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

AI Thread Summary
Silicon Carbide heating elements require careful voltage and current management, and while Solid State Relays (SSRs) can function similarly to SCRs, they cannot increase current like a transformer. The discussion emphasizes the importance of calculating the necessary power, voltage, and current for effective heating, with a specific requirement of 110V and 30A identified. Using an SSR with phase angle control is feasible, but safety concerns arise regarding line voltage isolation and potential failure modes. Ultimately, while the SSR could work, the recommendation leans towards using a transformer for enhanced safety and reliability.
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
 

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