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I have two separate voltage sources that I would like to control a couple of relays with. The first source is a 14V DC signal being sent by another device under certain conditions. The latter is a 5V DC signal that I have wired to a momentary switch as a kind of manual trigger for the relays. I assume it's not important what I switch on the relays. If I'm wrong let me know and I'll add more information.

The relays themselves take in 5V and have roughly 70 Ohm of resistance, resulting in ~5.1V across their coils with SW1 closed. R1 and R2 are in parallel to distribute heat dissipation between the resistors. That way they stay nicely under 1W when SW1 is closed.

I'm new to this electronics stuff, but I'd like to know if this is an incredible stupid approach? Some questions in particular:

  1. Should I absolutely not use a voltage divider to step down the 14V to ~5V for the relays?
  2. Should I absolutely not have the 5V and 14V circuits grounded together? If not, where should I "close the loop"? On the 14V circuit or the 5V circuit?
  3. Should I absolutely not have the 5V and 14V circuits "intermingling" with each other at all like this? If not, would I need more relays to control the 5V circuit from the 14V circuit or something like that?

Two voltage sources controlling two relays

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The way it is shown, the 14V supply can drive current into the 5V source. In particular it could cause the 5V rail to rise above 5V under certain conditions (one or more relays disconnected, high coil temperature, for example).

There's nothing inherently wrong with using resistors to drop the voltage- you could use a 7805 but it would actually dissipate a bit more power, and would be less robust.

I suggest using a couple of Schottky diodes (eg. 1N5819) to combine the two voltage source, which will also prevent much backflow (the diodes do leak a bit, but in most cases not enough to worry about). For example, if you had something connected to 5V that only drew a few uA (and nothing else) it could power up from the diode leakage when the 14V supply is present. If that's an issue in your case, you can just add some load to the 5V rail (a resistor to ground, for example).

You'll get a bit less than 5V with the 5V source only because the diodes drop a bit of voltage, maybe 4.4-4.5V, probably okay if the 5V source is regulated.

schematic

simulate this circuit – Schematic created using CircuitLab

You could also wire your switch to the 14V supply (with diodes) and avoid the 5V supply entirely (and perhaps use 12V-coil relays, which would save about 60% of the power consumption)

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    \$\begingroup\$ Thank you for answering my questions and for all the detail and additional context, especially suggestions for specific components. \$\endgroup\$ Oct 4, 2023 at 15:26
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Another simple way to isolate the two supplies would be to just use a double throw switch for SW1.

You would place SW1's common pin directly on the relay side. That way the 14V supply (with dropping resistors) could be selected by setting SW1 in the first position. With SW1 in the opposite position the push button SW2 has optional control of the relays via the 5V supply.

It would be best to also add fly-back diodes on the relay coils to prevent any high voltage spikes from getting to the switches, (call it finger safety).

If you don't have local access to SW1 you could alternately make SW2 the double throw switch (maybe a momentary rocker switch), then run the in-coming 14V line through that normally-closed pin. The current limiting resistors could then go on either side of SW1.
(See second diagram)
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schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thank you for these suggestions. The first design wouldn't work in my particular case because I don't actually have any control over SW1. It's actually a alarm system PGM (Programmable Output) signal that the alarm system control board will control and provide. The second design looks great though. \$\endgroup\$ Oct 4, 2023 at 15:21
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Probably the best way for me would be to add another relay and run it from the 14V switch. Parallel it's NO contacts across SW2. This way everything stays isolated.

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