Ground loop - Prevention --- +5V External PSU

  • Hi everyone,

    I got myself a duet recently, and already installed it in my printer with delighting results. My setup includes:
    -24v standard meanwell psu
    -12v step-down modune, common gnd for fans and water pump
    -AC mains bed controlled by SSR

    Then I use a contact relay to switch PSU and bed, which is on the normally open side. To turn it on I have to momentarily connect the Normally-open and Normally-Closed terminals of the relay and it keeps on when M80 is written in the config.g file. I can use M81 and/or reboot the printer but not M80 unless it is physically connected again.

    To solve this I thought about adding an spare 5V phone charger as always on to the board, but it's still unclear to me if I will have any trouble with it causing ground loops. It has been tried and seems OK, anyone knows a good answer to this?

    *I have measured voltage between booth gnd terminals and by the time it was checked that was not any significant magnitude

    A solution that comes to my mind is using the Normally closed side of the relay for the 5V phone charger, that turns itself off when the other circuit is activated.

  • Tie the negatives at the PSUs and only have one negative to the board (with duex5 daisy chained if necessary) to the VIN?

    The negative for the 5V and VIN are on the same trace on the duet last time I checked.

    Edit: Added 'on the duet' in last sentence for clarity.

  • administrators

    The usual solution is to use a SSR to control the power to the main PSU, and control the SSR from PS_ON. To start the printer you press a push button that bypasses the SSR, holding it down long enough for the Duet to execute the M80 command at the start of config.g.

    A 5V phone charger won't cause a ground loop unless it uses a 3-pin mains plug and there is continuity between the ground pin and the ground side of the 5V output. The phone chargers I have seen use 2-pin plugs, or in the UK the earth pin (needed to open the shutters) is plastic instead of metal, so no risk of causing a ground loop.

  • SSR manufacturers warn against the use of solely SSRs in safety critical switching.

    Coil and contact relays fused at an appropriate level to keep the current through the contacts within spec is preferred. They will require a flyback diode across the coil to protect the electronics (that are sinking the current from the coil) from back EMF.

    Edit: Straight from the Duet 3D fire safety page:

    A similar problem can arise with heated beds that are controlled through a solid-state relay (SSR). The manufacturers of these parts that they can fail as a short, that is, supplying full power to the attached device regardless of the input signal, and they advise against building devices whose only off switch is an SSR in case this occurs.

  • Many thanks for the clarification, so no risk of ground loops using a common 2-pin phone chargers.

    To join the V- terminals on the PSU socket instead of on the Duet looks better also

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