Relays and Timers

timer relay combination device

It's truly amazing how often expensive power contactor relays get thrown away because one of many sets of contacts has become intermittent or unable to make contact. More often than not, many of the contacts on that relay have never even been used. Such trash can quickly become treasure if they can be applied to a system where only one or two contacts are needed to switch the "Hot" side(s) on.

For one thing, such devices can be used to create a safety disconnect to prevent damage from power interruptions. If a piece of equipment wis running when a surge occurs, this safety measure can shut it down until manually restarted, to keep the equipment from restarting unmonitored when the power is restored.

Considering that some high power relays sell for a few hundred dollars, they may not be a bad thing to hold on to, even if they're partially disabled. But this is only useful to the experienced electrical do-it-yourselfer. If you plan a project around incorporating power relays, remember that the power MUST be off before you start tinkering.

relay device

Relays 101

Let's begin with the basic relays and how they work. A relay is an electrical switch that lets a low current circuit control an appliance, fan, heater, air conditioner or other device that draws a much large current, requires higher voltage, or both.

For example, when someone starts their car, as soon as the key is turned, 12 VDC is sent to a small square box under the hood and energizes a relay, making the connection between the two lugs on side of the relay. Less than 500 milliamp (or 0.5 Amps) are required to energize the relay, but then, 150 Amps coming straight from the battery is sent flowing through the starter to turn it over.

That tells you right there what has to be known when choosing the right relay or contactor relay—for the example above, the set of contacts has to have the capability of handling a minimum of 200 Amps and at least 12 VDC or more. For the coil rating, the voltage is all that is needed (in this case, the 12 volts from the car battery).

In a different scenario, a contactor relay might be needed to operate a table saw with that needs to draw 18 Amps if wired on 115 VAC or 9 Amps wired on 230 VAC. The motor would consume only half the power if wired on 230 volts, so this would probably be the option of choice, and require a contact rated at 230 VAC minimum with at least 10 Amps capacity, although a larger Amp rating wouldn't matter.

If the motor is wired with a 12/3 cable containing three wires plus the ground wire, a coil rating of either 115 or 230 VAC can be used to activate the relay. However, if it is wired through a 12/2 cable with only two wires (one red, the other black) and the ground, that means all you get at the motor is both of the "Hot" sides, thus providing a single option of 230 VAC for the rating of the coil.

relay device

For the past few years now, since I've been doing repairs on medical equipment running on battery power (most times two 12 Volts batteries hooked up in series) I've come across a lot more relays with a 24 VDC coil rating. 24 volts is not bad since a lot of gadgets we buy nowadays run off power packs or transformers that output that voltage.

Needless to say, I save any power packs, chargers, and step down transformers when disposing of the broken or obsolete equipment they were powering. Then if I need a relay activated by a 24 volts coil, I just use a step-down transformer or a Power Adapter with a current rating matching or exceeding the rating required by the coil in that relay.

If anything else, such as a solenoid valve, or a timer, runs on 24 volts, it can be all wired to that transformer, provided the power rating is sufficient. All those components can then be fixed inside an electric box where they can get properly wired. Again, this is not for the inexperienced DIYer in electrical work. If you are experienced enough to attempt such projects, DO NOT FORGET to turn off the power—the charge can be very dangerous, even lethal. If you're not a certified electrician, consult one before starting.

Various Types of power relays

Using Timer Relays

A time delay relay is another item I've been coming across and finding extremely useful. As the name states, it is primarily a relay. Their main purpose is to control the energizing of some components or equipment, but only at a very specific preset time and for a very precise length of time. They come in four basic modes of contact operation:

1. NOTC - Normally Open / Timed Closed

The coil is not energized - the contact is open.

Energize the coil - the contact stays open until the preset time duration has elapsed.

The contact stays closed as long as the coil remains energized.

De-energize the coil - opens the contact.

2. NCTO - Normally Closed / Timed Open

The coil is not energized - the contact is closed.

Energize the coil- the contact stays closed until the preset time duration has elapsed.

After the preset time delay has elapsed, the contact opens and remains open until the coil is de-energized.

3. NOTO - Normally Open / Timed Open:

The coil is not energized - the contact is open.

Energize the coil - the contact closes.

De-energize the coil - the contact remains closed until the preset time duration has elapsed.

After the preset time delay has elapsed, the contact opens.

4. NCTC - Normally Closed / Timed Close

The coil is not energized - the contact is closed.

Energize the coil - the contact opens.

De-energize the coil / the contact remains open until the preset time duration has elapsed.

After the preset time delay has elapsed, the contact opens.

Double Throw Contacts

Most power relays will have "double throw" sets of contacts, meaning that contact will make the connection with one side of a set of contacts while creating an opened circuit to the other side while inactive, but with the exact opposite occurring when the coil is activated. So if you have a NOTC relay that's double throw with a common contact-making connection with another contact simultaneously, the connected contact would be NCTO, but operating on the same set timed delay.

The same is true for the NOTO and the NCTC relays. Those can be set up to open a circuit, close a circuit, transfer from one circuit to another, all at a precise predetermined time and for a predetermined period.

These basics are just the tip of the iceberg when it comes to DIY electrical tinkering. If you're working on some cool creations, share them with our readers in our projects section!