Variable ON/OFF timer options

[Rocky88]

I'm trying to decide on a best option to control a water pump in my irrigation system. I need to be able to set on time to about 1 minute and off time to about 5 minutes or so. Most cheap timers you get from the shops can only have a few on/off settings or can be set to only 15 minute periods so that's not an option.

I've made a similar circuit using 555 timers and a relay before, but if possible I'd like to avoid using external power supply or batteries and it needs to be super reliable because it needs to be running 24/7 year round without failing.

I've been looking at timer relays such as this: **broken link removed**
If I understand the datasheet correctly it requires no external power supply when connected to the mains and can be set from 0.1s to 100h periods which is good. The only issue is it costs $150. Does anyone have an experience with this type of timers? How well do they work and are they really worth all the extra money when compared to a 555 timer circuit?

Any opinion is appreciated.

 

[panic mode]

it is common general purpose industrial timer like omron H3R series for example. the reason for high cost is marketing practices. if you have an account with any of electrical suppliers, you would expect to spend no more than $40 on items like this. you can get comparable products for much less. it is good idea to consider Omron, because they make this in larger volume and that means lover price. for example

https://www.digikey.com/product-detail/en/H3DS-ML AC24-230/DC24-48/Z2393-ND/819761

 

[alec_t]

it needs to be running 24/7 year round without failing.

In that case it will need to be a type which can be powered by a back-up generator as well as the mains, to allow for mains outages. Check that whatever you propose buying can cope with a supply (possibly modified square-wave) other than mains.

 

[panic mode]

how well do they work?

that is the only thing they need to do andthey do it well. many (not all) have built in universal power supply and can be powered from variety of sources, including mains. many have option to be both dinrail and panel mounted. panel mounted ones are handy where process requires frequent adjustments. this is industrial product and it is meant to be robust. unlike 555 timers, this is ready to use and can work in many modes (timer on, timer off, astable...). there is no search for other components, calculating delays, designing pcb, soldering parts, finding suitable enclosure etc.

 

[Rocky88]

Thanks for the answers. I'm worried about the life expectancy of the omron relay stated in the datasheet. 100,000 operations lifespan wont last long if it's switching approx every 5 minutes

 

[4pyros]

Thanks for the answers. I'm worried about the life expectancy of the omron relay stated in the datasheet. 100,000 operations lifespan wont last long if it's switching approx every 5 minutes

Yes that one has a built in mechanical relay that can fail over time. You need a solid state output model.

 

[panic mode]

then you didn't read the datasheet for the original timer that was $150 because it also uses relay and the datasheet clearly states that it is rated for 10^5 operations. note 10^5=100,000 which is the same life as for Omron.

in the original post the only issue with that one was the high cost so I offered an alternative that is more common and nearly 4x lower cost.

btw 100,000 of operations is common for relays, they all have life expectancy of this order of magnitude. but... this is guaranteed life, not actual life. it tells how many operations relay can do while interrupting rated current (8A for example).

this is what wears down relays the most. mechanical switching is much less significant factor and at lesser loads relay will easily do 10x more cycles.

i work in the industry and see bunch of stacklights (beacons) with 24v 10W incadescant bulbs flashing once every second or so. and that is on equipment that is never powered down - and the relays still tend to work for many years (because 0.5A is much less than rated 5A or 8A...).

now IF your process is so sensitive, why put your trust into one timer? there is something called redundancy.

also you can add monitoring and indicate if one of them is not working. the question is how much you want to spend?

one thing you can do is use so called smart relays (you can use them as timer or simple programmable controller). then you can use more than one output and (using your programming skill) you can alternate them so each of the outputs does only fraction of all cycles. using built in inputs you can also monitor each output and automatically skip the bad one and use only the healthy ones.

so how much you want to complicate...?