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

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#1 21680899 22 Oct 2018 17:51

Ruslan Tazhkenov Ruslan Tazhkenov

Anonymous

Post #1
21680899 22 Oct 2018 17:51

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.
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#2 21680900 23 Oct 2018 03:18

Rick Curl Rick Curl

Anonymous

Post #2
21680900 23 Oct 2018 03:18

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
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#3 21680901 23 Oct 2018 14:15

Ruslan Tazhkenov Ruslan Tazhkenov

Anonymous

Post #3
21680901 23 Oct 2018 14:15

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.
#4 21680902 23 Oct 2018 16:31

Rick Curl Rick Curl

Anonymous

Post #4
21680902 23 Oct 2018 16:31

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
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#5 21680903 23 Oct 2018 17:01

David Ashton David Ashton

Anonymous

Post #5
21680903 23 Oct 2018 17:01

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.
#6 21680904 23 Oct 2018 17:28

Rick Curl Rick Curl

Anonymous

Post #6
21680904 23 Oct 2018 17:28

Thanks, David.That makes a lot of sense.-Rick
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#7 21680905 24 Oct 2018 08:59

Ruslan Tazhkenov Ruslan Tazhkenov

Anonymous

Post #7
21680905 24 Oct 2018 08:59

Thanks a lot, David
#8 21680906 24 Oct 2018 09:19

Ruslan Tazhkenov Ruslan Tazhkenov

Anonymous

Post #8
21680906 24 Oct 2018 09:19

Thank you very much, Rick
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Topic summary

The discussion addresses the optimal firing mode for thyristor power supplies controlling Kapton heaters in a vacuum environment. Phase angle firing mode, while known to generate radio frequency interference (RFI) and harmonic distortion, is generally acceptable if no sensitive nearby equipment is present. Kapton heaters, typically with nichrome heating elements, exhibit low inductance and relatively stable resistance once heated, minimizing issues with phase angle control. Burst firing mode, defined as supplying whole half cycles of AC power at zero-voltage switching points, reduces RFI and harmonic distortion and suits heaters due to their thermal inertia. Both firing modes are viable; phase angle offers smoother power control but may cause more RFI, whereas burst firing minimizes electrical noise and distortion. The choice depends on the presence of nearby sensitive electronics and the importance of minimizing electromagnetic interference.
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Best Firing Mode for Kapton Heaters in Vacuum Using Thyristor Power Supplies

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