Category Archives: SCR Power Controls

NuWave Introduces New Solid State Relay Mount Temperature Controller

The new SSRTC mounts directly on the input terminals of a Solid State Relay and provides a low cost, conveniently packaged temperature control solution. The input accepts J or K type thermocouples and can be used on heat or cool processes with on/off or proportional control. The setpoint can be controlled by a potentiometer or analog voltage or analog current. The temperature setpoint is proportional to the voltage or current at the setpoint input.  Status indicators indicate how close the temperature is to the setpoint.

SSRTC-XXX Solid-State-Relay-SSR-Temperature-Control

Open vs Closed Loop Temperature Control

We get many calls from customers that want to control the temperature of a process without a closed loop temperature controller.  In this mini article we will briefly discuss open loop temperature control vs closed loop temperature control.  First, lets simply define closed loop and open loop as it applies to common electrically heated processes:

Close loop temperature control utilizes a temperature sensor that monitors the process.   This feedback is fed to a temperature controller which controls a Power Controller that will apply power to the heater precisely and maintain the process temperature.  Some of the advantages of closed loop control are accuracy, repeatability, fast warmup times and fast recovery times.

Open loop temperature control does not utilize temperature feedback from the process.  The power delivered to the heater is set by the user, usually with a potentiometer and SCR Power Controller or a Variac (variable auto transformer).  Some of the advantages of open loop control are simplicity, low cost, reliability.


One of the major advantages of NuWave SCR Power Controllers and SSR Driver Boards is line voltage compensation.  Line voltage compensation corrects for changes in line voltage.  Performance improvements can be observed in closed loop processes, but especially in open loop processes.


If you have a process where the temperature accuracy is not critical, open loop control may be an option.  The only way to really tell if open loop control is suitable is to try it in your process and see if the results are acceptable.

Phase Angle SCR Power Control with Current Limit – NEW

SCR Power Control with Current Limit
SCR Power Control with Current Limit

The new 1P-CL-N SCR power control offers features only found in much more expensive power controllers. The compact package can control up to 80Amps at 40C and offers current feedback. The current is sensed using an external current transformer and is used to limit the current on inductive loads such as transformers or resistive loads that have a sharp positive temperature coefficient such as quartz heaters or molybdenum heaters. Reliable firing / commutation of inductive loads are no problem with the on-board rugged SCR snubber.

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4-20 mA Solid State Relay SSR Pulse Module

The SSRMAN-1LP 4-20 mA Solid State Relay driver provides 0-100% PWM of any standard SSR.  The PWM cycle time of the time proportioning is 350mS at the center but gets longer toward the end of the range to optimize the resolution of the output of the 4-20 solid state relay system.  The calibration of the time proportioning converter can be adjusted by ordering option -ZS (zero and span pots). The SSRMAN-1LP can drive up to 4.2V @ 14mA offering a high performance, low cost solution to 4-20mA SSR systems.

4-20mA Solid State Relay
4-20mA Solid State Relay, Low Cost, high Performance Control

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New Low Cost RC Snubber for SSRs

Introducing the new RCSB RC snubber board for Solid State Relays SSRs.

RCSB Snubber Board

The RCSB snubbersuppresses dV/dt and provide reliable commutation. This is especially important when controlling transformer coupled loads as improper commutation can result in huge inrush currents and blown fuses.  The RCSB snubber is also used on our SCR power control assemblies. The RCSB is available in 120, 240, 480 and 600VAC versions.

The RCSB Snubber Board mounts directly on the output terminals of a solid state relay. The RCSB provides overvoltage protection from transients and limits dV/dt.

For more information: SSR Snubber Network 

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New Current Limit Phase Angle Controller Module SSRMAN-1P-CL

SSRMAN-1P-CL Phase angle control

The new SSR Mounted Phase Angle Controller Module SSRMAN-1P-CL mounts directly on the SSR input terminals and provides true linear power phase angle control of the load. The output power is based on the command input and the current feedback from the current transformer. The command can be dip switch selected from over seven different industry standard inputs. Using the SSRMAN-1P-CL with the appropriate SSR for your load will provide a modular high isolation SCR power control at a very low price.  An extensive filtering scheme and proprietary zero cross detection algorithm improves noise rejection without sacrificing accuracy.

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New low cost SCR gate driver

The new ZERO CROSS SCR GATE DRIVER NWZC-SCR, is a low cost SCR driver for driving back to back SCRs in resistive load applications.  The driver board is zero cross fired and has on board MOV protection.  It provides optically isolated 5V, 24V inputs that can easily be interfaced to temperature controllers and PLC Time Proportiong (PWM) outputs.  The packaging is available in panel mountable or din rail mountable snap tracks.

NWZC-SCR SCR Driver Board

SCRs requiring 200mA of gate current or less are a good fit for use with the ZERO CROSS SCR GATE DRIVER NWZC-SCR.  Wiring of the Module is simple and requires no external power supply; the gate drive power is derived from the line supply of the SCRs.

SCR Driver Board Wiring

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Phase Fired SCRs and Line Harmonics

Phase Fired SCRs can generate significant 3rd, 5th & 7th order harmonics above the line frequency due to the thyristor firing occurring between the AC zero crossings:

Phase Angle Control Waveform
Phase Angle Control Waveform

When Phase Fired SCRs must be used due to the load characteristics described in our previous blogs, EMI filtering can be added to the line to attenuate the harmonics.  Several companies makes suitable high current filters for Phase Fires SCRs:

By using the line filters with SCR Power Controls it is still possible to obtain suitable EMC performance and allow sales with a CE mark.

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Let’s Talk About Thyristor (SCR, TRIAC and SSR) Snubbers

Commutation Problems: When an SCR or TRIAC is used to Phase angle control an inductive load, the load current lags the mains voltage. When the device turns off at zero current, the rate of rise of the reapplied voltage can retrigger the device and produce half cycling and blown fuses. To limit this rate of rise and obtain reliable commutation, an R-C (resistor–capacitor) snubber circuit should be connected in parallel with the SCR/TRIAC.

dv/dt Problems: When voltage transients occur on the mains supply or load of an SCR/TRIAC it can cause the device to turn on unexpectedly due to the fast rate of rise of voltage (dv/dt). This can result in false firing and half cycling of the load that can cause blown fuses when driving inductive loads. An R-C snubber circuit will help to limit the dv/dt seen by the device and will produce more reliable thyristor firing.

Snubber Suppliers:  Check out our new RCSB Snubber: RCSB SNUBBER which is also available for SCRs and well as SSRs.

Snubber Sizing: Although most designers use an empirical approach to solving the aforementioned issues with snubbers, a number of great articles have been published on the mathematical basis for calculating snubber circuit values.  When an SCR/TRIAC using an R-C snubber turns on, the capacitor is discharged through the resistor into the device resulting in high peak currents. It is critically important when sizing your snubber to make sure that the resistor value does not become so low that the ratings of the SCR/TRIAC are exceeded when the capacitor is discharged.
MOVs and TVSs: Metal Oxide Varistors and Transient Voltage Suppressors are both used on TRIACS/SCRs to “clamp” voltage spikes that can occur across the devices and damage them. Snubbers are not a substitute for MOVs/TVSs and vice versa. Snubbers and MOVs/TVs should be used together to get reliable performance and long life from the SCR/TRIAC application.
Solid State Relays (SSRs): AC output SSRs use either SCRs or TRIACS internally, so this information applies to them as well. Even though many SSRs have internal snubber networks, we have found these to be insufficient for use in many inductive load applications. The addition of external properly sized snubbers has improved performance in many applications we have encountered when using our Phase Angle Controllers and SSRs for driving inductive loads.

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When to use Phase Angle Control vs Time Proportioning, Soft Start and Line Voltage Compensation

Phase angle controllers should be used when the following conditions exist:

  • The load requires voltage control
  • The load may not be able to tolerate full line voltage so a phase angle w/voltage limit may be employed
  • The load has high inrush characteristics and required soft start
  • The load is inductive
  • The load is sensitive to the thermal shock of time proportiong (burst firing)
  • The load is very fast
  • The load is a phase controllable motor or vibratory feeder

  Burst Firing (time proportioning or PWM) at the zero crossings is a good method to control the load power when the following conditions exist:

  • The load is a heating load and has sufficient thermal mass that the on and off pulses will not ripple through the process
  • Low EMI is required

Soft Start is a method of limiting the rate of rise of the output of a phase angle controllers power output generally during start up only.  Soft Change is a method of limiting both the rise and fall of of a phase angle control’s power output.  Both of these methods are used mainly to limit the load’s inrush due to either the resistance temperature coefficient of a heater lamp or because the load is inductive.  Soft start is useful on high inrush heaters such as Quartz, Molybdenum, Tungsten, or Graphite heaters.  Soft Change can be a good choice for phase angle controlling inductive loads, where rapid changes in the phase angle can induce a DC imbalance in the in load and thus cause misfiring, heating in the inductor, or blown fuses.

Soft start phase angle control limits inrush

Line Voltage Compensation in a Manually Controlled Heater Process:  In a simple manually operated phase angle control arrangement the user sets the power level of the phase angle controller and this delivers a percentage of the full power to the heater.  Ruling out any other process disturbances, the line voltage can change at any time affecting the power delivered to the heater dramatically (don’t forget P=E^2/R).  Line voltage compensation effectively cancels out the change in line voltage keeping the power delivered to the load exactly where the user sets it.

Line Voltage Compensation in a Closed Loop Heater Process:  In a closed loop heater process, a temperature controller such as a CAL 3200 sets the output level of the phase angle controller.  The temperature controller (if it is a PID controller) will correct for changes in line voltage, but these changes have to ripple through the process lags before they arrive at the controller.  This can affect the product quality, because often times the sensing point of the controller isnt perfect.  Line voltage compensation cancels the affect of line voltage changes before they arrive at the temperature controller. 

Line Voltage Compensation improves process stability

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