Best Firing Mode for Kapton Heaters in Vacuum Using Thyristor Power Supplies

Hello to everyone,I'm going to use thyristor power supplies to control the power delivered to kapton heaters which will be located in vacuum environment (big vacuum tube). What kind of firing mode of power supply is best suited for such application? I already know that the main disadvantage of phase angled firing mode is its vulnerability to RFI and harmonic distortions, however I'm still confused is it essential factor in our application or not. I've already adviced to use Burst firing mode as  Kapton heaters (Polyimide) are probably not inductive or very low resistance on start up while I still don't understand what's the best firing mode for sure.Thank you very much in advance.

Hi Rusian-It's not that phase angle triggering is susceptible to RFI and harmonic distortion- is that it causes both of those. As long as you don't have nearby equipment that would be affected by some RFI (and it's probably not very much RFI), then I think you'll be fine. Phase angle triggering uses a very simple circuit. By the way- the insulating material in the heaters is Kapton. The heating element will be another material, perhaps nichrome wire embedded in the Kapton.  you're correct in that the heaters will not be very inductive. When you mention that the resistance will be low on startup- that is often true, but the biggest change in resistance happens when the heating elements begin go glow red. If you won't be getting the heaters that hot then the resistance change may be minimal.-Rick 

Thank you very much for your response Rick,Does it mean that phase angle firing mode is more preferable as it gives smoother power distrubition in case there are no any  electrical devices nearby which could be affected by RFI? The fact is that we want to buy thyristor supply and we need to choose its firing mode while the cost is the same for all firing modes. 

When you refer to "burst firing", I **think** you're referring to  a scheme where multiple pulses are fed to the gate of the triac during the time it should be on.  This is sometimes done when there is a heavily inductive load and there is the possibility that the triac could turn off when it is not supposed to. I could be wrong about that, though.Perhaps someone who is more knowledgeable than me will chime in on this.I think like either firing method will be fine since the load is not inductive.-Rick

Rick - I have heard the term "burst fire" referring to power control with whole half cycles fed to the load, implying zero-voltage switching at the crossover point of the AC waveform.  It results in very low RFI and distortion as there are no large steps in the current.  It's particularly suitable for heaters as they usually have a fair bit of thermal inertia and feeding them X half cycles out of say 50 or 100 gives adequate control, and the high currents of heaters usually give rise to RFI and other problems with phase control.I found quite a good explanation here.

Thanks, David.That makes a lot of sense.-Rick

Thanks a lot, David

Thank you very much, Rick