Put my trust in relay? - Page 1
Forum section: electronics / general

author: Lotus
author_rank: Contributor
timestamp: March 10, 2013, 12:25:17 am
content: Hello homiesMy digital temperature controller is switching my heating element on/off with an internal relay,The heating element is operating near the max tolerance of the relay specs. (Watts)The relay has failed me one time, but showed no sign of damage, and it works again.Can relays be tricky and fail when being pushed to hard - and then start to work again as if nothing ever happened?

author: c4757p
author_rank: Super Contributor
timestamp: March 10, 2013, 12:51:35 am
content: Yes. Interrupting the current can cause brief arcs which oxidize the contacts, then when the contacts smack together a few times they clean themselves off. Not usually a good idea to run power stuff close to its maximum ratings.

author: tylerl
author_rank: Contributor
timestamp: March 10, 2013, 01:04:23 am
content: Remember that rule-of-thumb about choosing a product rated for twice your expected load? Yep, now you know why. The product may work, but you should replace it with one rated for higher capacity. Running it consistently at its rated limit could mean:It is significantly more likely to failYou're decreasing your product's overall lifespanYou are less capable of handling temporary excursions outside your typical rangeYou may be creating a fire hazard

author: SteveyG
author_rank: Unknown
timestamp: March 10, 2013, 01:10:25 am
content: Quote from: tylerl on March 10, 2013, 01:04:23 amSorry but no, if a component is specified for use up to a certain rating it is guaranteed to work safely at that rating with a MTBF as specified in the datasheet. If you have a 16A relay and you need to switch a 16A load then it's not going to be creating a fire hazard or anything else. Definitely no need to choose a device rated for TWICE it's required rating. That's just throwing away money and wouldn't get you far in the real world.

author: c4757p
author_rank: Super Contributor
timestamp: March 10, 2013, 01:16:40 am
content: No, if 16A is the maximum, this means you must never exceed this ever, and if you are running "at 16A", I promise you are occasionally exceeding it. Double might be a bit much, but I would never run 16A through a 16A relay.

author: SteveyG
author_rank: Unknown
timestamp: March 10, 2013, 01:22:14 am
content: Read the datasheet. If it says 16A then it's suitable for use at 16A otherwise it wouldn't say this as it's typical rating.

author: SteveyG
author_rank: Unknown
timestamp: March 10, 2013, 01:29:10 am
content: No need to de-rate. They even give you the endurance at the relay's specified current:

author: smashedProton
author_rank: Frequent Contributor
timestamp: March 10, 2013, 05:01:01 am
content: you can put a really low value film cap in parallel with the relay. That should keep the contacts from arcing.

author: SeanB
author_rank: Super Contributor
timestamp: March 10, 2013, 05:40:59 am
content: Normally a 10nF class Y capacitor in series with a 10R 1W resistor works well. The class Y capacitor is designed to fail open circuit so it will not allow power through to the load side if it fails.With relays the mechanical life is very long, but the electrical life is a lot shorter at full current. Derating will make it last longer, and if the load is even slightly inductive you must use the inductive load rating of the relay, which is generally much lower. The 16A relay shown above has a life of 100 thousand cycles at full power, and at low power through the contacts it will typically do 1 million cycles.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 10, 2013, 06:16:51 am
content: Use a SSR or thyristor/triac instead of or in addition to a mechanical relay and you'll get longer life and better temperature regulation. (It makes the most sense when your thermal time constants are relatively short.) It's also possible to do near zero voltage switching of the relay although it would take some characterization work.A snubber won't do much unless your load is significantly inductive.

author: ignator
author_rank: Regular Contributor
timestamp: March 10, 2013, 06:54:58 am
content: Quote from: Lotus on March 10, 2013, 12:25:17 amWhat is worst case scenario if the relay sticks ON?Will it cook fish in an aquarium? Or burn your building down? A single channel device like this with electronics would generally be given a failure rate of 1 per 1000 hours. Unless you can compute a better number based on FMEA, and known failure data of the components. Clearly you can design a single channel device for much better then that.If you want fail safe (probability of 1 failure per million hours, assuming 10e-3 failures/hour for each channel), then you need another isolated relay contact on another controller that acts as a "ARM" monitor shut down. Where the existing thermostat controller is the "CMD".This in fail safe avionics world, is a ARM and CMD redundancy system.I'm using one of the STC-1000 thermostat off eBay to control SSR to a 3000 watt overhead infrared quartz heater in my shop. I have an inaccurate bi-metal mechanical thermostat in series with the SSR. I don't trust the SSR or the STC-1000 to not shut off when expected. As well the antique Honeywell thermostat. I am trusting that both will not fail at the same time. A large surge like lightning could cause all series switches to weld close.

author: Lotus
author_rank: Contributor
timestamp: March 10, 2013, 08:18:16 am
content: The worst case scenario , which actually did happen, was that my eggs in the incubator got 50C degrees and died.There is no real fire hazard because im running the heating element at a low setting....well unless that fail too, then my house burns down.I did not consider another safety-relay, so now i will use an overrated relay, and have a similar relay as safety-relay as ignator said.thanks

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 10, 2013, 09:49:03 am
content: Quote from: SteveyG on March 10, 2013, 01:10:25 amIn almost all cases, under-running a component will reduce the probability of failure.In "The real world", the cost of replacing a failed component, or the consequences of its failure will be many times the cost of that component, so over-speccing will often be a good investment in the long run.Also bear in mind that some manufacturers can be a little "optimistic" with their specs to make them look competitive with others (who may also be being optimistic), or have certain caveats on the spec which may be hard to guarantee avoiding in practice (e.g. peak inrush current spikes, mains transients).Unless a design is very cost-sensitive, over-speccing parts, especially those with known wear-out mechanisms is generally a good practice.One minor point with relays, is that some contact materials are designed for a minimum current, to guarantee that oxide layers will be broken, so extreme under-speccing (e.g. bypassing with an SSR) can potentially cause problems.

author: ddavidebor
author_rank: Super Contributor
timestamp: March 10, 2013, 10:35:47 am
content: Use a triac...