Current rating of Faston / Lucar connectors

[Adrian Brentnall]

HI All I'm thinking of replacing the main relays in my 7.5kw glass-fusing kiln with these relays

- which are Potter&Brumfield DPDT 40A relays.

Currently I'm using domestic contactors, which last a few years (not through being overloaded, simply through the number of on/off cycles they go through during each firing schedule). The contactors have screw-down clamps for the connections.

The Potter&Brumfield relay uses 1/4" 'Faston / Lucar' connectors. Out of curiosity, I looked on the web for the current rating of these connectors - Wiki claims "24A (continuous)" with 10AWG wire (about

5.2mm-squared).

Any thoughts on this - or how a relay rated at 40A has connectors rated at 24A?

My plan was to find the sturdiest Lucars I can locate, crimp them onto the mains feeds and (?perhaps?) run some solder into the crimp - just to be sure.

What does the team think? Thanks

[Harry Bloomfield, Esq.]

Adrian Brentnall has brought this to us :

Why not look at converting over from relays, to triac switching?

BTA40-600B is rated at 40amps 600v

[Adrian Brentnall]

HI Harry. I've looked into the whole 'Solid State Relays' thing, and, to be honest, I'm not convinced.. When you examine the specs, these things need some serious heatsinking (as they have a finite 'on' resistance) and, call me old-fashioned, but I rather like to hear the relay clicking on & off, as it reassures me that all is running properly. Retro-fitting the SSR plus the heatsink and probably a small cooling fan would be difficult in the existing enclosure.

The big worry with glass-fusing kilns is that the main relay fill fail 'on' (welded contacts) - as when this happens the kiln will continue to heat uncontrollably until something melts or catches fire.

Seems that the solid-state 'switches' can fail 'on' or 'off' - just like the electromechanical relays. I know that they have a longer predicted life - but the high-powered ones are also several times more expensive than the equivalent relay.

I think relays are the way to go - and to treat them as a 'service item' and swap them out every three years or so (based on the predicted cycle life).

I was more concerned about the best way to terminate the wiring - given that the Faston connectors are apparently rated at 24A and the kiln's pulling 30A or so while the relay's rated 40A... Maybe it's a non-problem, and Faston are just being cautious?

Thanks

[jrwalliker]

The relay is two-pole, so why not parallel the two poles and share the current across two sets of Faston connectors? John

[Harry Bloomfield, Esq.]

Adrian Brentnall used his keyboard to write :

If I'm reading the datasheet correctly, it suggests 1 watt dissipation. It will be smaller than you contactor. You could always add an indicator light and even a small relay just to make the reassuring noise.

I've had such a SSS switching in my garage every few seconds, for the past 20 years. Zero switching, they generate no switching surges.

[Adrian Brentnall]

That might be a plan.. In fact, the 7.5kw is actually made up of two separately-wired heating elements - with two cables running back to the control box - so switching each one through its own set of contacts would reduce the current on each Faston. The contacts themselves would probably appreciate it too...

The currently-installed contactors are also double-pole - and I was switching both Live and Neutral - but I guess there's no real need to do so...

OK - thanks!

[Adrian Brentnall]

I did the sums a while back, and I seem to recall it was nearer 60 watts.. and the warmer you run them the more you have to derate them

The SSRs I was looking at were the same form factor as the contactors (DIN rail mounting), before you added the heatsink.

Zero-switching is about the only advantage I could see in using SSRs...

I think I'll stick with good old relays! (Fully-paid-up Luddite, me!)

[T i m]

<snip>

I have similar reassurance on the (12V / 120W) heated bed of my 3D printer as that's powered (Bang-bang) via a HD automotive relay.

The extruder is electronic (PWM) driven by the main unit but is rarely on continuously, only during initial heatup (probably 60 seconds or so).

If you tell it to preheat, you see the outputs for both the extruder and bed via their LEDs (and there is an LED on the bed itself) but it's the click of the bed relay that you hear when you aren't looking at the printer. Ok, when the printer is actually printing you hear that as well but once running I'm less bothered than during the startup.

I believe I did try a SSR on the bed but I also think I remember it getting very hot but I can't remember if I was using it BB or PWM (although I'm not sure that would have mattered during warmup as it would have been on continuously in any case).

I think the idea of splitting your load of one heater per connector on a dual pole relay or how about one heater per relay (again, just switching the lives, assuming they aren't that expensive and the input current for the coils low)?

I think you wire contacts in series if you want them to last longer (less arcing during disconnection) and you shouldn't parallel contacts to the same load, other than for redundancy and assuming one contact can still carry the required load.

Cheers, T i m

[newshound]

Personally I'd be inclined to go over to solid state. I did this many years ago for the hot liquor tank on my mate's brewery (which is heated by 3kw immersion heaters).

Which ever route you go, personally I would add a "Policeman" to trip the system on significant overtemperature.

[T i m]

<snip>

I wonder if there is a practicality issue re switching AC over DC? When I tried a SSR on my 3D printer switching 12V DC at ~10A it got very hot quite quickly (and it was rated much higher).

Agreed.

I have a thermal fuse on the extruder (~250 DegC) and I think there are watchdogs and other protections in the software for both, as I'm not sure the hot bed could get hot enough to be a risk?

Cheers, T i m

[Dave Plowman (News)]

Vehicle Wiring Products sells the best quality I've found. No need to solder if you have the correct crimp tool. But that isn't cheap.

[Adrian Brentnall]

I've looked into solid-state switching, but I think I'll stick with good old 'clickety-bang' technology.

OK on the 'policeman' - the rather snazzy touchscreen controller has an output for a 'safety' relay (which cuts power to the heaters when the controller hits an error condition). I'll be implementing this via a second relay when I change the first relay..

Sadly - most manufactures of kiln controllers include this 'safety' output in their controllers, and most kiln manufacturers ignore the feature! This creates a situation where the failure of a single component (the main relay) can cause the kiln to heat uncontrollable - which isn't a Good Thing.

Sure - the controller can get itself confused, even with a safety relay

- but short of having a separate temperature-limit circuit, it's the next best thing..

Thanks

[Adrian Brentnall]

In glass-fusing it's all about a series of segments - each one has a ramp speed, a target temperature and a hold time - up to 7 or 8 per schedule - so the relay is worked fairly hard by the controller as it turns the power on & off to achieve the desired profile...

I had thought about running two relays, one per heating coil - but I'm not sure that achieves much advantage over running the two could from two poles of the same relay.. Sooner or later, the switching contact will fail - probably in the 'on' position (ref Murphy's Law!)

Running the two poles to the two heaters gets over the slight worry about the current-handling capacity of the Fastons...

Thanks

[Adrian Brentnall]

I'm pretty sure I did the maths, and the on-resistance of the SSR resulted in quite a lot of heat that needed to be lost 'somewhere'. The detailed spec sheets show some 'interesting' de-rating - depending on the current flowing and the devide temperature.

See the comments on my other post. Ideal would be a separate hardware watchdog - they actually make a chip (I've had two of them on the desk looking at me for four or 5 years now!

- waiting for a round tuit) that accepts a thermocouple input, and will switch an output at a preset temperature. The kiln controller also has a 'safety' output - which can drop out a secondary relay if the controller thinks that something's gone pear-shaped (like uncontrollable heating 'cos the main relay's fused shut).

I plan to wire that up as well!

I'm probably worrying without due cause - but having a 7.5kw heater in a timber workshop, running for up to 10 hours at a time (often overnight), and operating routinely at temperatures around 800c - it did strike me that it's better to be safe than sorry!

Thanks

[Adrian Brentnall]

I've got a good ratchet crimper and some nice-enough crimps.. It was just the allegedly low current rating on the Fastons that was worrying me - but if I do as others have suggested and split the feed to the two heating elements then that'll sort that.

Thanks

[newshound]

Interesting. First came across policemen when using electrically heated test rigs involving pressure vessels in the 70's. Always implemented by a physically separate independent temperature controller with its own thermocouple. And in a summer job in the 60's I had to build a regulated power supply with crowbar protection (although I didn't actually design it).

[jrwalliker]

I have come across furnaces with a thermal fuse for over-temperature protection. A gold wire fuse (driving a relay) might be suitable for your temperature range. Without a relay it might need rather a lot of gold to get the needed current rating.

John

[T i m]

<snip>

Other than the load is then carried over two mechanically (thermally) different modules, something that could be of value if the temperature rise de to connector and contact resistance may be concerned?

I think the normal reason for contact welding is a high resistance built up from a mixture of contact bounce on connection and arching on disconnection. I think the suggestion of running two contacts in series is supposed to help alleviate that.

And even better to have them on two complete different relays (and only because your load is naturally formed by two separate devices). ;-)

Cheers, T i m

[Harry Bloomfield, Esq.]

Adrian Brentnall has brought this to us :

Wouldn't a thermal fuse be better suited to that? No risk of any other attached circuit failing?

[T i m]

<snip>

I am fascinated by all those PID's (are they called) and I believe my printer firmware (Marlin) has the ability to create the PIDs itself by running the heaters up and measuring the response and noting any overshoot etc. Once done, that becomes the working profile. ;-)

As an aside, we know someone who used to work in a 'Glassworks' and it is interesting hearing about all the process and what they had to do when things went wrong.

Cheers, T i m