SSR vs SCR for heating element : what is the difference?

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Discussion Overview

The discussion revolves around the use of Solid State Relays (SSR) versus Silicon Controlled Rectifiers (SCR) for controlling Silicon Carbide heating elements in a furnace setup. Participants explore the feasibility of using an SSR in place of a transformer, considering the implications for voltage and current requirements, as well as safety concerns.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Exploratory

Main Points Raised

  • One participant suggests that an SSR could be used instead of a transformer, noting that it can decrease output voltage significantly, similar to a transformer.
  • Another participant points out that while SSRs and SCRs operate similarly, they cannot increase current when voltage is lowered, which could be problematic if the supply voltage does not match the heating element's requirements.
  • A third participant mentions that the SSRs described in the Celduc guide are SCR-based and capable of analog control, recommending the use of a 'zero crossing' variant for larger heaters.
  • One participant calculates the necessary power and voltage requirements for the heating elements, indicating a need for 110V and 30A to achieve the desired temperature of 1400C.
  • Another participant outlines the pros and cons of using SSRs versus transformers, emphasizing safety and reliability concerns, particularly regarding line voltage isolation and potential failure modes of the heating elements.
  • A later reply acknowledges the safety concerns raised and expresses a desire to avoid using a transformer due to logistical issues, but recognizes the necessity of carrying one for safety reasons.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility and safety of using SSRs versus transformers. While some acknowledge the potential for SSRs to work under certain conditions, others emphasize the importance of following manufacturer instructions and the associated safety risks. The discussion remains unresolved regarding the best approach for the specific application.

Contextual Notes

Participants mention the need for careful calculations regarding voltage and current, as well as the importance of safety measures when dealing with high voltage equipment. There are unresolved considerations regarding the specific requirements for the heating elements and the implications of using SSRs in place of transformers.

Panthera Leo
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Hello,

I have Silicon Carbide heating elements which should be used with a Step Down Transformer or SCR as mentioned in the guide from manufacturer.

I currently have a SSR available at hand, and was hoping it could be used instead.

The Solid State Relay when connected to PID can decrease the output voltage very significantly and seems like it's doing similar job as the transformer!

Any chance I could connect the SSR? I don't see why not?! I will highly appreciate your guidance.

This is the SSR's manual:

http://www.celduc-relais.com/all/pdfcelduc/PrincipeUK.pdf

This is Kanthal Silicon Carbide heating elements guidelines:

http://www.kanthal.com/Global/Downloads/Furnace%20products%20and%20heating%20systems/Heating%20elements/SiC%20heating%20elements/S-KA011-B-ENG-2011-06.pdf


Many thanks in advance.
 
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Hello Leo - The SSR works essentally the same way as the reccomended Thyristor circuit. ( Compare fig 9 Celduc to Fig 12 Kanthal).

If your AC source is approximately the same voltage range you want for the heater element, you idea can work, But if you have a 480 VAC source and the elements max V is 100V - then the Thyristor or SSR will not regulate very well - or if the source is 120V but 20 A and you need 40 -50 A to get the heat necessary, same issue. The thyristor or SSR can not "increase" the current when the Voltage is lowered the same way a transformer will.

You need to do the math and figure out how much heat you need, which element you want to use, then calculate what voltage needs to be applied and then how much current is needed.
 
The Celduc relay guide describes several styles of their SSR. They all appear to be SCR based. Apparently they are capable of analog control as you have suggested.

If these heaters are large you'll want to use a 'zero crossing' variant of the SSR as described on page 3 of the Celduc guide.
But read carefully that section on "fast cycle firing" starting on page 26 of the Kanthal guide.
 
Many thanks for your contributions... I highly appreciate it.


Windadct said:
You need to do the math and figure out how much heat you need, which element you want to use, then calculate what voltage needs to be applied and then how much current is needed.

I have calculated 5 watts/cm^2 to be the necessary power. The resistance of each element is 0.6 ohm & 6 of them will be used.

"A total of 110V & 30A" will be required to get 1400C

My supply is 220V & I can draw 30 Amps from this supply.

Can this work?
 
With phase angle control - yes "it can work" - the question is why not do per the Manufacturers Instructions there ar +s and -s:

+ Cost
+ Complexity ( mixed on this one Low voltage could use zero crossing control, to do 220 to 110 conversion - you need phase angle control)

- Line voltage Isolation
- Series (single point failure) vs Parallel operation ( some added reliability)
- controller failsafe(er) - if the controller runs foll 220 to the rods they may die.

There are MANY factors in determining the best solution for your application - in particular SAFETY has value and justifies added expence - ALWAYS. So - I would prefer the Isolation and lower voltage that the transformer provides. Below 50V for any equipment that people would touch - it is nearly impossible to predict the failure modes of systems.

When an electrode (heater) fails - what will happen electrically. I had the heater element in my home oven fail, and it almost killed me - because I "THOUGHT" I knew what happened - but I didn't KNOW. An I have 15 years field experience and was the shop safety coordinator working in power!
 
I see your point windadct... Very precise indeed. I do have an AC transformer but unfortunately I was willing to set up the furnace in a remote location, therefore I was hoping to set it up without a transformer...

But apparently there is no way out, I have to carry the transformer somehow :D

Many Thanks for the contributions