Couple of Basic Relay Questions

[Ian MacGregor]

Hey Guys,
I'm using some relays to add a hard bypass to a new project. The box is stereo and balanced, so I need 4 relays to do this. My first question is:

Can I use 24V relays and use the +12V and -12V rails as the supply voltage for the relay?

Second question: Do I need a diode for each relay, or just one since the coils will all be in parallel anyway? I'm thinking I can just one, I just need to make sure it can handle the operating current of all four relays, right?

Thanks for the help fellas!

Ian

 

[Ethan]

Most relays can handle a pretty decent swing of about 3V+/-, but 12 to 24 is pushing it. IF the relay works at all, you will most likely have switching noise or constant buzzzzzzz.

You can use one diode, a 1n400X will do fine. It would also be good measure to use a 0.1uF cap in parallel with the diode.

Chae gave me some really cool, compact relay bypass boards. I'll see if he wouldn't mind posting the layout.

 

[PRR]

I don't understand Ethan's remark "Most relays... about 5V". Relays are available in many specs. 24V is a perfectly common spec for coil voltage.

Yes, this can come from a +/-12V supply, the relay does not care since it has no ground connection.

Note however that some +/- regulators can "get stuck" with a large ungrounded load. If the + regulator comes up before the - regulator, the - regulator's output pin is pulled positive, which isn't good.

If your wall-voltage is pretty constant, I would be inclined to go to the raw +/-20V power feeding the regulators to get relay power. Relays do not need great regulation. This avoids strain on the regulators, and isolates relay coil surges from the clean +/-12V rails. You will need to figure a dropping resistor to keep the coil voltage in the 20 to 27V range.

I assume you will use a mechanical switch to engage the relays. If you use a transistor, it may be awkward to reference it to the rails.

If you use a switch, there is no great need for any diode. The diode is there because when you turn the relay OFF, the voltage across the coil spikes VERY high, maybe 200V for a 24V coil. This will punch holes in transistors. But 240V-rated switches won't mind.

If you do need a diode, IF all four coils are in parallel, you use just one diode. Its current rating should be higher than the relay coil current (or sum of currents for parallel relays). Since you can trip a lot of contacts with 24V 100mA, I'm sure a 100V 1A diode is plenty.

 

[Ethan]

[quote author="PRR"]I don't understand Ethan's remark "Most relays... about 5V".[/quote]

I thought he was asking whether he could power a 24V relay with 12V? In which case, usually a 3V(the 5 stated above is a typo) difference more or less is OK. But I don't think a 24V relay will work with 12V without problems.

Did I misunderstand the question?

 

[Ian MacGregor]

Ethan,
I was asking if I could use a 24V relay with +12V on one end of the coil and -12V on the other end (so 24V total across the relay). I think PRR pretty much answered all my questions! Thanks a gain for the help...

Ian

 

[Ethan]

Ah, OK

Looking at your post now, I'm not sure how I misread that...somehow I thought it read 12V difference instead of 12+/-.

Too much blood in my caffeine system.