Simplest electromechanical relay circuit

If the relay coil is rated for the voltage applied to it, there should be no reason for a series resistor. If the relay coil is rated for less than the voltage of your power supply, then choose a series resistor such that the voltage applied to the relay coil is within its specified rating.

That's the simplest circuit _equivalent_ for analysis purposes. The real-world relay winding has resistance that limits to current to its nominal operating current.

The diode across the coil is to limit inductive voltage spikes when the relay is switched _off_, needed to prevent switch pitting or over-voltage with active-device switches. The diode orientation should be such that it _doesn't_ conduct when the relay is activated... it only functions to "capture" the inductive current when that current can't flow thru the switch any longer.

The world has been "scroogled"... all searches result in advertisements dominating... so you have to learn to be clever and exclude certain words to narrow your searches. ...Jim Thompson

Which will usually resemble a camel. ;)

Cheers

Phil Hobbs

Thank you for your prompt, verified correct, answer. Although I half suspected as much, given the length of the coil winding, I was unwilling to gamble with one of my four relays (all of which are needed for my project) to find out.

My bench power supply only goes up to 15 VDC. But, my intuition told me that 15 VDC might just be enough to energize the coil to the point of contact. This time my intuition was correct. 11 VDC is enough to close the contacts, which then exhibit hysteresis and remain closed until the voltage falls to 3.7 VDC.

My intuition also tells me that, for safety's sake, to place the relay contacts into the (white) neutral side of the AC circuit instead of the (black) hot side of the AC circuit.

Chances are, you have a 12V coil.

--^^^^^^^^

Um, no. You want to switch the *hot* side, not the neutral. Ditto with fuse placement, "power switch", etc. Always interrupt the *hot*, not the neutral.

HTH,

--don

Don is correct. *Always* switch the hot leg; never the neutral leg.

A 24 volt relay doesn't need a resistor when driven from a 24 volt supply.

You don't really need a diode, either, if the switch is a mechanical pushbutton. It will reduce arcing and EMI, which may not matter. There is industrial relay logic that's still working, and was built before diodes were invented. Our elevator is like that: relay logic and a water-powered hydraulic ram.

button.

Or you could use a resistor instead of a diode across the coil. This would limit the transient peak voltage to the relay coil current (as it was just before turn off) multiplied by the resistor value plus of course the supply voltage. The disadvantage is the added power dissipation during the relay on time but the technique can be useful if polarity cannot be assured or fo r AC relay coils. For further refinement add a capacitor in series with the resistor, creating a classic RC snubber, then steady state relay on loss i s reduced but the transient is still caught at the instant of turn-off.

Plenty of other solutions exist too.

On a sunny day (Sun, 22 Dec 2013 12:01:38 -0800) it happened John Larkin wrote in :

mm must be AC relais, or a running from a DC generator if before diodes were invented. :-)

The relay case clearly says 24V. It surprises me that a mere 11 VDC energizes it.

Of course! That rule of thumb is second nature whenever I replace wall switches and outlets. AC power always spooks me. Something got lost (the potential) when I downshifted back home to the low voltage electronics domain where things are infinitely safer (for me). :)

Thank you for all of your great advice. It allows to start my project on a sure foot without the nagging doubt of overlooking something basic.

I should qualify that statement. Always switch at least the hot leg. If you have a double-pole relay, switch both hot and neutral.

Thank you for your practical advice. One or more of _The Art of Electronics_, _The ARRL Handbook_, and _The Digital I/O Handbook_ (co-written by Jon Titus if that name rings a bell) says as much. But, none of those good books really get into the elementary aspects of relays.

Yes, it's all 120 volt AC logic.

I see some code violations!

Jamie

It gets load tested and inspected by the State of California every 2 years and it has a valid inspection sticker posted in the cab.

It's noisy and a little cranky, so we were thinking of having it converted to a modern oil piston system. They would fab a custom cylinder assembly and drop it inside the old low-pressure water cylinder. But the building is zoned for 320 feet, a 40 story tower, so we might just sell it first, let someone turn it into a quarter billion dollars worth of apartments or condos.

SF is swarming with cranes and cocktail bars and Google busses, the Fourth Gold Rush.

They are trying to put this up a block away:

and people are complaining that it will make San Francisco look like Dubai. But on the other hand, hardly anyone would miss the All Star Cafe.

Why are you re-engineering what has already been engineered ?

OK, I take it your new to electronics.

If you want to question the engineering of parts that have been on the market for years, get an engineering degree and challenge those existing products with your own designs.

IF your designs are better, you will take over the market.

It is good to verify those parts that may be questionable, but a relay has been on the market for over 120 years.

You'd think they would have got it right by now.

On the other hand, people how don't read the specifications will have problems. Or those that do not have a reasonable engineering understanding, will also have problems. ( does not necessary mean a degree, just a good/reasonable understanding )

Good Luck on your future projects.

hamilton

This is where a basic/reasonable understand of electronic components would be nice.

Then you would not have to ask such a simple question.

A relay do not just have contact parameters. ( Contact Voltage/Current )

There are also timing parameters.

The speed of contact closure is a parameter that is based on the voltage of the coil.

Think old time relay dialing circuits.

hamilton

I just saw my first up-close-and-personal google van... mapping our new street... co-o-o-ol >:-}

It's about time... I had to get nasty with both local UPS and FedEx dispatchers who didn't have the brains enough to call before they sent back Amazon orders due to "non-existent address" ;-) ...Jim Thompson

• One CANNOT "burnup" a relay unless one applies at least twice voltage rating for a goodly period of time. 24V across the coil of a 24V relay can be applied "forever".