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We have a system at my work that controls a furnace via a NI cRIO. This circuit currently uses a SCR (model listed below) with the cRIO to switch power out to the furnace. I don't want to get into the weeds but I would like to swap out this SCR for an SSR (model listed below). Currently the SCR signal is limited to 25% of the maximum possible output via the cRIO. My coworker claims that at full signal, this SCR actually decreases the max amount of current that will pass through the furnace heating elements by 25% while the SSR will give 100% current for 25% of the time. This will require a full HAZOP because our max current draw will be 4x now.

I don't think this is true from my reading. I think the SCR would give 100% current 25% of the time, it'll just do it at a much higher frequency than the SSR and with lower thermal shock effects. Can anyone comment if my assessment is correct? It would be much more convenient on my end to use an SSR b/c that's what the rest of our equipment uses.

Link to SCR: WATLOW DIN-A-MITE DA10-24F0-0000

SSR part: CRYDOM D2425

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    \$\begingroup\$ The SCR is just a switch. It either gives you 100% or 0% for variable amounts of time, just like the SSR. There is no functional difference. \$\endgroup\$
    – Jonathan S.
    Commented Oct 30, 2022 at 18:57
  • \$\begingroup\$ The thing that makes me go in circles about this is with SCRs fast switching, is it possible it actually is reducing current by only using the high voltage section of the waveform (see Figure 1 in electronics.stackexchange.com/a/339759/325080), verses an SSR which can't do 60Hz and therefore has the full period of the waveform when it's switched on? \$\endgroup\$
    – user3430541
    Commented Oct 30, 2022 at 19:06
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    \$\begingroup\$ @user3430541 Note that SCRs cannot be turned off except by waiting for the AC power to reach zero. So that sort of switching isn't possible with an SCR, though it would be with a MOSFET or IGBT. However, that won't equate to lower current draw except when driving some specific types of loads. \$\endgroup\$
    – Hearth
    Commented Oct 30, 2022 at 19:09
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    \$\begingroup\$ Thanks for the link to my answer in the other question. The Watlow literature is a little vague but you have the F0 version which is analog input. I can't see how the time proportional control is done on the output. Is it zero-cross giving an integer number of full cycles during each period or is it adjustable phase angle. Their diagram shows it internally as two back-to-back SCRs but the Crydom module may be the same. I don't think your question can be answered without more detail on the control circuit. \$\endgroup\$
    – Transistor
    Commented Oct 30, 2022 at 19:20
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    \$\begingroup\$ @user3430541 There are functional differences between an SSR using a TRIAC and an SSR using two SCRs. (I feel 2 SCRs is better and it is what I use in my own discrete SSR designs. Won't debate that here, though.) I'm not sure where you are getting your 100% and 25% figures and I don't have time now to try and understand the misunderstandings that are likely involved. But just keep in mind that not all SSRs are the same. \$\endgroup\$
    – jonk
    Commented Oct 30, 2022 at 19:21

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It looks like your 'SCR' controller has analog phase control, while the SSR only has zero voltage switching on-off control. If so then your coworker is correct.

With phase control the heater receives an adjustable portion of each half cycle. At 25% max the rms and peak currents will both be much lower than if it was switched fully on.

Perhaps you can reprogram the temperature controller to produce a pulsed phase control output, and use an 'instantaneous turn-on' SSR which permits phase control. If not then you might need to reconsider your plans.

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