I am developing a home automation project in which I'm using relays to control appliances. I need to control devices with 220V and 6A rating.
Should I use relays to control these appliances as a long term solution?
Relay I'm using is mechanical and is rated 220V 7A. If I keep it ON to control e.g. a fan, for more than a few hours on a daily basis, will the relay cause any problems? If yes then, what are other possible solutions?
Relays tend to be quite reliable in benign environments, however they have a limited lifetime. Typically something like 50,000-100,000 operations at full rated load. At lighter loads, the life will increase, generally up to many millions of operations with a negligible load (the so-called mechanical life).
All this information will be clearly given in any decent datasheet. The markings on the relay are only limits for safety agencies and have little to do with the relay life.
Not all datasheets show the life vs. switched current, even for resistive loads, so you may have to test samples to determine that characteristic if you are say, using a 30A relay to switch 5A maximum. Inductive loads, incandescent lamps, and motor loads will also shorten the life.
Solid-state alternatives to relays have no easily defined wear-out mechanism, however they can easily die suddenly due to voltage surges, current surges (including momentary shorts) and from thermal cycling. They are also less resistant to heat, and tend to create a lot of it (a ballpark number is 1W per ampere of load current).
Most remotely switched outlets and similar consumer devices (where the consumer can plug anything into them) use relays. If the load is relatively light and well defined (perhaps a lamp) then solid state may be a superior solution.
This is an old post but I am a controls engineer who programs industrial machines so I have 2 cents. I have a machine that does 20,000 cycles a day and I must use solid state relays even if they will fail in a short circuit situation and need to be replaced.
A general rule of thumb for me in my undergrad education many years ago is that if it turns on and off more than 100 times a day make it solid state. Typically mechanical relays are cheaper so I choose them by default.
You must also account for failure of the solid state relay when it has a short to add to that rule of thumb.
Also, someone said mechanical relays live to be 50,000 to 100,000... This is wrong I am pretty sure most of the datasheets I have seen with my mechanical relays are 500,000 to 1,000,000. The relays I use now are 2x10^7 in lifecycles so I typically say 1 million life cycles but it depends on what you buy. Mine are low end industrial relays.
My company uses relays in our HVAC products for several reasons.
1) They are reliable. Based on past and current experience, I expect them to last decades.
2) Relays tend to be significantly less expensive than triacs and their drivers.
3) They waste less energy as heat than solid-state devices such as triacs.
This is important for several reasons:
1) getting rid of excess heat is expensive.
2) energy wasted in the controls degrades the allowable 'green' energy rating that the unit has.
In general, I've had far more triac failures than relay failures.
We use quality relays from reputable manufacturers.
Normally the relay will be specified by number of contact operations. Apart from mechanical wear the contacts may become prematurely worn due to the nature of load (inductive, capacitive or resistive) that is being switched.
The mean time to failure (MTF) of most devices follows a 'bath tub' function - high failure rate at the beginning of life (due to faulty manufacture, poor assembly etc.), then a period of low failure and then a rising failure rate due to wear, heat fatigue and so forth. {see http://en.wikipedia.org/wiki/Bathtub_curve }
Failure of electronic devices tends to be sudden death (it works, then it doesn't). Relays tend to get a bit 'sticky' before failure (a temporary cure being a 'tap' in the right place.
Its one of those questions along the line of "How long is a piece of string?" so there isn't a precise answer yes or no. All I can say is that relays have been and are used quite successfully in equipment over a number of years.
Advice overall is generally good.
Relays are 'hard to beat' if proper attention is given to ratings.
Note that resistive versus reactive (L or C) loads make a major difference and manufacturers specs must be carefully noted.
Note also (not applicable in this case) that DC is very demanding compared to AC. Manufacturers specify DC voltage ratings that are much lower than for AC.
Do take good note of Dwayne's comment re using quality parts from a known reputable manufacturer. In a serious application you MUST use a product of known quality. Unknown brands and equipment whose "provenance" is uncertain (ie may be fakes or out of spec parts) MUST NOT be used.
Specifics: A 7A rated relay at 6A load is probably OK, especially at low switching rates, but if possible I would use a higher current rated relay and/or look carefully at the specifications. When they say it is 7A rated, do they specify resistive or inductive load or other conditions?
