I have an AC-DC DIN rail PSU (Mean Well HDR-60-24). I'm pretty sure it's +24V and 0V (GND). From the 24V PSU I have a 5V isolated single output (Traco power TMH 2405S) PSU to power electronics. My goal is to detect a 24V (high signal) input. I'm currently using a darlington array. However, it seems that the 5V PSU is a 2.5V and -2.5V (GND). Therefore, my darlington array is outputting not 5V or 0V, for the next components, but 2.5V and 0V. The 0V however is going to -2.5V, so in essence, there is still a 2.5V difference, which makes my next components think that it's a high signal.

Given that there will only be passing a very low current through the resistor, can I connect the -2.5V instead of the 0V (GND from 24V)? This should only increase the potential to 26.5V on one side. In essence, would it be safe for the PSUs to connect them, with a large resistor as in the attached image?

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  • \$\begingroup\$ The Traco is producing an isolated output so there should be no problem connecting its output V- to the ground of the 24V PSU. Then its V+ would be at 5V instead of 2.5V. Does that help? \$\endgroup\$
    – ErikR
    May 31, 2021 at 8:49
  • \$\begingroup\$ I agree with ErikR. "It seems that the 5V PSU is a 2.5V and -2.5V (GND)." This is very unlikely and can be proved either way by either tracing out the ground track on the PSU or/and by metering through. \$\endgroup\$
    – Transistor
    May 31, 2021 at 8:51
  • \$\begingroup\$ @ErikR, yes that helps, thanks \$\endgroup\$ May 31, 2021 at 9:58


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