Anyone know if there's a suitable mains timer that'll do this?
When the (mains) power is applied, a timer starts and closes contacts to a 15A AC mains resistive load. Timer switches off after (user settable) time up to 2 hours. If mains fails, then when it comes back on, the timer starts over again with it's timing cycle. ie the timer ensures an (up to) 2 hr ON time after a mains failure.
Additionally, a user-start-button available so the timer cycle may be started at any time (provided mains power is available of course)
Yes I could make something with a 555 chip and a battery and a box and some R's and C a mains relay etc etc but I just wondered if there's some beefy timer-thing avaiable somewhere off the shelf I can just use. Accuracy is not an issue +/- 15 mins would be ok . It's just for some garage heaters I'm "experimenting" with.
ps Last week I finally scrapped a metal box and the circuit within that I made
20 years ago to bring an "outside" light on for a couple of minutes. It was (I think) a unijunction transistor + R/C driving (eventually) the single phase coil on a 3 ph contactor! A very satifying "thunk" when it came on and off. It worked without fail for all these years. Now, though I can buy one for £4.99 from B&Q. Ahh nostaglic about a security light - how anorak is that? Any other anorak stories out there (thus generating an OT thread)
It's just the timer that's the "problem" - not where it is. Perhaps I didn't describe it very well in OP.
I wasn't sure if they were sure to startup on power up - although admittedly all the built-in ones on ext lighting I've tried have. Good idea - I'll check that out.
However. They are very prone to failing shorted, not to mention that some will, if the power fails twice in quick succession, they may turn on permenantly.
I can vouch for the siemens logo, as I have used a on few projects at work. Maybe a little ott for this application, but a din rail mounted timer can set you back £30!. Download the software and have a play with it. You can design and simulate the software before even spending any money!
Yes, I did download the software and play with it a bit. The documentation is quite well written as well, with worked examples as explanation.
I was prompted to look at this for much the same reason. This was wanting to have something to control various lighting, heating, extraction, battery charginf,..... systems in my workshop, I realised that this was going to result in quite a lot of timers, relays, etc. to do it which seemed to be becoming quite expensive as well as inflexible.
All of the various logic controllers seem to have +/- quite similar functionality in terms of the logic blocks etc., and using a PC, it would appear that it's pretty quick to prototype and test befoe implementing.
A few questions though:
- What sort of projects did you do and were there any things that you wished had been present in the Logo! that weren't there?
- Did you find the amount of storage and the number of logic blocks (130) enough? Did you need to optimise and take stuff out to fit the programs in?
- Did you know if it's possible or did you use any of the external interfaces at run time? By this I mean use of the serial, EIB or AS buses to connect to another system. Here I'm thinking about if I wanted to connect to an external computer etc. for logging or other control... It seems that the PC serial interface is predominantly for downloading progrsms to the Logo! as opposed to run time use. I'm not that familiar with EIB and AS buses.
The lovely U6047B chip really designed for things like car rear window demisting was ideal for this. Could be toggled on and off or made to start at power up, etc. And could also drive the usual car 30 amp relay directly. Easily set from a few seconds to many hours via a couple of external components and very immune to PS problems. Its main virtue over a
555 is that you can easily set a slight delay at power up to allow everything else to stabilise and to prevent random triggering.
I've used them for a pump over-run on my CH system and also for a Ni-Cad charging station to give the required 14 hour charge for cordless tools which I've modified to not cook the batteries.
At the moment I'm going full circle to the 555 idea. I think just a 555 monostable triggered and isolated from the mains by small relay (series HV cap to get it to pulse) - contacts to trigger the mono. They O/P to another relay (via a 1A power transistor) to close the 15A contacts. The user input button is then easy too (and safe as is on the relay isolated side of things). Clumsy. Yes probably, but safe, inexpensive - and I think it may even work!
555's are not good for long delays. They stretch to 10's of minutes with the low current types, but beyond that, the charging current becomes small compared to the various leakage currents. You might do better with an oscillator feeding a counter or divider. A 9 stage divider fed with 50Hz (derived from mains) would get you a delay of 174 minutes. Tap off at stage 8 and you get half this, at stage 7 a quarter, etc.
555 is not especially reliable at 2 hour times. You want something like the 74hct4060 (there are probably more suitable chips).
It's a 14 bit divider, and oscillator, in one chip. Add a capacitor from VCC to the MR pin, and a resistor to ground (say
10K/10uF) to reset it when power comes up.
You set the RC network so that it oscillates at about 0.5Hz, then 7200 seconds later bit 14 goes high, conducts through the diode to RS to short out the clock generator at high.
From bit 14, you run a resistor/NPN transistor to the base of a 5V relay. (with appropriate snubber to stop it frying)
Total parts cost - apart from the relay, maybe 1.50.
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