Announcement

Collapse
No announcement yet.

Sending PWM through a relay

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

    Sending PWM through a relay

    Long story, but some of my parts for a reflow oven have arrived, but I think I can get it done with my remaining parts. I know a standard relay can't directly switch a PWM signal, but if I have one SSR controlling the PWM signal, and three magnetic relay downstream of that to control three seperate heaters, there shouldn't be a problem with the PWM flowing through the magetic relays, right?

    Said another way, the SSR will handle the PID/pwm and the three relays will be turned on or off based on how fast the heat needs to rise in that section of the heat profile.

    To my understanding, arcing of magnetic relays is caused when the contacts are close, but not touching. Would sending a PWM signal through a closed relay cause any issues?

    #2
    Re: Sending PWM through a relay

    Im not sure what you mean. Through the contacts or coil?

    Through the contacts is fine. As a note, best practice would say to switch off at a zero crossing of the PWM circuit or AC line if you are using the PWM to modulate an AC signal.

    Through the coil isn't the best idea, as it can lead to arcing of the contacts
    Last edited by goontron; 03-15-2020, 09:18 AM.
    Things I've fixed: anything from semis to crappy Chinese $2 radios, and now an IoT Dildo....

    "Dude, this is Wyoming, i hopped on and sent 'er. No fucking around." -- Me

    Excuse me while i do something dangerous


    You must have a sad, sad boring life if you hate on people harmlessly enjoying life with an animal costume.

    Sometimes you need to break shit to fix it.... Thats why my lawnmower doesn't have a deadman switch or engine brake anymore

    Follow the white rabbit.

    Comment


      #3
      Re: Sending PWM through a relay

      Exactly, your verbiage is much better. I'm planning on sending PWM through the contacts of the three magnetic relays, not the coils. The coils couldn't handle such fast switching, we are probably talking about multiple hz. I think I have a zero-crossing SSR, so that would be better for the contacts also.
      Last edited by clearchris; 03-15-2020, 09:20 AM.

      Comment


        #4
        Re: Sending PWM through a relay

        Originally posted by clearchris View Post
        Exactly, your verbiage is much better. I'm planning on sending PWM through the contacts of the three magnetic relays, not the coils. The coils couldn't handle such fast switching, we are probably talking about multiple hz.

        Much clearer!


        Some more notes to make:


        Contact "Bounce" will mess up anything that "tracks" the PWM signal. For a modulated load, such as a heating element, this is not a concern. Usually only when timing is involved, so this doesn't apply if you are switching load(s) that are being fed PWM modulated power, however for SIGNALS it does.



        The best time to switch on or off the relays is when the PWM is low/off as it will eliminate arcing of the contacts on the mechanical relay
        Things I've fixed: anything from semis to crappy Chinese $2 radios, and now an IoT Dildo....

        "Dude, this is Wyoming, i hopped on and sent 'er. No fucking around." -- Me

        Excuse me while i do something dangerous


        You must have a sad, sad boring life if you hate on people harmlessly enjoying life with an animal costume.

        Sometimes you need to break shit to fix it.... Thats why my lawnmower doesn't have a deadman switch or engine brake anymore

        Follow the white rabbit.

        Comment


          #5
          Re: Sending PWM through a relay

          Good point on the contact bounce issue. Fortunately, I can configure the magnetic relays, then start up the PMW through the SSR, sending the power through the closed magnetic relay contacts.

          I'm not planning on directly tracking the PWM in any manner aside from a k-type thermocouple to measure the resulting temperature of air.

          Comment


            #6
            Re: Sending PWM through a relay

            People even send RF through relay contacts...

            Comment


              #7
              Re: Sending PWM through a relay

              You can't use high frequency PWM to control a heater, that is kind of a notion from the Arduino crowd.
              This is because the heater has a big thermal mass and slow response time, it doesn't react to say 1kHz. If a mains SSR is zero-cross it needs up to 8.67msec to turn on/off anyway which limits your PWM carrier frequency.

              The common practice is (including using a mechanical relay) to turn on the heater for a few seconds, then off for a few seconds. You don't need high speed switching on/off. It can also be many AC cycles on, many AC cycles off I have seen from some PID controllers.

              Long life is to have an SSR in series with mech relays, and first turn on the mech relays then say 50msec later turn on the SSR. Then turn off SSR then mech relay. This way a mech relay never switches any current and lasts a long time. But who needs over 100,000 cycle life on their reflow oven

              Comment


                #8
                Re: Sending PWM through a relay

                redwire: Well, we do PID control at least on 3d printers with heaters.

                You could be right about only needing mechanical relays to control the oven, but I haven't seen a single person implement that, so I'm hesitant to buck the trend. Everyone uses SSR for some reason.

                And yeah, I don't need super long life on my reflow oven, if I use it 100 times, that's going to be a lot.

                Comment


                  #9
                  Re: Sending PWM through a relay

                  Probably because SSRs are "the thing" since it's newer and quieter, but I do wonder about losses. I probably suspect newer SSRs are possibly using MOSFET switches nowadays but even them and those using TRIACs probably have significant VI and IR losses where mechanical relays are pretty consistent until they burn out.

                  Comment


                    #10
                    Re: Sending PWM through a relay

                    Originally posted by eccerr0r View Post
                    Probably because SSRs are "the thing" since it's newer and quieter, but I do wonder about losses. I probably suspect newer SSRs are possibly using MOSFET switches nowadays but even them and those using TRIACs probably have significant VI and IR losses where mechanical relays are pretty consistent until they burn out.
                    I would think using an SSR to make the connection, pull in the mechanical contactor, then dropping out the SSR would still prevent the losses and save the life of the contactor. Inverse for disconnection of the circuit of course.
                    Things I've fixed: anything from semis to crappy Chinese $2 radios, and now an IoT Dildo....

                    "Dude, this is Wyoming, i hopped on and sent 'er. No fucking around." -- Me

                    Excuse me while i do something dangerous


                    You must have a sad, sad boring life if you hate on people harmlessly enjoying life with an animal costume.

                    Sometimes you need to break shit to fix it.... Thats why my lawnmower doesn't have a deadman switch or engine brake anymore

                    Follow the white rabbit.

                    Comment


                      #11
                      Re: Sending PWM through a relay

                      I suspect there would be some point a hybrid relay as described would make sense to use as both subunit relays need to carry full load without damage, but such device would add significant cost to the BOM and still no faster than the slowest part - the mechanical portion.

                      Comment


                        #12
                        Re: Sending PWM through a relay

                        A disadvantage of SSR's is they dissipate about 1W per amp, which is a lot of heat to get rid of. A mechanical relay wastes much less heat, no heatsink required.

                        Either way, use low frequency PWM with a carrier frequency in the seconds. Seconds on/seconds off kind of thing to control heat output.

                        Comment


                          #13
                          Re: Sending PWM through a relay

                          Yeah, I was wondering about that, I suspect the SSR's I pulled from dead devices are of low current rating and thus do not have heatsinks. But a 1KW+ heater in a reflow oven, unless you're running at 240V+, at 120VAC the IR and VI losses in a SSR will be significant (though less than 1% of total system energy usage.)

                          TRIAC power loss is on the order of 1.1*I + gate current which is usually minimal
                          MOSFET power loss is on the order of 0.7*I+0.02*I² (Note there's an intercept point between mosfet and triac depending on current...)
                          Mechanical relay power loss is on the order of 0.02*I² plus coil power loss.
                          Of course these constants dependent on devices used.
                          Last edited by eccerr0r; 03-15-2020, 06:24 PM.

                          Comment


                            #14
                            Re: Sending PWM through a relay

                            In other applications, I might worry about dissipation, but I don't think it's going to affect this one much. Either I'm going to have enough power, or I won't. Right now, it's a regular toaster oven (standard resistive heaters) and I'm adding in two 500w halogen lamps. I'll probably have to put an aluminum shield in so the dark chips don't get baked, but I think it will work, and all with very common parts. I'm trying to keep it under 20a, but I do have a 30a circuit in my garage, so if I need more amps it's inconvenient but doable.

                            Most quality SSRs (not fotek) have two ratings. One amp rating for no heat sink, and another with a large heat sink. My carlo gavazzi SSR is rated at 5amp no sink and 25a with a large heat sink. If there's only one rating on the SSR it's usually for the large heat sink.

                            Incidentally, the carlo gavazzi SSR did fail in it's prior application, I opened it up, and there was a bad solder joint on a small transistor.

                            Comment


                              #15
                              Re: Sending PWM through a relay

                              I have one like this:

                              https://www.aliexpress.com/item/32656181441.html

                              Comment


                                #16
                                Re: Sending PWM through a relay

                                When dealing with 120V 25A (3KW) circuits it's best to consider 240V across-both-phases for new design to reduce I*R losses.

                                Honestly after reading this thread and doing some analysis I'd stick with a mechanical or at most a hybrid relay (if it were mission critical) for 3KW applications, if I were forced to use 120V. At 240V I may dabble with SSR but possibly not even that, 480V SSR is a consideration but I don't have 480V readily available in my house.

                                Comment


                                  #17
                                  Re: Sending PWM through a relay

                                  Well, if it's available, 240v can be better. I'm trying to do this with off the shelf readily available parts though. 240v heating elements in a small size are a bit harder to come by.

                                  At any rate, my latest testing indicates that I'm going to need to close the gap with a lot of insulation. My temperature rise with 1200 watts of resistive heaters and 500w halogen was fairly abysmal, but I could feel the heat radiating from the outside of the oven. I may not need the second 500w halogen bulb (I probably will), but for sure I'm going to need to seal the oven better.

                                  If this does work, it should be a lot easier to make a reflow oven, as these halogen bulbs are very cheap and widely available.
                                  Last edited by clearchris; 03-16-2020, 10:00 PM.

                                  Comment


                                    #18
                                    Re: Sending PWM through a relay

                                    I also worry about the halogen bulbs whether they need cooling for longevity?

                                    Anyway agreed, that's another thing I notice about re-purposing toaster ovens as reflow ovens is that the insulation of toaster ovens is miserable. On the positive side it helps cooldown phases ... by the way how are you doing the cooldown phases and are they even fast enough, which would be hurt with insulation unless you have some other way to cool down?

                                    Comment


                                      #19
                                      Re: Sending PWM through a relay

                                      Instead of increasing the power, I would look at where the heat is going. Heat can be transferred any of three ways - conduction, convection, radiation. Learning those three makes life much easier. You might need a convection fan or insulation. Stainless steel reflects a lot of heat and not great for an oven.
                                      I think halogen bulbs are very high temperature over a small area and would make hot spots in the oven, instead of nichrome wire heating elements which run cooler over a larger area.

                                      I don't know what size your oven is but 240V is like a range baking oven.

                                      Comment


                                        #20
                                        Re: Sending PWM through a relay

                                        Yeah, I did some research about halogen bulbs. They are basically the same as "quartz heaters" except the quartz heaters are red instead of clear for halogen bulbs. Halogen bulbs give off about 3.5% of watts as light, the rest is "waste" heat. It just so happens that I'm pretty happy with 96.5% heat for a few bucks. I may end up putting the halogen bulbs in some perforated metal tubes, that seems to be common for quartz heaters.

                                        I just hooked up the fan, I thought it was going to need a speed control, but the fan isn't terribly strong and there are apparently guides in the unit that take air in from the middle and send it out the top and bottom, near the heaters. I couldn't ask for a better setup. I'm curious how much it actually affects the heat though.

                                        You can see in the pic my first cut at sealing the oven with high temp RTV silicone. You are certainly right about these ovens being poorly insulated. There's zero insulation and the seams aren't well sealed either.

                                        If it doesn't cool down on it's own fast enough after being insulated, I could easily rig up a stepper and leadscrew to open the door.

                                        I'm planning on running this with a 3d-printer software, klipper. Having gcode specified heat profiles sounds very appealing to me.

                                        Attached Files

                                        Comment

                                        Working...
                                        X