A Chinese Stat for a Garden Propagator

[AlanBeckett]

I have a couple of Propagators which I use to raise Seedlings in early Spring. Actually I have two but one doesn't have a Thermostat yet but more later.

This Spring I was struggling on two fronts. First the peculiar Weather - Sunny Days and freezing Nights which is unhelpful. Second, keeping a stable Temperature, particularly over night, was near impossible. Anyway, I struggled on and managed to raise enough for my needs in time for the Weather to stabilise. So, what's the problem?

The Propagator came with a Stat, one of these:-

   

With this Propagator I'd long since rewired it to add an external Joint Box and Neon so that I could see what was going on. It soon became clear that although the Stat was demanding Heat the internal Relay hadn't closed. Twiddling the Knob would make it happen but only with a lot of over Temperature demanded. So what's the problem?

Taking it to bits was easy enough and changing the Relay didn't make one iota of difference.

   

However, now it was possible to sensibly measure the Relay Voltage it was clear that it was woefully low. Eagle-eyed Joe spotted that the Yellow Capacitor in the middle had to be a Dropper as there was nothing else. Sure enough it was about a third of its supposed Value. After some scratching about I found a couple of decent Caps that I could substitute.
Not very elegant but that sorted it so I put the original Relay back.

   

The final problem was the Knob. That engages with the Skeleton Pot on the PCB which has a Hex hole. In all honesty it's not very positive so I threw it away and plugged an Allen key in. Not very safe bearing in mind that there's Mains in there but as everything is protected it would do in the short term.

   

So what now for the long term and my other Propagator?

'ode 'ard Lawrence while I continue

Alan

[AlanBeckett]

So, to continue .......

Once again Joe came to the rescue by finding a suitable Chinese one:-

   

They are, of course, ludicrously cheap, less than £4 Post Free. Just to put that into context a replacement for the original is over £20. So I ordered a couple.
The downside is that it's 'Bare'. However, this is where all those useless bits that you collect come into their own. It needs 12Volts at 60mA Max so one of Jeffrey's little Torroids is hopelessly overrated but eminently suitable. A couple of Diodes, and couple of Electrolytics and a 7812 does the biz, and it gave every impression of working fine.

   

So, an Enclosure. My guess is that it's designed to fit into a Cavity in the top of some sort of moulded Housing, and I have plans for a better one in the future. But for now I've used a plain Plastic Box that I had to hand. I made a Cardboard Template just to make sure it would all fit.

   

... and made sure that the bits would go in. Then it was just a matter of cutting out an Aluminium Plate and mounting everything.

   

Drill Holes, Grommets, Tywraps, Silicon Sealer and that's about it.

   

The only other shortcoming was that the length of Cable on the Probe isn't over-generous, so I extended it bit - Heatshrink and Silicon Grease.
So, a good dose of Silicon Grease - DC-4 - round the edges, screw it together and give it a go.

   

On this version I simply brought the Relay Contacts out on a Pair as that matches the Connections on this Propagator.
I've installed it in the Greenhouse and let it roast for a couple of Days and it survives the trip OK, which isn't surprising bearing in mind how over-rated everything is.

   


A final comment would be that whereas on the original the Hysteresis is an unknown quantity on this you can set it to whatever you want.

Onward and upward

Alan