Posted 07 June 2011 - 05:56 AM
Howard, it looks good.
Some points though.
The enclosure. Its a difficult choice. And constrained by availibilty! There's no need for a weatherproof box, if you are using indoor-type power connection sockets. Also a well waterproofed box has no ventilation for the heat sink! A plastic box is easier for cutting the various holes, but a metal box (earthed of course) can better conduct away the heat from the heat sink.
I wanted to adopt the same simplifying approach of attaching everything to the box lid, but given the size of UK power outlets and plugs, that wasn't possible for me. I couldn't find a small box with a big enough lid!
I chose to put the power distribution and SSR in a different box to the controller, thinking I might use it on other projects. At $10 delivered (eBay) for the SSR, the simplicity of putting everything in one box and minimising flying-lead clutter is something I'd now strongly recommend. The heat from the SSR being less than I'd feared also supports the all-in-one approach.
The weight and tension of mains electrical leads are significant in the stability of the box(es), so choosing the best place for all external mains cable connectors is the first layout consideration.
My power box has a non-controlled (manually switched) outlet used for the controller box, but also catering for a constantly running pump or fan. Without it everything could have been smaller! The power box also has an isolation switch with a power-indicating neon. Again, I'm not sure I'd bother in future. With an all-in-one-enclosure design, the PID display shows its on!
My power box has its control input in association with the controlled output. It shows which outlet is which. With an all-in-one, I'd similarly choose to have the probe input signposting the controlled power outlet.
The other design consideration is keeping all the external cables away from the PID and its control buttons.
The SSR. I have the 25 amp version of your SSR, on a large (antique Mac 71/8100?) cpu heatsink (which barely gets at all warm), stood-off from the walls but within a vented plastic box. On UK 240v mains, my 1800 watt heater pulls less than 8 amps. So my SSR is rated at over 3x my highest current. Is there any advantage to using an even higher rated SSR (40 amp would be 5x standard load) - like less heat for the same current?
Worth noting is that as a 'zero-crossing-switching' SSR, it produces no mains or radio interference from switching. But it does 'quantise' the power control. On a 50Hz mains, with a 2 sec PID cycle, the SSR restricts the output to 200 steps - adequate resolution, I think.
Your probe (and its float). Could you give some more detail? I'm using a (not really cheap) "fully immersible" Pt100 from Auber, which I have mounted to the rack in the bottom of my tank. I have been thinking that having the probe mounted in a consistent position was important to the measurement being a consistent indication of the tank temperature - surely if it can float about in the tank, it might sometimes be measuring near the centre and sometimes near the edge (which would be cooler)?
The probe connections. For a Pt100 ptobe, its nice that we can easily have a connector giving us a break-point -- for a K-type (for example) you need to take account of the connector. Noting your smoker intent, I've seen an Auber gas temp K-type probe but not a Pt100 gas probe. Any ideas there? (I've been wondering about a homebrew hot air chocolate tempering control...)
Any comments about your choice of PID?
I have an N2006P via eBay. One thing it lacks is a manual over-ride, but one (undocumented) feature that it has is the ability to display the output power demand. Stable at 56C, my tank needs about 8% heater on-time.
The N2006P auto-tune isn't perfect, but its good enough (I generally reduce twitchiness by reducing the D term).
Especially if you are going to use different containers and heaters, auto-tune should be a must-have feature when choosing your PID. (The P I & D factors need to be set differently for each combination of tank and heater - even changing the water level in the tank changes the response and needs a different tuning.)
My PID has two 'alarm' outputs, which I don't use.
I do have a completely independent thermal safety cutout, in the form of the mechanical thermostat on my tank. I don't turn it all the way to maximum - instead I set it above my working temp, but below boiling, so that if my homebrew control kit 'failed on', the mechanical thermostat would prevent boiling, and particularly any possibility of 'boiled dry' dramas.
And Howard, your workmanship is much neater than mine!
"If you wish to make an apple pie from scratch ... you must first invent the universe." - Carl Sagan