Sorry for hijacking this thread, but this might also help others. I am planning to use a very similar relay module (as pictured in the first post), for my printer (supplied from 24V power supply). The difference is that on mine, the trigger for the relay is the center pin. So recapping: it is non opto-isolated 5V coil side, though driven from the 5V via a transistor and not the signal pin and the relay coil have a diode as support.
I just want to confirm that my planned schematic for a small board is going to work as I expect it to.
0_1523304616058_DuetWifi_PSON Inverter_schem.png
Basically in short, we have 24V coming in and then being regulated to 5V (7805 regulator). The 5V is then supplied to the 5V near the PS_ON pin. The 5V is also used to pull the PS_ON to 5V (using a 10k pull-up resistor) [I understand that the PS_ON is not actually pulled high inside the DuetWifi, since the ATX PSU usually does it]. The R1, R2 and Q1(547B transistor) works to invert the signal (since the DuetWifi PS_ON On signal is actually LOW, and my module needs a HIGH). Finally the 5V and inverted signal is fed to the Relay Module. Note that R5 is simply a link aka 0Ohm resistor (needed in the board design).
The second part is an optional part in which I try to increase the time the printer can perform the save/resume when power is lost during a print. The SuperCap is a 1F capacitor for 5.5V, which is charged via a 220Ohm resistor (this means it takes 220s to fully charge upto 5V, but should not blow the regulator). If you want to do something similar, you will need to use the External 5V jumper, otherwise it will simply not work.
Now my few questions:
Should the relay circuit work? Are there any required changes for this?
The super cap, will it be able to power the board via the 220Ohm resistor, or how can I ensure it charges via the resistor, but not discharge via it (I considered a diode, but then the voltage drop [though the 220r resistor might give an even greater voltage drop]).
Some extra info that might be useful: in my setup I have the 5" PanelDue, I am using only a single extruder (PT100 sensor) with a heatbed (10k thermistor) and the IR Probe. I have a bypass switch, with which the PSU will get power, startup and enable the relay.
As to the question of the SSR above, I considered using one, but decided against it. Ultimately it is your decision, however you should keep in mind that an SSR fail closed (i.e. it lets the full power through). SSRs are not recommended for important circuitry, especially as the only switch (thus you need at least a switch before it, so that you can safely shut down the printer in case of a failure). That said, failures of the SSRs are not very common (as long as you have a reputable brand) and chances of it failing with a mosfet for one of your heaters are extremely slim (that is mostly the worst case scenario*).
*The firmware does not currently allow a shutdown if it detects such an issue, but it have been discussed as a possible feature for the future (will need a debugging log that can persist after shutdown).
