Interface 240v to 5v / clean contact ?

HI All As part of my glass fusing kiln project, I've a need for my Raspberry Pi to 'know' when one of the big, 240v-operated contactors has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power from the 7.5kw heating elements in the event of a fault condition being detected. The safety contactor is in series with the main switching contactor - which, because it's switched quite frequently, is probably more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was able, at the start of a fusing session, to pulse the safety contactor on/off - just so's it knows that, if it wants to, it _can_ drop the contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on the switched contacts of the contactor into somethaing that the RPi likes - either clean-contacts or a 5v logic signal?

Thanks Adrian

Reply to
Adrian Brentnall
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6 - 25 quid depending on number of channels required

See also

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Owain

Reply to
spuorgelgoog

easiest simplest cheapest is a wallwart. But doubling contactor workload will halve its life, very roughly, causing failure.

NT

Reply to
tabbypurr

Thanks - that's useful.... I'd thought of a good old mains-coil relay - which would provide me with clean contacts - but the ebay link is interesting, and looks to be nicely-built... for 7 quid plus postage. Sounds good, thanks. Adrian

Reply to
Adrian Brentnall

Optocoupler I'd say.

mains->cap->bridge rect->LED

5v>resistor->pi input AND phototransistor, phototransistor->gnd.

Reply to
The Natural Philosopher

Yes - a wallwart's an idea.... space is a bit limited, but I guess I could extract a wallwart from its 'shell' and mount the gubbins inside my enclosure....

Not sure quite what you mean about the 'doubling contactor workload'..?

There's two contactors - in the revised design the 'safety' contactor will be pulled in for the duration of each fusing schedule (about 7 hours - and probably no more than 5 schedules per week. The other one, the 'heat' contactor is a bit more stressed - a single schedule can see it cycling on/off 900 times.... so I was planning on making that one a 'replace every 2 years' item - they're only ?20 each, so it's not the end of the world, cost-wise.

What would be embarrassing would be if the 'heat' contactor were to stick 'on' - which is where the monitoring RPi comes in (has its own thermocouple and will drop the 'safety' contactor if it doesn't like what it's seeing. Hence the (slightly paranoid, I'll admit !) aim of checking that the monitoring RPi can turn off the 'safety' contactor, and that the mains actually disappears from the 'safety' contactor's output when it's de-energised...

Anyway - thanks for the idea - that's the background! Adrian

Reply to
Adrian Brentnall

That's certainly a possibility... but I might just have a spare 5v wallwart kicking around somewhere here.....

- which would have a similar effect.

Thanks Adrian

Reply to
Adrian Brentnall

Be aware that the output voltage from your wallwart may take several seconds to decay. Solved with a load resistor.

Cheers

Reply to
Clive Arthur

Thanks for reminding me, Clive! As you say - solved with a load resistor - or even a load resistor and a green 'safety' LED. (You can never have too many indicator leds! ) Thanks Adrian

Reply to
Adrian Brentnall

No volt-free AUX contacts on the contactor I presume?

Reply to
Andy Burns

I think if it's running a kiln, the extra load of a 5V wall-wart isn't going to trouble the contactor :-)

Reply to
Andy Burns

Sadly not! FWIW, the contactor is being driven by a 12v relay, that _does_ have spare clean contracts.... but knowing that the 12v relay has picked/dropped isn't quite the same as knowing that the contactor has done the same.

I'm trying to get to the point where the 'safety' circuit knows that, if it wants to, it's capable of shutting off the main power to the elements.

Thanks Adrian

Reply to
Adrian Brentnall

And if it knows it has turned off the relay, but detects the contactor is still on?

Reply to
Andy Burns

As in 'Never check for an error condition you can't handle...."?

Well - the plan is that _before_ a firing schedule starts, the monitoring RPi will pulse the 'safety' relay - and reassure itself that the mains went 'on' and then 'off'.

No guarantee (of course) that this means it'll actually work in the event of an error, but it does prove that the safety wasn't welded 'on' at the start of the exercise.... which is probably better than nothing...?

Adrian

Reply to
Adrian Brentnall

Easiest of the lot is a simple 240vAC relay. Check the contacts though - don't go for one with simple heavy duty contacts as they will eventually oxidise up on 5v inputs to the Pi - there isn't enough voltage or current to keep them clean. You need at least 24v and 50-100mA really, unless you have serrated or (preferably) silver/gold plated contacts.

Also, remember that you must NEVER depend on software to open that protection contactor if there is a problem. There should be some sort of crude manual or non-resettable over-temp switch on the kiln to look after that. Possibly even something like the melting link drop-wire switches that they install over diesel generators to shut the fuel off in case of a fire. Software can fail to open it as well as fail to close it. You have to assume that all control has died and the kiln has caught fire.

Reply to
mick

Thanks - 'wetting current' I seem to recall.... Just wondering - perhaps a reed relay might do the job.... I've got some of those somewhere - simple capacitive dropper to reduce the mains down to whatever the coil wants - and (unless I've remembered it wrong) the reed is in an evacuated glass envelope so no worries with oxidation? Think that they used reeds in early computers...?

Hmm - well - I'm only trying to produce a system that's as safe as the usual kit that's sold to glass fusers. Example - the commercial controller I'm using has a 'safety' output from the controller board - designed to do the same thing as my safety system. In the commercial kiln it came out of - this output was simply not wired up - so when there was an odd fault on the thermocouple, the kiln continued to heat, and was somewhere around 950c when I caught it! (Normal operating range would be up to 800c)

I've looked at ultra-simple over-temperature safety facilities - and then it dawned on me that I'm already planning to have an RPi doing some other monitoring / logging activities, so it might as well do this function as well. It'll probably involve a very simple fail-safe hardware watchdog as well (including a set of contacts in series with the control to the safety relay) - just to make sure that the RPi hasn't gone walkabout..

If we get to the point of there being actual fire outside the kiln then I've lost the game anyway - as it's installed in a timber shed I've not managed (so far) to find something mechanical that would drop the power at (say) 950c - and the RPi system I'm proposing will be a great improvement over most commercial kilns...

Thanks Adrian

Reply to
Adrian Brentnall

How big is the switch movement. Opto device actually in line with the moving bit so its blocked at either on or off? Brian

Reply to
Brian Gaff

HI Brian

Good idea... but it's a din-rail-mounted contactor - fully enclosed... I think that either the 'gutted' wallwart idea or the 'proper' opto-isolated interface board (from ebay) are probably the best ways to go...

Thanks Adrian

Reply to
Adrian Brentnall

Could you use a changeover contactor, and detect the changeover from L1 to L2? That gives you a safeguard against a broken connection to the sensor, in that either L1 or L2 must be live, and not both.

Owain

Reply to
spuorgelgoog

Probably could.... but I've got the sp/no ones (and a pair of spares!) at the moment...

My thought was along the lines of ... RPi waits for 'start' button to be pressed

RPi checks that 240v isn't present on contactor contacts RPi commands'safety' contactor to close RPi waits a while & then checks that 240v appears on contactor contacts RPi commands 'safety' contactor to open RPi waits a while & then checks that 240v appears on contactor contacts RPi waits a while & then checks that 240v is no longer present on contactor contacts

RPi then commands 'safety' contactor to close - lights an 'all OK' led, and operator presses the 'go' button on the separate kiln controller - secure in the knowledge that the safety monitor RPi is capable of dropping the safety relay if it detects a fault condition. Most likely/hazardous fault condition is main contactor stuck 'on' - which would otherwise lead to kiln overheating, possible damage to kiln and fire!

Adrian

Reply to
Adrian Brentnall

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