newbie 12vdc control 110vac

I'll say it up front: I'm a software guy, and I might be over my head.

The goal:
For my deaf son, create an alarm clock (with snooze) that turns on a table fan (which, aimed at his face, wakes him up without disturbing his roommates, unlike the vibrating alarm clocks on the market). I've currently got the mechanics in place, controlled by a standard digital timer switch from HomeDepot. It works, but it's cumbersome (tedious to program/change, and there's no snooze).

On his vibrating alarm clock output to the vibrator is labeled 12v dc, 0.5 A. This currently goes to the vibrating disk. I'd like instead have that output trigger a relay to switch on the fans, N.O. so that when snoozed, the fans go off. Just like the vibrator.

So I think I need a relay with a 12vdc control that won't draw more than .5A, and will operate a N.O. switch on a 110vac 15A circuit.

I've googled a lot, and searched through this forum, and have the following questions:
1. Am I high? Missing something? Is this a feasible application?
2. Where, specifically, are the risks (e.g., presumably I need to make sure I don't try to draw more than 0.5 amps for my relay control) and how are they best mitigated (e.g. do I want a 500mA fuse between the alarm clock and the relay?)
3. Assuming I can buy a relay that meets this need, how would be sure I'm getting what I need (I'm not sure which specs are important, specifically how to make sure it won't draw more than 500mA).

Obviously, I don't have a lot of experience (oh, really? :-D), and suspect I've gotten several things wrong above. I'm looking for folks who know what they're doing to set me straight (or tell me to give it up).

Any help much appreciated!

12V @ 0.5A means a max load of 12 x .5 = 6W so if the relay coil (solenoid) consumes less tan that you should be fine.

Relays coils are sometimes rated by power or current, but usually resistance. From V=IR (Ohms Law), R=V/i 12/0.5 = 24 Ohm, hence a coil resistance of AT LEAST 24 Ohms should be fine.

Many automotive 12V relays are ~60 Ohms upwards (ie, under 250mA) but you must use a relay approved for 110VAC use.
[ I'm was going to say that is your greatest risk - using unapproved parts or performing unauthorised works. Many things like that are Regulated here. (Then again, we have scrapped mandatory pre-approvals so now we act & sue after death has occurred!). ]


I'd suggest a spike protection diode across the relay coil or clock output - eg, a 1N4004 or 1N4007 diode with Cathode/Line towards +12V (or relay 86) and the other end to 0V (relay #85). That's to protect the clock from the high voltage spike when the coil de-energises.


The relay's AC rating needs to exceed or suit your local AC voltage and contacts be rated for inductive/motor loads else be preferable at least twice the fan's current rating.


Just make sure wiring is approved and safe.


You could add a fuse but it may not protect the clock. Maybe no larger than a 250mA fuse (suggesting a relay coil of 200mA or less) because even that might blow after the clock has blown. It all depends on the clock's circuitry.
Or a current limiting circuit (LM317 voltage regulator and a 2.7Ohm 1W or 3.3Ohm 1/2W resistor).
But I'd probably risk it unfused/unprotected unless the clock is expensive.

As to being high, probably - you are a programmer.

Hi,
How much current does the fan draw? You may want to look into a solid state relay. Shouldn't load the circuitry in the clock radio. The relay could be mounted in an electrical box for heat dissipation, outlet on the box. Pigtail cord to plug into the radio.
Mark

Making sure it's approved - ie, it has the isolation between 12VDC and the AC supply.

RIBU1C

https://www.supplyhouse.com/Rheem-RIBU1C-Enclosed-SPDT-Relay?gclid=CKKKi9PExsACFZCUfgodLYcALA

This relay can take a 12v to trigger an ac circuit.

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Gary Sather

Wow. This is great stuff, thx!

- Ween, there are two 6" fans drawing 0.28amps each (about 34 Watts each at 120V). Could you clarify why a solid state relay might be preferable?
     
- I assume this from Ween: "Shouldn't load the circuitry in the clock radio" and this from OldSpark: "isolation between 12VDC and the AC supply" are saying the same thing. Sorta seems obvious, but since you both mention it, I guess some folks are selling relays that don't isolate the circuits... In any case, if I see stuff like "UL Listed", "CE", etc, can I assume I'm safe?

- Regarding the RIBU1C (thanks, tonanzith), I found this, which seems to indicate it's just what I need. Anybody think of a reason this isn't the way to go?
     
Thanks!

So a relay with 120VAC rating of at least 1A should be fine.

The "loading" and "isolation" are not the same.
Loading refers to the LOAD - ie, its current/power draw.
Isolation refers to "galvanic" isolation - ie, there is no electrical connection between the input and output.
Tho many 12V relays maybe have isolation, they may not be sufficient nor rated for 120VAC. Hence under certain circumstances (dust, moisture, failure) 120VAC may feed into the input side - ie, 12V DC or the operator.
The latter tends to involve jail terms even if not death.


As to your linked relay, provided it meets your local regulations etc and the unused NC output is properly isolated and all work/wiring carried out IAC local regs it seems ok.
I prefer such relays to have AC wiring totally separate from the DC/coil in case there is any insulation breakdown.

Ah.

So I wasn't thinking the terms meant the same thing... I just thought Ween's "Shouldn't load the circuitry in the clock radio" was also a warning about making sure what you describe above doesn't happen, ie: "120VAC may feed into the input side - ie, 12V DC". If that's not what Ween meant... then I guess I don't understand what "Shouldn't load the circuitry in the clock radio" is referring to..

Thanks!

It's a bit ambiguous, else maybe confusing?

Certainly non-isolation between the clock (its 12V output) and the AC supply may well cause what some of us refer to as a catastrophic failure (ha ha). It could be considered a loading as in overload - ie, the clock output could not handle the 60VAC or 120VAC voltage it saw.

But that IMO is tied up by the safety requirements - ie, an approved DC relay to switching mains/wall/domestic AC.


I - and I expect we - would mean loading wrt not overloading the clock output - ie, keeping its avarage and maybe peak current below 0.5A @ 12VDC.


Keep in mind that the relay's output voltage and current/power has nothing to do with its input. The 2 are usually quite independent and are only related or connected via external circuitry. (3 pin horn etc relays being an exception, and that there is a limit as to the maximum voltage separation of solenoid & contacts - maybe 500V, 4kV, etc.

oldspark wrote:

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I - and I expect we - would mean loading wrt not overloading the clock output - ie, keeping its avarage and maybe peak current below 0.5A @ 12VDC.

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Oh. Duh. Of course.

Thanks again for all your help.