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Need help with an auto switch project

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SailingBarry

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Hi, I could use a little help for a project I’m working on. I am looking for a way to switch the flow of 5A DC current from one DC device to another, depending on the presence of 110V AC current (the AC current will be controlled by another switch, which is a given in this scenario). To be clear, I’m not looking for a way to create the various currents mentioned above, only to be able to switch DC current between 2 devices, depending on the absence or presence of AC current (One DC device would only ever run if AC is detected, the other would only ever run when AC is not detected). Any help or suggestions would be greatly appreciated, thanks!
 

SailingBarry

New Member
Thanks! In re-reading my post, I see I misrepresented my situation (sigh) I stated 5 A current, when I really meant 5V (with closer to 1.8 Amps, such as one would expect from a phone charger). My bad. At any rate looking on Amazon for SPDT Relays, I see most are rated at 12V - 30/40 Amps (for automotive as you suggested). Would these be suitable for my situation (sorry for the confusion)?
 

Diver300

Well-Known Member
Most Helpful Member
You can always use relays where the contact rating is higher in voltage and current than what you are trying to switch*.

The coil voltage has to be close to what you are using, so an automotive relay with 12 V coil voltage won't work on 110 V ac. You need a relay with a 110 V ac coil, but 120 V ac would be fine.

The other method is to find a 12 V dc power supply, an old router power supply or similar, that runs from 110 V ac. Connect the output of that to the coil of an automotive relay, and the relay will energise when the 110 V ac is supplied to the power supply.

*(Within reason. If you are trying to switch really small currents or voltages, there are other considerations.)
 

Grossel

Well-Known Member
What kind of current values of 110VAC should you measure ?

Can you use current transformer and rectifier to do the job (regarding ripple currents)?
 

SailingBarry

New Member
Thanks Ian, I would agree, but didn't try to. When I went back to try to edit my original post, I found no evidence it existed... except under my "Member Activity" it had what appeared to be a redacted attempt to post (a few words of the first line, with strike through font). That, plus the fact it was showing 0 posts for me. I figured I must have accidentally deleted it, and therefore posted again. Until you made this comment, I did not know two existed (and for what it's worth, I haven't received any replies to anything other than this one).
 

SailingBarry

New Member
What kind of current values of 110VAC should you measure ?

Can you use current transformer and rectifier to do the job (regarding ripple currents)?

Hi Grossel, I appreciate the feedback, but I'm afraid your level of understanding is way deeper than mine, and I wouldn't even be able to provide valid responses to your questions. I will try to go with something somewhat simple like AnalogKid and Driver300 have suggested. Thanks!
 

Mickster

Well-Known Member
Most Helpful Member
The simplest solution, where you don't have to mess with 110V AC, would be that of Diver's "other method", IMO.
A 12V wall wart is plugged into 110V AC and the output wires are connected to the two terminals which power the relay coil of an automotive relay - 85 and 86.
Terminal 30 supplies 5V.
Without power supplied to the relay, terminal 30 and 87a are connected. Terminal 87 is open-circuit.
When power is supplied to the relay, terminal 30 and 87 are connected. Terminal 87a is open-circuit.

A picture may help:
SailingBarry..png


Regards.
 

ChrisP58

Well-Known Member
Most Helpful Member
While I agree with others that using a separate AC/DC power supply would be the simplest, I see a possible problem.

How quickly do you need the relay to transfer after AC is lost? Most DC power supplies have output capacitors that may keep the relay energized for a few seconds after their AC input power is lost.
 
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