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I am trying to make some lighting resilient to breaker trips. (I.e. You trip the breaker and the lights don't go out.)

Naturally it's all LED, and I am favorable toward low voltage DC lighting.

I want to have a DC always-positive and return bus (e.g. Red and black) then lighting wired in the usual way with a switch interrupting a switched-positive line to the light.

My question is -- can I use multiple DC power supplies to feed this DC bus off multiple mains AC circuits? Idea being if an overload or GFCI trip knocks out a circuit, the other one keeps the lights on. Supplies would be oversized with that in mind.

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  • So, a redundantly powered DC bus in building wiring, basically? – ThreePhaseEel Dec 16 '17 at 22:15
  • @ThreePhaseEel yes, sort of like dual power supplies in a server, but in building wiring. – Harper - Reinstate Monica Dec 16 '17 at 23:04
  • i dont understand the scenareo. Just put the critical lighting on its own circuit. Are you going DC just for this redundancy concept? The DC supply itself becomes an extra potential failure point. – agentp Dec 17 '17 at 14:30
  • @agentp adding a dedicated circuit is not an option, I need all 4 possible circuits for other loads. MWBC is also not an option. – Harper - Reinstate Monica Dec 17 '17 at 20:22
  • Another approach to this problem would be to use a relay activated by the main supply that would switch over to the backup circuit in the event of power failure. – agentp Dec 18 '17 at 16:11
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I parallel power supplies all the time. The only problems I have found is with switching supplies sometimes they don't work well together but this is on loads that are inductive in nature. If I want to be 100% sure it will work I use a transformer based regulated supply set at the same voltage. So both supplies under normal load are being used. Setting 1 supply at a different level has the higher voltage one doing all the work and possibly failing earlier than if both are set at the same level. No diodes or anything else is needed. It is much like hooking 2 batteries in parallel for more amperage.

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I know you are favorable to DC lighting ..Put those circuits on a UPS ?

The problem you have with multiple DC power supplies is that they are switching power supplies.T hey regulate in either constant voltage or constant current mode. They are really turning on and off rapidly to regulate either current or voltage and have a sense circuit to determine regulation.

I said all of that so you will understand there are two things you require in your scenario current and voltage regulation - You will need to make sure you have the right power supplies to do this - although I will say the DC power supplies probably use AC - although you can find DC to DC supplies.

"Two or more power supplies being capable of CV/CC automatic cross over operation can be connected in parallel to obtain a total output current greater than that available from one power supply. The total output current is the sum of the output currents of the individual power supplies. The output of each power supply can be set separately. The output voltage controls of one power supply should be set to the desired output voltage (CV); the other power supplies should be set for a slightly higher output voltage. The supplies with the higher output voltage setting will provide constant current output, and will drop their output voltage until it equals the output of the CV supply. " copied from the link below:

https://www.keysight.com/main/editorial.jspx?cc=US&lc=eng&ckey=520808&nid=-11143.0.00&id=520808

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    Constant voltage only. – Harper - Reinstate Monica Dec 16 '17 at 23:04
  • @Harper I believe you will be required to regulate both. The constant current ensures you maintain proper current in order to regulate the voltage. As you know when your load increases if there is not enough current the voltage will drop. The sense circuit will pick that up and try to regulate more voltage but it will not be able to achieve it because the current draw will be too great. In your instance I think Auto-Tracking is what you need.Keep in mind Switching Power supplies a Load must be present in order to regulate. – Ken Dec 17 '17 at 8:01
  • you are way, way, way overthinking it. Disregard the LED thing, I wasn't gonna use discrete LEDs anyway. Pretend my DC loads are a dog's breakfast of incandescent lights, routers, a TV and other random things which expect 12V constant voltage. – Harper - Reinstate Monica Dec 17 '17 at 20:23
  • @Harper The load type does not make a difference, We are talking about. DC switching power supplies and the way they operate and regulate voltage and current is different than a battery. They turn on and off at a high frequency to regulate with a certain max voltage/current. These power supplies have a sense circuit (resistor) which in CV either ups the voltage (more on time) or lowers the voltage (more off time) based on current draw (ohms law at work) up to its limit (100% on time) after that the voltage cannot regulate and it will fall off as more current is required. – Ken Dec 18 '17 at 4:06
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Yes, you can. Connect them via diodes. You will have a small voltage drop (0.7V) over the diodes, but it ensures that only one power supply will be feeding the system at a time, and that it will not feed the other PSU.

There's several products that does this, for instance this Trio Diode from Phoenix.

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