Problem with thyristor circuit control

[alec_t]

Unlikely.
Have you tried removing (or disconnecting) the preset as RJ suggested?

 

[draculic acid69]

No I haven't.the thing I was getting at is one thyristor circuit worked at an acceptable temperature but when the potentiometer was changed to a different corded one I got a different result in temperature.both the potentiometers were turned down to zero so there's not really any other possibilities as to why two different results from technically the same parts

 

[alec_t]

Do the main pots and the preset ones have identical values on both controllers?
Have you adjusted both the main and the presets to minimum on both controllers?
Is there a fixed resistor in series with either pot on either controller?

 

[draculic acid69]

All preset trimmers,pots, resistors,diodes,and transistors are the same.

 

[rjenkinsgb]

Likely just component tolerances between different makers.

At the price the whole units sell for, they could even be rejects / out of specification parts.

 

[alec_t]

Something must be limiting the minimum setting. Can you trace the circuit to see if there is a resistor in series with the pots?

 

[draculic acid69]

The two circuits are wired the same.

 

[draculic acid69]

The pot seems to be the cause of variance

 

[rjenkinsgb]

Something must be limiting the minimum setting

The multi-turn preset on the PCB is in parallel with the control pot. Maximum resistance = later firing & minimal output.
The preset limits how high the overall control variation can be.

 

[Nigel Goodwin]

The pot seems to be the cause of variance

Tolerance on pots is huge, often +/-20%, so it could be as simple as normal tolerance of the pots.

EXACTLY what is it you're trying to control?, does it have thermal inertia?, and to what degree are you trying to control it?.

 

[alec_t]

Maximum resistance = later firing & minimal output.

So a resistor added in series (or increased in value if one is already present) should reduce the output further, if that's what's required.

 

[rjenkinsgb]

So a resistor added in series (or increased in value if one is already present) should reduce the output further, if that's what's required.

... Or just remove the shunt preset, as I have already suggested.

A series resistor will limit the high end as well.

 

[alec_t]

A series resistor will limit the high end as well.

I don't think that matters, if the aim is just to reduce heating element temperature.

 

[draculic acid69]

I'm going to try the branded potentiometer on a cord and see if that makes a difference to when it was soldered directly to the PCB.will report back

 

[draculic acid69]

Soldered the better pot to the board and then turned the preset trimmer down till it started clicking and now the minimum temp when set to zero
is 50'c. That's awesome now this thing works as it should.so the pot was the source of variance.now I know how to get them working how I want.thanks for the help guys.

 

[Pommie]

A simple thing to try is to cut the track to the trimpot. If your board is the same then cutting here should work,

You can normally score a groove with a suitable knife or screw driver.

Mike.