Hi All,
I have been trying to come up with a cheap way to control the temp on a heating element that I have been using. Its 240 volt 2200 watts but I need to keep the temp of the fluid I am heating to around 78.4-80 oC . I have spoken to Pete about it and have looked at Variacs to lower the voltage to control the output. I was going to make a picaxe to turn on and off the temp probe but I have been doing a bit of reading and I need to keep the fluid more constant then turning it off and on as there is a lag time when this happens. It takes a bit of time to heat up again and then to cool down again so it is all over the shop. Any

Amack

Sounds like you need a PID function, but don't ask me how to write one

If you apply a very slow PWM signal (maybe a 5 sec period) to the heating element that is inversely proportional to the temperature I don't think you would have fluctuations that are too wild.

You could use a picaxe to control the temp running a program something like this (untested and not syntax checked)



Once it reached 73C it would start to cycle, and at the most it would be on for 60 sec and off for 10 sec.
OR
On for 1 second and off for 10 sec.
OR
Somewhere in between with a 10 sec off period.

You would need a solid state relay (AC) or a triac to do the mains switching.

Its One option following on what Rob suggested.

I was going to take the easy way out and got to (JUTE) and buy a Veriac and be done with it but stuff it, I might give the Picaxe a go as you learn nothing from spending money in a store.

Amack

So do that mean you want a circuit diagram for a picaxe design?

Its really not that hard, except needs a few parts like a DS18B20 temp sensor (digital in 1 degree output) or a thermister (harder to dial in but not fixed to 1 degree stages), and a solid state AC relay suitable to the current draw. (cheap on ebay)

A 08M chip would do the job, and if you use the DS18B20 sensor then, the commands are in picaxe to read it directly.
Although limited to 127C max temp, which is inside your temp range.

Pete.

Found a water proof temp sender and at $3.57 I think it's not a bad price.

http://www.ebay.com.au/itm/NEW-Waterpro ... %26ps%3D54

Amack

Not a bad deal for the price, although you can buy 10 senors for @ $10.00 for just the sensor chip, then you need to seal it up.

The important part is to ensure the chip used is a DS18B20 (the "B" being the important part)
There is a DS1820 chip that is not compatible with the commands in the picaxe chip.

The DS18B20 is the same chip used to monitor the stator temp in Phills AXFX mill, so if you want to go the 10 chip route then you have a use for a few spare DS18B20 for your mill, or get a few spare cabled sensors for the mill too.

Pete.

Hi Pete, I got two of them for $7.08 They are DS18B20 Waterproof Digital Thermal Probe or Sensor, to the door. They are sealed and water proof so might be OK. The second one was planned for the mill but might see if these are ok before I get any more. For $3.54 each by the time I got the chip and some wire and srink rap and put them together I thoght it might have been quicker to get these.

Amack