Thyristers fordriving a heating coil

[binojkumar]

Thyristers

Hi friends im doing a project that controls temperature of a heating coil.
the heating coil power supply is connected through thyrister and a PWM out put is controlling this Thyrister. This is my application. the heating coil is operated in AC. So what kind of thyrister i should use? please suggest thyrister and a driving circuit.

VC

 

[pranam77]

Re: Thyristers

Hi friends im doing a project that controls temperature of a heating coil. the heating coil power supply is connected through thyrister and a PWM out put is controlling this Thyrister. This is my application. the heating coil is operated in AC. So what kind of thyrister i should use? please suggest thyrister and a driving circuit.VC

I got confused a bit with the question. First of all if you are operating the load with AC, the PWM technique will not work. You need to go with Phase controlling technique. You may use BT136 or BTA12/600 Triacs for operating heating loads on AC. Cheers

 

[Naveed Ahmed]

You can use solid state relay for that purpose.

Use MOC3023 optocoupler and drive a triac BTA16 through that optocoupler. And control PWM through it.



Regards
Naveed Khowaja

 

[betwixt]

I agree with pranam77, what is the purpose of using PWM to control a Triac, it only needs one pulse per quadrant to turn it on and it turns itself off at zero crossing point.
You must use phase control not PWM.

Brian.

 

[FvM]

EMC rules require higher power heating controllers not to use phase control but full wave switching.
It can be either operated as a slow PWM with a few seconds period, which is the classical method, or in a
SD modulator style.

 

[betwixt]

I think we need Binojkumar to tell us more.

If their intention is to control the current flow as accurately as possible, they have no option but to use phase control and trigger the Thyristor once per cycle. If they intend to maintain an average temperature but it can be allowed to rise and fall around a center point, they can use slow PWM to emulate a mechanical thermostat.

I suppose both solutions are applicable under the right circumstances but which is optimal depends on the project requirements.

I would still use a Triac in preference to a Thyristor though.

Brian.

 

[FvM]

both solutions are applicable under the right circumstances

In the EU, EN61000 restricts the application of phase control for heating applications to power up to 200 W.
I guess, that similar regulations apply for the US and other countries.

I also don't agree, that PWM neccessarily involves temperature fluctuations. Even in analytic instruments, that are very
sensitive to temperature variations, e.g. gas chromatography column ovens, full wave control works with good results.
The reason is, that only few heatings "plants" have time constants below 1 sec. and effectively none below 200 ms.
In the latter case, the said SD modulator method still produces acceptable control quality.

 

[betwixt]

Binojkumar doesn't tell us what the coil is used for. Perhaps the load it imposes is below the 200W restriction.

Do you know why EN61000 was introduced? I have no idea although I suspect it is something to do with asymetric loading of AC lines. I remember trials many years ago when applying a DC offset to street lighting supplies was tried as a method of turning all the lights on or off from a central control box. It worked fine in theory but at the time it was commonplace to use SCR PSUs in domestic TVs and the half cycle loading they made on the mains supply was enough to unbalance the +/- AC cycles and the street lights followed TV program popularity!

Brian.

 

[FvM]

The title is Limits for harmonic current emissions. Avoiding possible interference with ripple control receivers is only one of several motivations.

Because phase control can't be easily eliminated in lighting control, it's still allowed there, but harmonic limits require some filtering. For applications, that effectively don't need phase control, the harmonic limits are so low, that the filtering effort would be inpracticable.