mini control unit (small but perfectly formed : )

[julianf]

I cut this for a friend yesterday.  I wanted to make a small control unit in a neat package, that was heat-sunk, and could be affixed out of the way easily.

Its nothing compared to the home-automation systems built by others here, but it wasnt supposed to be either! : )

[Tony]

Very tidy

[Julian]

Could you make the economically enough to sell?

I bought a batch of these and they sold very well for around £90.00, second hand.



This was a diagram I made to explain how easy it was to connect them, which I think helped to sell them ...



Several went to the home brew market and were bought by people with no technical knowledge, who really liked the "plug and play" aspect.

I would imagine a new, less archaic looking unit could command a far higher price.

[julianf]

I built the one in the photos for a friend, but, i must admit, that the idea of recouping some of the design time on some sales would be appealing.


I was considering offering them as, like you say, a plug and play unit, down to the 2.5mm flex, and the coupling for the thermocouple (ie a tapped fixing for the odd thread that all the units seem to have) - but then its parts, cnc work, lathe work, and it all starts to get difficult to put an attractive price on!

So, yes, id like to sell them, but im unsure of what the uptake would be if i priced it at anywhere near my usual electronics / cnc rates.

[Julian]

I know it would be adding complication and a little cost, but the following would give you a couple of selling points ..

Make the heater circuit latching ... safety feature, and coupled to using the hi temp alarm as a hi temp cut out, you can set the process temperature to say 62°C and the hi temp to 60°C and the unit will heat to temperature and then shut down.

I've used my controller this way for many years and it's a big advantage.

[julianf]

Im unsure that i follow?

I dont understand why you would want to reach a set point, and then not maintain the setpoint with the 'PWM' function of the PID?

[Julian]

Im unsure that i follow?

I dont understand why you would want to reach a set point, and then not maintain the setpoint with the 'PWM' function of the PID?

My be it was a bit garbled, sorry.  Setting the cut out a degree or so lower than the process temperature is useful when dewatering oil or drying bio.

For oil drying I set process temperature to 92°C and cut hi temp cut out at 90°C, pumps and cooling water running, then walk away.  Several hours later I return and the oil is hopefully back down to circa 60°C ready for processing, having been up to 90°C and been dewatered.

[julesandtash]

You can use the high alarm limit to trip the heater too.
Mine are set that, should the process temperature exceed the set point by more than 5C, the high alarm output activates a latching relay and the heater is locked out until manually reset. That means that a SSR which has failed in the ON state or just a failed output stage in the PID contoller will not cause an overheat situation.