WiFi board revision 1.0 or 1.01 or a white Duet WiFi pre-production board, then fuse F1 will blow. The symptoms are:
- All thermistors will show as open circuit
- The resistance between a VSSA pin on the back of the board and a ground pin, which should normally read less than 5 ohms, will be very high
If this happens to your Duet, you should first identify and fix the short. After that, do one of the following:
- Replace the original fuse with any fast acting 0603 size fuse having a rated current of between 125 and 250mA and a resistance of between 2 and 4 ohms. Suitable part numbers are Kamaya FCC16151AD-B and Littlefuse 0438.250WR. If you choose the Littlefuse part and the old fuse is also the Littlefuse part (recognisable by its metal end caps), you may be able to solder the new fuse on top of the old fuse. Otherwise, you will need hot air soldering equipment to remove the old fuse.
- Bypass the original fuse with an external cartridge fuse, fitted between one of the VSSA connections and one of the ground connections.
- Bypass the original fuse with a self resetting fuse as is fitted on Duet WiFi board revisions 1.02 and later. Suitable part numbers for the self-resetting fuse are Multicomp MC36207 from Farnell, Bourns MF-MSMF014 and Bel Fuse 0ZCJ0020FF2E. Connect the fuse between the E0 thermistor VSSA pin and the E1 endstop ground pin. Optionally, connect a 10K resistor between the TCK pin of the JTAG pads and the VSSA pin of the E1 thermistor connector, which will allow the firmware to warn you when there is a short to VSSA. Here is a photograph showing this modification.
The small green component bearing the letters 'bF' is the self-resetting fuse, and the component with stripes is the optional 10K resistor. A piece of Kapton tape was put on the board first to provide additional insulation.
