Another power wiring question



  • Hello!

    Sorry, I've been searching through the forum to try to find an answer but haven't found it. In my build I will be using a 24V Prusa i3 heatbed and duet wifi. I'm looking at using a Mean Well 5V 3A PSU to power the duet wifi (EXT 5V) and a Mean Well 24V PSU to go to the POWER IN and HEATED BED. I will have a power switch (with fuse) for mains to plug into.

    I plan on using an SSR for protection for when I'm away from the house while I'm printing. My question is, can I use an SSR on the L and an SSR on the N lines going to the 24V PSU? My thought is, if one of the SSRs fail, it'll fail short circuit, but the chances of both failing would be pretty low.

    If I do the above, I'm assuming I run PS_ON to the negative terminal on both SSRs, as well as the 5V (or 3.3V as my SSR-40 DA is 3-32VDC) to the positive terminals; is this correct wiring?

    Secondly, if one SSR fails, how would I ever know that it failed? If one fails short circuit, then the other SSR will still turn on/off the PSU and I wouldn't know any better. Any way to hook something up to identify a failed SSR?

    I already have two SSR-40 DA relays as well as both PSUs.

    Thanks in advance!



  • @ringo1508 I've never tried 2 SSR in series with the load but I think that'll work. To tell which is failed hook neon bulbs across the line terminals of the SSRs. When both SSRs are ON or OFF at the same time (normal conditions), both neon bulbs will be off. If one SSR is ON and the other is OFF (which should be an abnormal condition), the bulb across the SSR that's ON will be off and the one across the SSR that's OFF will be on.



  • @gtj0 thank you! I wasn't thinking about running lines between the two to check.

    When an SSR fails, is it correct to assume the short circuit condition is only on the VAC side? I only ask because I rather do logic with low voltage, and I think looking into an XOR circuit of some type and if it's possible to get that value back to the duet to monitor that condition. If I could do this, then the duet could tell me when there's an issue, as well as not let me turn the printer on (the 24V PSU.)



  • I think I might be overthinking this.

    If I'm powering the heated bed via the duet wifi board, it has a fuse that will blow if something shorts correct?

    If there's a thermal runaway, the duet wifi board will disable the power to the heated bed, correct?

    So, if I am using an external 5V PSU to power the board, and then using the duet wifi PS_ON to control the 24V PSU that is powering the board and heated bed, if the SSR that's feeding the 24V PSU from mains fails in a short circuit condition, is there any risk of a fire anywhere? (I wouldn't think so if the duet board is handling the hotend and bed's safety)



  • @ringo1508 said in Another power wiring question:

    @gtj0 thank you! I wasn't thinking about running lines between the two to check.

    Well, not between the two. You'd connect a neon lamp across the LINE (~) terminals on each single SSR. When the SSR is ON, there's no potential difference across it's line terminals so the lamp will be off. When the SSR is OFF, then there WILL be a potential across it's line terminals so the lamp will be ON. This is how old style lighted wall switches worked.

    When an SSR fails, is it correct to assume the short circuit condition is only on the VAC side? I only ask because I rather do logic with low voltage, and I think looking into an XOR circuit of some type and if it's possible to get that value back to the duet to monitor that condition. If I could do this, then the duet could tell me when there's an issue, as well as not let me turn the printer on (the 24V PSU.)

    Yes. Only on the LINE side. The control side is opto-isolated so there's no indication of the actual state of the triac other than from the load. The little red status LED only indicates the state of the control logic, not the actual state of the triac. The only way to get status back to the duet would be if you added a line powered relay of some sort.

    I think I might be overthinking this.

    Yes I think you are. 🙂

    If I'm powering the heated bed via the duet wifi board, it has a fuse that will blow if something shorts correct?

    Only on the low voltage (5v or 24v).

    If there's a thermal runaway, the duet wifi board will disable the power to the heated bed, correct?

    Only if the Duet is powered up and operating and only if at least 1 of the SSRs are operating normally. If the Duet is powered down or halted, it can't do a thing.

    So, if I am using an external 5V PSU to power the board, and then using the duet wifi PS_ON to control the 24V PSU that is powering the board and heated bed, if the SSR that's feeding the 24V PSU from mains fails in a short circuit condition, is there any risk of a fire anywhere? (I wouldn't think so if the duet board is handling the hotend and bed's safety)

    I would not bet my house on the Duet preventing a fire. Too many things can go wrong. Anyone using a mains powered heated bed should have a fail-safe in the form of a thermal fuse that's attached directly to the bed. The fuse should be rated at least 20C higher than the max temp you expect to be printing with but that's just a rough rule. I have a PrintBite surface that only needs 70C to print even ABS and Nylon and my fuse is a 15A 110C part. Also, you want it to be a fuse and not a circuit-breaker. Once the bed reaches the critical temp, you want it to permanently blow and not keep cycling.



  • The bed is 24V. The SSR is only to turn the 24V PSU on/off so I can leave the board on all the time via the 5V PSU that powers it through the external 5V connector on the board.



  • @ringo1508 said in Another power wiring question:

    The bed is 24V. The SSR is only to turn the 24V PSU on/off so I can leave the board on all the time via the 5V PSU that powers it through the external 5V connector on the board.

    Oh duh. You did say that in your first post. I just forgot. 🙂


 

Looks like your connection to Duet3D was lost, please wait while we try to reconnect.