Announcement

Collapse
No announcement yet.

Relay outputs capable of PWM?

Collapse
X
  • Filter
  • Time
  • Show
Clear All
new posts


  • Relay outputs capable of PWM?

    Hello All,

    I am planning to buy a Do-More BRX PLC to drive a stepper motor since it has the High speed I/O. One of the other tasks I want to do with the PLC is to control a resistive heater cartridge using PWM and PID control. The temperature sensor will be wired to the analog input.

    Initially, my idea was to use a solid-state relay and to switch it ON and OFF using the PWM digitial output from the PLC to achieve PID control of temperature. The digital outputs of BRX PLC are capable of PWM.

    PLC ---> Solid State Relay -----> Heater

    I also noticed that PLCs have relay output. Can I switch ON/OFF relay output in a PWM fashion and avoid the use of solid state relays?

    PLC with relay output -------> Heater

    Please advice


  • #2
    If you are switching slowly, you won't kill the relay as fast, but I would definitely use SS.

    You don't really need PWM for PID temp control. Use the TIMEPROP instruction.

    Comment



    • #3
      Right. So all on/off control ends up as PWM whether you need it or not. If your heating load is at 32% of available wattage, and you log on time over an hour or whatever, even on an on/off control, then shazam, it will come out to be 32%. So to decide on your PWM timebase (whether explicit in a PID or implicit in on/off control) is a question of the balance between the load thermal inertia and the acceptable control window for temperature. If the load changes temperature very quickly and you have a tight window, you're going to need a short timebase, which would kill a relay. If the load changes temp slowly, like 1 deg F per minute or something, and/or you have a broad window of temps that are OK, you should be OK with a long timebase or on/off control.

      But....if you need high speed outputs for your stepper driver, I don't think they're available on the relay out PLC's anyway. At least that was the case in the 05/06.

      Comment



      • #4
        On BRX all built-in outputs are homogeneous. So yes, if you want high speed, you need DC.

        Comment



        • #5
          OTOH if you need a 60HZ output from your BX-DM1E-36AR3, you can get that even when the output is not on. Plenty enough leakage to light an LED pushbutton lamp from Automation Direct. Imagine our delight!

          OK, I am not delighted at all. First and last relay output for me.

          Comment



          • #6
            Originally posted by Mike Nash View Post
            OTOH if you need a 60HZ output from your BX-DM1E-36AR3, you can get that even when the output is not on. Plenty enough leakage to light an LED pushbutton lamp from Automation Direct. Imagine our delight!

            OK, I am not delighted at all. First and last relay output for me.
            It's to prevent noise from causing far worse issues. Sorry it was a problem.

            Comment



            • #7
              Originally posted by BobO View Post

              It's to prevent noise from causing far worse issues. Sorry it was a problem.
              I get it (protecting your product), but it isn't "nice" and it's one of the reasons for skipping triacs in the first place. Passing current of any value for a "relay contact" output is unacceptable. It absolutely does not play well with LED indicator lamps and is not a negligible amount of current if it is difficult to know if a status indicator is showing on or off unless another nearby is brighter or dimmer. And a green LED is much, much brighter than a white with this leakage.

              Comment



              • #8
                Originally posted by ControlsGuy View Post
                But....if you need high speed outputs for your stepper driver, I don't think they're available on the relay out PLC's anyway. At least that was the case in the 05/06.
                BRX PLCs have inbuilt high speed I/O capable of PWM. My understanding is that this can be used for both stepper driver and PID control through SSR. I am planning to use AC load switching SSR driven from DC output from PLC.


                Comment



                • #9
                  Originally posted by BobO View Post
                  If you are switching slowly, you won't kill the relay as fast, but I would definitely use SS.

                  You don't really need PWM for PID temp control. Use the TIMEPROP instruction.
                  What is the difference between TIMEPROP instruction and PID instruction? I need precise temperature control at setpoint. I have used ON/OFF or Bang-Bang control before. It is a little lousy for my application.

                  Comment



                  • #10
                    Originally posted by RockB View Post
                    BRX PLCs have inbuilt high speed I/O capable of PWM. My understanding is that this can be used for both stepper driver and PID control through SSR. I am planning to use AC load switching SSR driven from DC output from PLC.
                    Originally posted by RockB View Post
                    What is the difference between TIMEPROP instruction and PID instruction? I need precise temperature control at setpoint. I have used ON/OFF or Bang-Bang control before. It is a little lousy for my application.
                    PWM is a high speed function, generally faster than the AC line frequency. If you aren't careful, the AC line freq and the PWM freq can phase weirdly, and for temperature control, you probably don't need a 1Khz PWM. That's why I said TIMEPROP. TIMEPROP is an output function that takes 0-100% from a PID function output, and converts it to a time proportionate output, which is essentially slow PWM. PWM is a microseconds to milliseconds period, while TIMEPROP is milliseconds to seconds.

                    If you want to use a high speed output with a PWM function, that's perfectly fine, but it probably isn't necessary for temperature control.

                    Comment



                    • #11
                      Originally posted by Mike Nash View Post

                      I get it (protecting your product), but it isn't "nice" and it's one of the reasons for skipping triacs in the first place. Passing current of any value for a "relay contact" output is unacceptable. It absolutely does not play well with LED indicator lamps and is not a negligible amount of current if it is difficult to know if a status indicator is showing on or off unless another nearby is brighter or dimmer. And a green LED is much, much brighter than a white with this leakage.
                      I will flog engineering and suggest they consider the error of their ways.

                      Comment



                      • #12
                        Originally posted by BobO View Post

                        I will flog engineering and suggest they consider the error of their ways.
                        Well, in that case, I feel better. But I still won't use a relay output with AC again. This really is a serious issue though. I am going to have to order incandescent bulbs for these operators and field install them.

                        Comment



                        • #13
                          Originally posted by Mike Nash View Post

                          Well, in that case, I feel better. But I still won't use a relay output with AC again. This really is a serious issue though. I am going to have to order incandescent bulbs for these operators and field install them.
                          The nominal leakage is about 100uA at 60Hz, coming through a 2200pf snubber cap. It's hard to believe that would turn on a LED bright enough to matter. I wonder if there is some higher frequency noise on the AC.

                          Comment



                          • #14

                            LOL, I won't use anything BUT relay outputs with AC loads, for like 20 years now. I've found that they're plenty reliable, actually moreso than triacs, which I despise not only for poor reliability but the noted current leakage. I've had excellent success with relays, though I'm typically driving solenoids, contactors and other pilot loads with a 120vac relay output, but also including the occasional pilot lamp, with no issues. For me, though, I'd typically have a pilot light on a DC output, so I may not have done those enough to run across this issue.

                            Comment



                            • #15
                              Originally posted by BobO View Post
                              for temperature control, you probably don't need a 1Khz PWM
                              You certainly don't need a 1kHz PWM for temperature. Even IR lamps, the most demanding heating loads I'm familiar with for timebase, because of thermally-induced fatigue, do just fine on an 8ms time base (phase angle fired).

                              Comment

                              Working...
                              X