Build: HLT Controller - STC-1000 with relay and float switch

[TIm Boyd]

Currently building a new controller for the HLT.  We wanted a circuit that monitors the temperature of the water in the HLT and fires up the element when required, also with the added protection of a float switch to protect the element before the water level drops below it.

Below is my initial wiring diagram.  I'm far from a professional when it comes to electronics, so any input would be appreciated.

[Scott H]

Check the stc specs and make sure it can switch DLC. Otherwise switch the 230v supply to the transformer

[TIm Boyd]

Hi Scott,  I've already built a quick prototype to check the wiring and it does switch DC, so I think I'm good on that front.  Just wanted to make sure I'm not missing anything else.  thanks for the response.

[mattd2]

From what I have seen of photos of the STC1000 guts you should be good - looks like the relays are rated for 10A @ 277VAC / 28VDC http://www.hongfa.com/pro/HF3FF_en.html

What float switch are you using? And is all of this on a 15A plug/socket?

[TIm Boyd]

Thanks, yeah it looks like the DC side will be fine. I'm using this Float Switch. Running it from a 20A RCD socket.

[Scott H]

DLC ment to be DC

[Barry]

Any reason you wouldn't use a cheap Sestos PID over the STC? 

More accurate control, just as cheap, no DC transformer required as they have 12VDC output specifically to power SSRs.

[Barry]

With the exchange rate what it is right now that thing would be about $NZ35 landed.

[TIm Boyd]

All very good points, and to be honest I had not really looked into a PID.  I already have most of the parts from other projects so I'm putting them to good use. Thanks for the heads up.

[Peter Smith]

Have also seen people put a slow speed motor and stirrer into the HLT to make sure the temp is even.  I am going pid for HLT, I used my urn as an HLT the other day with no temp control other then me and a digital temp gauge,  it was so much better then heating water in kettles....

[TIm Boyd]

Project complete and all working well, albeit with the failings that were raised. The PID really is the way to go.  I get a 2-3deg overshoot after the element shuts off, the wiring is more complicated compared to the PID and I had to come up with a 24VDC supply.  Once Barry brought my attention to the PID's I decided not to spend anything on the project and made do with gear I had.  I ended up butchering two 12VDC power supplies and wired them in series to give me 24VDC.

All in all a fun project with a good learning curve and it gets me close to where I want to be for now, but I think I will be getting a PID sooner rather than later. 

Peter, I have read about people putting a stirrer in the HLT.  It makes sense as you often get a 1-2deg drop when you stir the pot.

Thanks for all the advice.

[Barry]

Nice work mate, sounds like a worthwhile part of the journey.

[TIm Boyd]

very much worth while.  got me head around quite a few things and now feel more confident to look into more comprehensive controllers.  let the fever begin!