Hi
Today I wanted to show you some of my oven developments in a new topic, maybe you could use that for inspiration.
But, ^@#%$%^#$%@#%#$
I was soldering the last parts to the oven that I have been working on in the last few days and was doing the first tests.
What I measured and saw did not feel good, there was "mental" behavior of this oven.
I have quite some experience in building this kind of oven, and I started by watching how the current drops as the oven warms up.
If the oven is close to the final temperature, the oven should start using less power quickly and should not become completely powerless.
If the current becomes almost "0" then the loop compensation is not good, that is one problem.
But with this oven I also connected a scoop channel to the output of the uA723. w.t...
A lot of "random" noise, lookt like the infamous popcorn noise, forgot to make a scoop picture of this.
But first a picture when the oven "was" ready.
I didn't have any more 0.1% resistors of the good value, so here are 1% resistors used this week the good resistors come back in here.
Without looking at my test setup, the noisy oven had stopped working.
At the bottom you can see that there are always bursts of current and at the end the maximum current and then nothing anymore....
The oven stopt working, i search myself silly...
Everything at the inputs seemed all right, also the reverence voltage was present with its 7.05V.
The +input of the uA723 also had nicely half the reference voltage as it should have been.
Only the output of the IC it has a voltage of about 1.7V, and then the MOSfets never conduct.
Noise at the output of the uA723 and an output that is far too low in voltage.
It must be a defective uA723, a pity of the neat wiring....
Out you go!
And I've soldered a new one in, "bad language" exactly the same behavior...
Grrr.
I had another hp 34401 on the bench and soldered one side of the thermistors off
and left the other side connected to the circuit and tried to measure the resistance of the thermistors.
Very strange results I got at the resistance measurement.
In the end I cut all four thermistors loose and measured them separately.
When I started measuring the resistance connections to the oven housing, the monkey came out of the sleeve. (This is a Dutch expression)
There was quite a lot of "leak" present to the aluminum of the oven.
This is the first time I have experienced this, when i mount/glue sensors in aluminium.
It's also the first time I've used the glue I've shown for this application.
To make a long story short, I have drilled out all four thermistors and placed four 5K thermistors in series so that I can all use 10K 0.1% resistors at the uA723 inputs.
4x5K makes 20K at 25C and at about 42C the total of the thermistors wil be about 10K.
This time it's not an ebay thermistors but beautiful from Vishay 5K and 0.5% not too expensive and very good.
The current is now nice and stable and the oven works fine, noise is gone.
The wiring is not as neat as it was, but it works fine.
I have to tune the loop just a little more for optimal performance.
The thermistors are put in the holes with termal compound, later i wil blok the hole with some hobby glue.
I can say that this was a learning day...
Kind regards,
Bram