For a heating application, I need to maintain constant temperature on a resistance heater (electrical heating rod) surface. I am planning to use a PID controlled thyristor based control . But not able to decide whether I should go for DC or AC voltage.
Can anyone help on this?
Hi A. V. Dhotre ,
if your decision for a thyristor is definite, then AC has the advantage that the thyristor turns off at the end of each halfwave, provided the gate current is zero. If you need to supply the heater with halfwaves of both polarities then you had to embed the thyristor in a rectifier bridge. Disadvantage of this rectifier: The potential at the cathode isn't constant. If your controller has to be connected to a constant potential you would need a transformer or a suitable optocoupler to drive the gate.
With DC, you had to provide a sufficient time interval with anode current = 0 A by discharging a capacitor, for example, in order to switch the thyristor off; this complicates the circuit.
Alternatives to thyristors are: MOSFETs (DC directly, AC with rectifier), IGBTs (DC directly, AC with rectifier), and solid-state relais = modules, suitable for AC, including an optocoupler and sometimes a zero-crossing detector in order to minimize the generation of voltage and/or current spikes during switching.
Thanks for the Answer, Mr Joerge.
I should have mentioned in my question that I am planning to use PLC as controller.
I think Joery provide good information regarding your question
Offcourse MrJoery did.
I'm trying to control the heater temp through PLC and thyrister but the temperature is shooting beyond set point. PLC was programmed for PID control function.
What could be the problem?
Hi A. V. Dhotre ,
if the temperature shows just a temporal overshoot, I would decrease the coefficient of the D component of the PID control or even try a negative one.
If the temperature does not meet the set value permanently, increasing the I component should fix the problem.
Thanks Mr.Joerg,
I have tried these things to get no results. I am giving Delta PLC output to tyrister directly. I received a suggestion that an digital to analogue module needs to be connectd between PLC output and thyrister.
Can you suggest on this?
Hi A. V. Dhotre ,
what kind of output do you use? As far as I can see, Delta PLCs offer digital outputs (relay or transistor) and analog outputs (0 .. 20 mA).
If I understand correctly, you use a discrete thyristor (no module) as power switch. In this case, I don't think that a DAC is necessary or useful.
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