SSR vs. Contactor vs. SCR: Choosing a Power Controller for Electric Heaters

How-To
Written By David Schwanke

Your temperature controller decides how much heat the process needs. The power controller does the switching. Whether you pick a contactor, a solid-state relay (SSR) or an SCR power controller affects heater life, how tightly you hold setpoint, and how much heat the panel has to get rid of.

Why cycle time matters

A time-proportioning controller turns the heater fully on and off in a repeating cycle. The longer the cycle, the more the heater element heats up and cools down each time. Watlow's Educational Series Book Six: SCR Power Control gives these typical figures:

Device Typical cycle time (Watlow)
Electromechanical contactor 10 seconds or longer
Mercury displacement relay 4 to 5 seconds
SSR About 1 second
SCR, variable time base burst firing A few AC cycles
SCR, phase-angle firing One half AC cycle

Watlow's SCR brochure states the tradeoff plainly: a longer time base extends contactor life but shortens heater life significantly. In Watlow's accelerated life test (Book Six, Figure 20), heater life rises as cycle time falls.

Contactors: slow, but a true break

  • Positive circuit break. Watlow notes that a contactor physically opens the circuit, while solid-state devices "almost always have a small amount of leakage current." Watlow also states that safety limit controllers need that physical break.
  • Finite life. Watlow puts the projected life of a high-quality mechanical relay at about 100,000 cycles at full rated load, and up to 1,000,000 at one-third load. A short cycle time uses those cycles up quickly.
  • Size on the right rating. Carlo Gavazzi's CC22 is rated 40 A for resistive AC-1 duty (690 VAC max) but 22 A for AC-3 motor duty at 380 to 440 VAC. Heaters are resistive, so use the AC-1 column.

Best use: on/off duty and high-limit disconnects.

SSRs: fast, quiet, and heat-sensitive

Watlow SSRs switch near zero volts, have no moving parts and are optically isolated. For nichrome heaters, Book Six recommends a 1/2 to 1 second cycle time. Three things shorten SSR life:

  1. Heat. Watlow calls ambient temperature the most important factor in SSR service life. Carlo Gavazzi's RM1A datasheet shows the effect: an RM..25 carrying 25 A needs a heat sink of 2.70 °C/W or better at 20 °C ambient, and 1.98 °C/W or better at 40 °C.
  2. Ignoring derating and spacing. The Carlo Gavazzi RGC..25, which has a built-in heat sink, is rated 30 A (AC-51) at 25 °C but 25 A at 40 °C. Its datasheet also has derating curves for side-by-side mounting.
  3. Wrong load or fusing. The same RGC..25 is rated just 5 A for AC-53a motor duty. The datasheet lists a minimum I²t for fusing (1800 A²s) for coordinating the protective device.

Both the RGC and RM datasheets list off-state leakage below 3 mA, so an SSR is not a disconnect.

SCR power controllers: longest heater life, most options

Watlow describes SCRs as able to switch loads up to 1200 A. They accept either a time-proportioned signal or a process signal such as 4-20 mA or 1-10 VDC. There are two firing methods:

  • Variable time base (VTB) burst firing switches at zero cross in bursts of a few cycles. Watlow recommends it for all Watlow heaters and reports heater life equal to phase-angle firing, without phase-angle firing's electrical interference.
  • Phase-angle firing trims every half cycle. Book Six says it is required for tungsten, molybdenum, silicon carbide and graphite elements, whose cold resistance can be a small fraction of the hot value (about 1/15 for tungsten), so soft start is needed. It is also required for transformer-coupled loads.

Book Six adds two cautions. Using an SCR only as an on/off contactor gains you little. And current limiting can't save an SCR from a short, so always fuse SCR controllers with I²t (semiconductor) fuses.

Watlow's DIN-A-MITE family covers much of this range. The Style B handles up to 40 A single-phase at 600 VAC. The Style C covers 30 to 80 A and offers phase-angle firing on single-phase models. The Style D handles up to 100 A single-phase at 600 VAC at 30 °C. Ratings fall as enclosure temperature rises, so check the rating curves.

Match the device to the controller output

On the Watlow EZ-ZONE PM PLUS (spec sheet WIN-PMPLUS-0125):

  • Output C, switched DC (22 to 32 VDC, 30 mA max per output): drives SSR DC inputs, or a DIN-A-MITE with the C0 (4.5 to 32 VDC) input. Add up the input current of every SSR on the output.
  • Output F, universal process (0-20 mA or 0-10 VDC): drives a 4-20 mA DIN-A-MITE (F0 input) for VTB burst firing.
  • Outputs E/J, 5 A mechanical relay (125 VA pilot duty at 120/240 VAC): drives a contactor coil. Check the coil rating against that figure.

Quick selection guide

Need Consider
Safety/high-limit disconnect Contactor driven by a limit controller
Standard resistive heater Heat-sunk SSR, 1/2 to 1 s cycle time
Longest heater life, higher current, 3-phase VTB burst-firing SCR on 4-20 mA
Tungsten, moly, SiC, graphite, transformer loads Phase-angle SCR with soft start

Wherever uncontrolled full power could cause damage, back up any solid-state device with a limit controller and a contactor.

Need help sizing a power controller, or want it built into a tested control panel? Entherm stocks Carlo Gavazzi SSRs and contactors and Watlow DIN-A-MITE and EZ-ZONE controllers, and builds custom panels in Burnsville, MN. Contact sales@entherm.com or 1-877-368-4376.

Manufacturer literature cited

  1. Watlow, "An Essential Guide to Power Controllers" (blog). https://www.watlow.com/en/blog/posts/essential-guide-to-power-controllers
  2. Watlow, Educational Series Book Six: SCR Power Control (WE6E). Fetched from a distributor mirror: http://www.i-marshall.co.kr/catalog/watlow/w_con02/acrobat_user_manuals/WE6E_16.pdf. Webster: link a watlow.com copy if one can be found.
  3. Watlow, "SCR Power Controllers" brochure. Fetched from a distributor mirror: http://www.i-marshall.co.kr/catalog/watlow/w_con02/acrobat_user_manuals/scr.pdf
  4. Watlow, DIN-A-MITE B product page. https://www.watlow.com/products/Controllers/Power-Switching-Devices/DIN-A-MITE-B-Power-Controller
  5. Watlow, DIN-A-MITE C product page. https://www.watlow.com/products/Controllers/Power-Switching-Devices/DIN-A-MITE-C-Power-Controller
  6. Watlow, DIN-A-MITE D product page. https://www.watlow.com/products/Controllers/Power-Switching-Devices/DIN-A-MITE-D-Power-Controller
  7. Watlow, PM PLUS specification sheet WIN-PMPLUS-0125. Fetched from a distributor mirror: https://watt.solutions/wp-content/uploads/2025/02/PM-PLUS-Controller.pdf. Official path: https://www.watlow.com/-/media/documents/specification-sheets/win-pmplus_rev-4_0125.ashx (it returned a server error to our fetch).
  8. Carlo Gavazzi, RGC..U solid-state contactor datasheet (01/11/2022). https://www.gavazzionline.com/pdf/RGC_U.pdf
  9. Carlo Gavazzi, RM1A solid-state relay datasheet. https://api.gavazziautomation.com/fileadmin/images/PIM/DATASHEET/ENG/RM1A.pdf
  10. Carlo Gavazzi, CC-series Contactors, Overloads and Manual Motor Starters brochure. https://carlogavazzisales.com/pdfs/ContactorBrochureCCseries.pdf
David Schwanke
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