Power wiring/crimp advice

[Adrian Brentnall]

HI Folks

This photo

shows the switch-box that powers my home-built glass-fusing kiln (26 Amps @ 240v).

(yes - I know the wiring's terribly untidy - but it works!)

The commercial digital controller outputs 12v, which is used to switch the small relays on the left, which, in turn, switch mains to the coils of the contactors. There's two of everything - because the long-term plan was to use one contactor as a safety relay, and the other as the actual 'control' relay

- which is turned on & off by the controller, to regulate the temperature in the kiln.

Power comes in on a 6mm2 cable top centre, and feeds to the heating elements in the kiln via the two flexible cables top right.

I used the 40A contacts mostly because I had them to hand. They last about 18 months in normal operation (somewhere around 200,000 switching cycles) before the contacts fail. (As evidenced by the sooty dust in the bottom of the enclosure!)

So - I'm looking to replace the main switching contactor with something more suitable

- options are

- Potter & Brumfield 40A relay T92P11D22-12 (used in many commercial kilns)

- Crydom 50A SSR - D2450

Either option would allow me to dump the two interface relays, as the P&B relay and the SSR will switch on a 12v signal.

The beauty of the contactors is that they have screw-clamp terminals which makes the wiring simple. The P&B relay uses 1/4" spades, the Crydom uses a screw/washer arrangement.

Are there any issues with using crimp connectors (either 1/4" receptacles or ring-terminals) with solid core cables? - or would I be better obtaining some heavy-duty multi-stranded cable... I have a good ratchet crimp-tool.

I would probably terminate the incoming cable on a Klippon terminal, and then run additional wiring to the relays - or I could retain one of the contactors as a safety relay, (which only switches once per firing rather than 750 times like the control relay) and run flexible leads from the bottom of that contactor.

Also - for anybody experienced in using SSR's - is this 50A SSR going to want significant heatsinking, when handling 26A or so? If so - what?

Thanks Adrian

[T i m]

<snip>

No real use to you as I've only used (or tried to use) one on the heated bed of my 3D printer and it draws around 10A at 12V (DC).

I think we used a 20A (or bigger) rated SSR on PWM and it got *very* hot, very quickly (no h/s) so we left it with the relay on bang-bang.

Coincidentally it has been printing most the evening (and fairly frequently) over the last ~5+ years and it's still going strong. ;-)

I think we used a 25A DC rated vehicle relay FWIW on (guess) a 50:50 duty cycle, switching every 20 seconds or so?

I think AC switching may be a different ball game (re SSR's)?

Cheers, T i m

[Adrian Brentnall]

Thanks Tim There's a part of me that rather likes to hear that relay going click-click while the kiln's firing - which you wouldn't get with a solid-state relay, of course..

Maybe the answer is 'better relays' - the contactors are somewhat agricultural, and probably aren't meant for frequent on/off cycles.. They're also unbranded - so probably Chinese Copies..

Was talking to a kiln manufacturer the other day and their latest 'Big Idea' is to run a small SSR and a conventional relay in parallel - the idea being that the SSR turns on, with its zero-voltage switching, and a short time later the mechanical relay also pulls in, at which point the ssr drops out. Reverse the sequence when turn-off is required.

Theory was that the relay only has to hold the current, rather than switch it, and the ssr is only on for one or two seconds, so it doesn't get a chance to overheat..

Sounded a bit over-complex to me!

Maybe I'll try the 'better relay' solution first.... Thanks

[The Natural Philosopher]

Unless you used it to drive a mechanical relay as well as the kiln...

Seriously, solid state is way more reliable than a relay. especially if select up for reliability and not down to a price.

[Adrian Brentnall]

Ha! - I had toyed with the idea of adding some kind of 'sounder' to replace the click!

Yes - that's why I was looking at the 50A SSR... The only thing I'm not sure about is what/how much heat-sinking it's going to want...

[John Rumm]

No problem with solid core wire, so long as the crimp is well made.

Yes you want a heatsink. Crydom make one designed for their SSRs:

[Harry Bloomfield, Esq.]

Dissipation will be minimal, but check the manufacturers spec for the required heat sinking. I assume you will be mounting it away from any kiln heat?

[T i m]

<snip>

Sorry it wasn't any use ...

Agreed, some audio feedback that it's actually working. If you leave the filament loaded in the printer the bit outside the extruder and toward the roll goes brittle very quickly (even overnight) for some as-yet unknown reason? So what I do when I've finished printing that day / colour is cut the (PLA, 1.75mm OD) filament off a bout 30mm above the extruder and put the roll back in it's plastic bag and in it's box on the shelf (I have maybe 10 rolls). So, when starting the next print I go into the printers own on-screen display (well, it's actually an Arduino Mega <g>) and set it to 'Preheat PLA, Extruder and bed' and (subconsciously now) listen out for the 'click' of the bed heater relay, confirming I have indeed selected the preheat routine.

Once the extruder is up to 200 DegC (that's cold to you! <g>), I press the spring loaded filament drive lever and pull the remains of the filament out and thread the end of the new filament in.

I have often gone to walk away, not heard the click ... and gone back and done it again (rotary encoder 'step' rate not quite right).

I used something like this:

12vDC driving the input switching 12VDC on the output.

I can't say we wired it 100% correctly other than it did 'work' but I'm not sure we were 100% sure from the markings on ours where the relay should sit in the circuit (or if it mattered). The output trigger for the heated bed of the 3D printer was pulled high (12V) on active so we connected that to the input +Ve pin on the SSR and I think we connected the output -Ve to PSU -Ve and SSR +Ve to the heated bed (and then to +12V on the PSU). Being isolated between input and output, I'm not sure it matters what 'leg' it's in, but that the polarity is right?

Maybe.

That makes loads of sense (from my experience of 'pushing' semiconductors when switching heavy loads).

I designed zero-crossing into all the light units I designed and built for my mobile disco but in most cases the switching devices were handling far less power than they were rated for and in 8 years regular use I never lost a single one. The lamps seemed to last well as well, considering they were flashing most of the time, possibly helped by the zero crossing.

Yeah, good.

I think it might be one of those things that when you have it installed, you wonder why you didn't do it sooner? ;-)

I think the trick might be to find something that is *well* overrated for the job. I think I also understand that if you want to uprate things like relays, you join the contacts in series? The idea that the contacts open faster (as in how quickly the gap forms) and so how much time the contacts may be exposed to any back EMF?

But if the kiln people are pushing the SSR / relay combo I'm guessing they have had good results from that?

In fact, if you go back 45 years when we were racing RC electric cars in Ally Pally Palm Court Hall (normally £1000 / night to hire but we knew someone .. <g>) a mate and I designed a hybrid speed controller that used an 2N3033 power transistor driven by a pot (and some other electronics) connected to a servo that then shorted the transistor out with a micro switch when the servo was on full speed (to minimise voltage drop). ;-)

I don't know if this would be any use:

Cheers, T i m

[Dave Liquorice]

Well is 30 odd W is minimal...

Spec has forward voltage drop of 1.3 V at full rated current. The forward drop won't be much less at 26 A so erring on the safe side

1.3 x 26 = 33.8 W.

The spec doesn't give a max case or junction operating temperature only the thermal resitance junction to case of 0.45 C/W so the junction at 26 A will be about 15 C above the case. Say 60 C max for junction case is at 45 C ambient 25 C. Does that give a heatsink thermal resitance of 0.6 C/W?

The makers sell a heatsink which one would assume is up to the job at full rather than half the rated current.

I quite like the idea of the SSR doing the actual switching with a relay/contactor shorting it out after the SSR has closed and opening before the SSR opens. Even though the current being switched by the relay contacts is still 26 A it's only at the forward voltage drop of the SSR not full mains, much less arcing...

[Adrian Brentnall]

Not at all - all inputs are welcome!

<snip>

Perhaps you're right. Their justification seemed to be that it would save wear on the relay, and that they didn't have enough room in their controller to fit a heatsink to the SSR. Just sounded to me a bit like fitting two components where one would do....

The heating elment is pretty-much a resistive load - so should'nt be any back-emf to speak of - just the initial 'oomph' from switching 25A or so at some random point in the mains cycle - about 1,000-times per firing schedule.

So far - apparently... but early days.

Thanks - I'll take a read.

The SSRs are often touted by the kiln-manufactures as being 'fail-safe'

- but the manufacturers warn that the SSRs can fail 'on' as well as 'off' - hence the need for a 'safety' relay that can kill power to the kiln should the main switching device fail 'on'

Thanks A

[Adrian Brentnall]

Thanks for that - I did do the maths but it's reassuring that you cam up with a similar answer. It's a small enclosure - but I could pop a small fan in there to vent any hot air.

I think I was looking on the cpc site - couldn't find a heatsink there (but the search facility is dire) I've a box-full of handy 'spare' heatsinks upstairs - it'd be a matter of finding something that would fit inside the case..

Hmm - I see what you're saying. Just the Luddite in me doesn't like fitting extra components that increase the 'failure' potential of the circuit.

The big worry in this application is that a relay that fails 'on' will result in a runaway kiln. This can be countered to some extent by running a 'safety' relay in series - controlled by the main controller. If the main relay fails 'on' then the controller will error out and drop the 'safety' relay, thus isolating the heating element.

An alternative plan is an independent hardware circuit that monitors kiln temperature and drops the 'safety' relay should it rise above a preset temperature. There's a single IC that takes a thermocouple input and does all the necessary...

All interesting info - thanks.

[T i m]

<snip>

It sounds like it would, where the 'wear and tear' is typically to the contacts, rather than the general mechanics? I check because you mention the '1000 times per cycle' and that is a far few times.

If I guessed my relay operates an on-off cycle every 30 seconds and let's say the average print job lasts 1 hour (can be seconds or the best part of a day) then that's 7200 times. As mentioned, it is only a basic (it might have been a Lucas or some such) car type relay and I feel it will carry on for ever. ;-)

Is their box metal and could you bolt it to that on the inside with a heatsink on the outside (or wouldn't that help if it's near the furnace)? Even a length of Ally angle cut to length and bolted to the (presumed metal) case (with maybe a bit of thermal compound) would act like a heat pipe?

Well, in this day and age you would think one should do but it seems that it can't always, or at least not without other 'costs' (like excess heat). ;-(

<snip>

I think that's the thing that might make quite a difference, especially in making the circuit (the SSR doing it). Is the idea that the SSR would likely still hold in till after the relay had released as well somehow, or are we assuming *zero* contact wear (arcing) when breaking the circuit, even at peak voltage?

I'm guessing it's quite a chunky device?

Ok.

<snip>

Understood, a good idea irrespective of the main switching gear.

Cheers, T i m

[Dave Liquorice]

B-)

There was a link further up the thread.

opening

Personally for the power switching I'd go for an SSR, PWM and PID control circuit. Get the PID values right for the PWM the kiln will heat up as fast as it can, with no or minimal overshoot (there is some trade off between no overshoot and rate of change), hold at the desired temperature(s), and cool naturally.

Link in the power circuit that melts a 100 degrees or so above the maximum temperature expected? What temperatures are we tallking about? Nickle melts at 1453 C, Palladium 1555, Stainless steel

1510...

Maybe a bimetal switch but that would self reset so would need to latch off some how.

Lots of additional bits to go wrong and shared with the main control circuitry.

Lots of additional bits to go wrong but independant of the main control circutry.

[Adrian Brentnall]

My box is plastic - but I'm sure I have some suitable heatsinks upstairs in a box..... if I can but find it!

Yes

I presume that's the operating sequence they were planning...

The one I was looking at was rated 50A - which seemed suitably 'chunky'

Yes - very necessary. Especially in a timber-built workshop!

[Adrian Brentnall]

Sorry - I'll backtrack and take a look.

The control bit is all taken care of with a very nice little touchscreen controller

It's only the power-switching bit that's exercising me at the moment.

There's the problem - don't go much higher than 820c with a glass-fusing kiln. Last time I looked I couldn't see anything that would carry 30A and fuse at that temperature.

There is that - independance & all that

'Lots'? - one IC, a couple of passives and a relay. A switch to ensure that, once 'tripped' the over-temp lockout remains locked-out...

I may still do it that way - but, short-term, I can let the controller's own 'safety' output do the work... as that'll be safer then nothing...

The 'safety' relay only switches once per firing schedule, and with no load on the contacts - so it should be reasonably reliable.. - more reliable than the 'main' relay, which switches 1,000 times per schedule.

>

[T i m]

<snip>

<snip>

(Any reason, if you were to go for a more chunky relay / SSR it couldn't be mounted outside the main controller box, say nearer the heater terminals?)

Flicking though the instruction manual for your controller reminds me very much of the Marlin software I use to drive my 3D printer or even the programming / functional options on the two mobility scooter speed controllers I've been playing with recently.

Needing a pod to take a couple of little digital voltmeters to allow me to monitor the voltage on both batteries on either of the two mobility scooters I'm currently playing with, (yesterday) I opened Sketchup on my PC and designed the parts (bezel, body and rear). After designing it and exporting it as an STL file, I opened it in Repetier-Host and sliced it into .gcode using RH's built in Slic3r. I then saved the file to my Open Media Vault NAS (running on a RPi) and then opened Octoprint (running on another RPi connected to my 3D printer by WiFi in a different room), uploaded the file to the printers SD card (on a 3D printed case containing the display, controller screen and input selector knob) and then selected it to print using the Arduino Mega running Marlin.

It probably took longer to type than to do! ;-)

What I was getting to is that I normally print either directly from the PC (over USB) or save the print file to the SD and then select it from the controller (locally). When printing remotely using Octoprint ...

I saw different things on the printers display, features that have been sitting there all along but I've not used before.

The point is the huge respect I have for the patience, skills and efforts of all the work the coders put into making this stuff work, specifically (in my case) because all the software mentioned above is free / shareware (donations normally made etc).

Marlin comes as a series of modules that you select via a config file (to suit your particular hardware) and that it compiles when you upload it into the (Arduino) controller.

The beauty of all the above for me is how nearly all of the hardware is either generic and / or open source, as is much (all?) of the soft / firmware.

I recently *nearly* bought a small commercially made 3D printer off a mate, till I realised I probably wouldn't be in full control of it as I am with mine. ;-)

Cheers, T i m

p.s. It looks like you can get open source and even RPi based kiln controllers. ;-)

The indestructible uses an SSR that might be of interest to you?

"Note: the SSR can get quite hot when in operation so you might want to put it on a heat sink. I sit my plastic box on a tin can when firing." ... ??

[Adrian Brentnall]

It _could_ be mounted outside the box - just be neater of it was _inside_

- will depend on how much heatsink it wants/needs

Don't really want to mount it any nearer to the kiln, as the outside of the kiln approaches 100c when it's running.

Clever stuff - never got into 3-d printing.... but I can see the fascination.

Toyed with the idea of laser-etching (to 'engrave' on glass - but there are easier / cheaper ways of achieving the same end)

I'd not seen that one before. The one I had seen, and this one, both require the schedule to be input in a strange format..

They do

Target time : Target temperature : Hold time

while most glass-fusing schedules do

Ramp speed : Target temperature : Hold time

You can use a spreadsheet to calculate between them, but the earlier RPi controller was driven by time (i.e. I'm at the target time) rather than Temperature ( as in 'I'm at the target temperature') - which is a bit different. In glasswork, a difference of 5 - 10c can make a large difference to the effect that you get - so temperature-driven is really a necessity...

Yes - we I don't have a spare biscuit tin <grin>

Actually - I have a RPi hooked up to my kiln - it monitors the temperature in the kiln through its own thermocouple and draws a real-time graph that can be viewed over the LAN.

I must confess I'm not confident in the RPi's ability to run things like kiln controllers, which need to run, unsupervised, for 8 - 10 hours at a time, often overnight.

For me - the RPi is just too 'full-featured' - it's go too much going on within the o/s for me to trust it to keep its mind on the kiln-controlling job. But then my background is real-time, safety-critical systems....

So - I did look at making a home-brew kiln controller - but, in the end, I let the programmers in the USA do the hard work, although it does seem that I've been helping them debug the thing over the past year or so...

The important part of the process is making things in the kiln - as that's what brings in the cash - so I decided to concentrate on that...

[T i m]

<snip>

Bigger box ... or I was thinking of another box fitted to the back / side / bottom of that one with the extra stuff in?

Ouch.

<snip>

It is indeed. 'Standing on the shoulders of giants' etc.

Well it is fascinating (still to me, when I think about it or when I'm holding the physical object I only thought of a while earlier) but for me it's more the practicality. To be able to just think of some 'thing', design it (or download it if it's been done before, from the likes of

and then be able to use it soon after. ;-)

Be it something new or repairing or enhancing something you already have. I look at it as an easy n/c variant of my lathe. ;-)

I don't think there is a cheaper / easier way of covering the range of things you can with a 3D printer (even if you already own a lathe). ;-)

<snip>

Oh, I wonder why they did it that way when I would have thought that you could do it any way you could imagine?

I think there are automatic PID configuration routines you can run on these diy 3D printers (and commercial ones I should imagine) and then you can use the output / calculated values to set / tune the printer.

I would have thought you would want to put the SSR directly on the tin, not the plastic box it was sitting in!

Oh, neat.

Ok. FWIW, I've had a RPi3B running a NAS for a couple of years now and touch it so rarely I've forgotten everything adminny about it. ;-(

It's similar with the one running a multi-TV tuner ... they just seem to work (and work)?

How about the micro controllers, like the Arduino then? My 3D printer runs on an Arduino Mega and as mentioned, runs print jobs that can be a day long and where even a fractional error would stand out like a sore thumb?

with a RAMPS controller board: drives the steppers, the extruder and bed heaters).

As you say, just as well with a kiln in a wooden shed. ;-)

And that's the thing ... one thing I like about all this popular / open source stuff is how many bodies are often working on stuff. It's not always the case of course, you can sometimes pick something that happens to then become obsolete or depreciated but if it was working ...

Oh, absolutely, I only got involved with the 3D printer because a mate wanted one and didn't have the confidence to be able to cover all the facets (not that I had ever built or even used one but I had done most of the parts either myself or commercially). We built it together, used to run it in his shop (handy to have somewhere to run it were you could keep an eye on it for 8 hour print jobs) and he sold it to me when he retired (with the proviso I still printed stuff for him as required). ;-)

Initially we printed all sorts of stuff (because we could), spiral vases, figures and personalised key fobs etc but now it's really only used as an engineering production tool.

Brackets, equipment boxes, special jigs, templates and tools ... the last serious thing I did before this voltmeter pod was spice rack ends for my daughter. Oh, when the filament she ordered arrives, I'm due to print 10 forked 'hooks' (for hanging big 4/ Stihl strimmers up on the wall). Not for money though, just a returned favour. ;-)

What sort of glassware do you do in yer kiln?

Cheers, T i m

[Dave Liquorice]

Hum, have more confidence....

pi@PiZ-StoveB:/etc $ uptime 10:04:57 up 73 days, 9:38, 1 user, load average: 0.76, 0.63, 0.52

A Pi Zero (sans WiFi or Bluetooth) running a python script to control the speed of the pump for the circulation between woodburner and thermal store. The aim is to maintain a few degrees difference between flow and return irespective of the actual temperature values and if the woodburner is going flat out or dying down. Flat out it boils rather too easyly...

Has half a dozen or so 1-wire devices connected that it monitors over two 1-wire buses, an SSR using PWM to control the pump speed on a GPIO, a small graphical LCD display SPI IIRC, (ex Nokia phones) to show what it's up to and provide a menu system for set up controlled by a rotary encoder on three GPIOs. Ethernet port via an ENC28J60 SPI(?) board.

It's also running nginx as web interface to the logged stove temperatures pump speed etc (may also at some point, provide a webinterface to the stove control script) and PiHole to block web advertising crap for the LAN.

OS is Raspibian Lite (buster) so no GUI, completly headless, no monitor, keyboard, mouse. Only way to talk to it is via the command line from SSH over ethernet. The display/rotary controller is confined to the menu system of the stove control script.

There is "real time" as in must react within 10 ns and and "real time" as in must react within a minute... The stove system is slow to react, pretty sure the period between each temperature measurment is a minute. I suspect your kiln is also suitably "laggy".

[John Rumm]

I have had Pis running for years uninterrupted without any problem... (normally only get a reset if there is a power cut!)

It does not typically run a true real time os, however for kiln control there are no hard real time requirements.

If you want a "simpler" option, look at an arduino - much less baggage to worry about.

Yup, often a good plan, it can be easy to get side tracked onto something "interesting" but ultimately not rewarding (financially at least!)