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DIY Controller?

edited February 2015 in Accessories

Hi all I have had a bit off a look on the forum after I was sent here from the shop as they could not quite answer my question. The shop said someone here would know.

I have a 3.6kw element which is over kill for what I need. Long story short if i use the DIY Controller and I run it at say 50% my element will be drawing 1.8kw. Will the other 1.8kw of power not going to the element be burned up in the SCR as heat? or will my total draw to the controller and onto the element only be 1.8kw? I like having the large element to heat up the boiler nice and fast but I don't need to use so much power during the whole time I'm running it. I can see in the drawings the POT controls the SCR. Does the POT switch the SCR on and off at a high frequency to control the element? I though POT's were just variable resistors. I would like to not have to waste extra electricity and further more Id like to run my cooling water as a slower rate and save some water too being that I am on rain water tanks.

This might be a silly question but Thanks in Advance

Travis

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Comments

  • No the voltage is blocked by the SCR. Just like turning a tap on and off really fast, what doesn't flow out when the tap is off doesn't get used.

  • Hi Mickiboi

    Thanks for the response It was doing my head in. So its basically chopping up the sin wave then. Its been a while since I played with SCR's I never controlled them with POT's though just microcontrollers and to shame myself being a mechanical electrician I should know this and I do just my mind was drawing a blank today big time. So the POT is chopping up the sin wave and that is turning the SCR on/off at what ever high frequency I dial the POT to. The 3.6KW element is still drawing 3.6KW for that very small time the SCR is on and not drawing when the SCR is off so the consumption of electricity overall is reduced and heating output is reduced.

    Thanks again Mickiboi :)

  • Sorry mickiboi, Nitris was right with his original way of thinking.

    I believe that the SD controller uses "phase angle control". Take a look at this pic which shows the output of a sine wave input at various phase angles / duty cycles: http://www.practicalcontrol.com.au/images/graphs/phase_control.png

    The power has to go somewhere. The portion of the sine wave that goes to the element (red in the pic above) will be used to generate heat in his boiler. The portion of the sine wave that is blocked (green in the pic above) is dissipated as heat - hence why we need heat sinks on the controller.

    Nitris, The pot simply a variable resistor that adjusts the controller duty cycle ie the ratio of time power is passed through to the element vs the time it is blocked & disposed of as heat. A pot cannot "chop up the sine wave."

  • Cheers Crozdog

    so if I run a 3.6kw element and run it at 50% the element will get 1.8KW but over all the current draw from my switchboard will still be 3.6kw? The extra 1.8KW will be burned up in the resistor and SCR heat sink as heat? Like I was saying I should now this but its been a while and I was doing my head in. Thanks in advance Crozdog

    Let me also say that this forum seams to have nice friendly members that don't chew your head off :)

  • Not all dragons breathe fire... B-)

    “Do I have to explain everything? Can’t you just be amazed and move on?”

  • edited February 2015

    I wonder weather it would be better for energy saving if I run a 240v to 120v step down transformer. On the 120v side where the element will be it is 15ohms it can only draw 8 amps because of the resistance. so I=v/r, I=120/15, I = 8A. If i convert that back through the transformer to 240. 120 x 2 is 240v and 8A / 2 is 4A. So on the 240v side id only be drawing 4amps. Does this look right? in this case i would only have to settings low and high but thats ok for me. It seams like I may be saving some power this way? Please correct me if I'm wrong. If I only get 8amps at 120v to go through the element at 15ohmes i can only get a max of 960w in the element. Fuck its confusing me.. maybe i'll have a good think and c if any suggestions come along.

  • Nitris, idk where you are located but in the USA I made up some pigtails that let me run a 5500 or 6000w element at 110vac so I get 1250 to 1500w with no controller for dialing in low heat.

  • edited February 2015

    He's in Australia running 240v guys.

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  • Whoaaaa, back the truck up a bit here guys.

    1st - the heat dissipated in the SSR is switching (internal I2R) losses and only occurs during the period of the sine wave that the device is actually switching (some micro seconds) as the internal resistance goes from bugger all of an ohm to meg ohms.
    When the SSR is fully conducting there are minimal losses. When the SSR is not switching there are NO losses as no current is flowing.

    If all the 'Non-switched' current was dissipated in the SSR then it would get red hot very quickly when the pot was at a minimum.

    The power consumed is the time average of what actually flows in the heater element. IF it is switched on for 1/2 a cycle it heats up for that period, and cools down for the other 1/2 cycle.

    It doesn't control the voltage at all, just the period that what voltage is available flows.

  • Whoops - That was a dumb statement "It doesn't control the voltage at all, just the period that what voltage is available flows."

    Voltage doesn't flow, current does as a function of applied voltage amplitude.

  • I was thinking to myself that if the kit is running a 5500 watt element and as some have stated the heat sink is dissipating whatever of those watts aren't going to the element then it would be white hot.

    Some of the statements in here don't make sense.

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  • Thanks for the responses so far guys, I'm keen to get to the bottom of this :)

  • It doesn't really matter if your controller is a phase angle controller or a burst fired controller. They are just different methods of doing the same thing.

    Switching the current on and off. Your controller is a variable switch. Also importantly it is not linear.

    Take my phase angle controller as an example. When you measure the output current and output voltage across the element, they BOTH change as the pot is adjusted. 50% ON may not be the same as 50% power. It doesn't really matter to some folks because they just wish to be able to repeat settings next run. They may not need to know the ACTUAL power being applied. If it is important, then you need to calibrate the controller - and repeat the calibration if you change an element. Or have a display that shows the current and/or voltage at the element.

    You need the heat sink because the current flowing through the controller WHEN IT IS CONDUCTING will heat up the components. If you don't dissipate that heat somehow you let out the magic smoke.

    The current drawn from the supply circuit does increase as you pass more of it through to your boiler.

  • edited February 2015

    No I'm not wrong. When a phase angle controller does not conduct the load voltage is zero. If the load voltage is zero then the power consumed must also be zero. The heatsink is used to carry heat away from the semi conductor when it is in the conduction phase, just like a transistor. ALL semi conductors produce heat when condcting. It makes no sense that a triac that is NOT conducting would consume 100% of its available power doing nothing. Im sorry but when its not supplying power to the load it is not consuming anything. See NXP website. When it is not in the conduction phase it is simply blocking the voltage just like a diode.

    To properly measure the power being used by a thyristor or triac you need a power meter like the EU9.

  • Please always keep the possible risks in mind when dealing with electricity, which makes me quote our disclaimer from our Terms of Service once again:

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  • @Mickiboi said: No I'm not wrong. When a phase angle controller does not conduct the load voltage is zero. If the load voltage is zero then the power consumed must also be zero. The heatsink is used to carry heat away from the semi conductor when it is in the conduction phase, just like a transistor. ALL semi conductors produce heat when condcting. It makes no sense that a triac that is NOT conducting would consume 100% of its available power doing nothing. Im sorry but when its not supplying power to the load it is not consuming anything. See NXP website. When it is not in the conduction phase it is simply blocking the voltage just like a diode.

    To properly measure the power being used by a thyristor or triac you need a power meter like the EU9.

    This makes sense to me and is written by someone who is employed directly in the filed.

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  • edited February 2015

    Cool so just to confirm if i run this DIY controller at say anywhere less then full load then the element will also be receiving less then full load. Further more from my switchboard the current draw to the circuit running the controller will be pulling less then full load which should be less the the full rating of the element in my case being 3.6KW? I know I said it up a bit but I am a Mechanical electrician I do this for a job. Just never used a POT to control a SCR so I'm having trouble getting my head around it. I normally have the gear lying around to build it just that I don't at the moment otherwise I would be able to test it all and get back to you all. Normally I use microcontrollers or a series of relays to turn scr's on and off. I will be careful with the advice given and use it with respect as doing the 50hz shuffle sucks. I guess in USA its the 50Hz shuffle.

    Thanks guys for the responses Keen to work this out. Travis

  • edited February 2015

    My controller is a SCR based unit, with a voltage and current meter on the output, it certainly adjusts both with turning the knob. Shown here is max output, close to 15amp or 3600w, can go down to under 100v and 3 amps or so, the box only gets warm.

    image

    I have a meter measuring my total household usage - it matches my still runs almost spot on. Run 2 x 3600w during warm up and household draw is usually 8.6kw or so, (7.2kw still, rest household stuff) switch off one element and bang it drops, turn down controller, same thing. I use it to estimate cost/credits on a solar system.

    fadge's Live Output

    The red line is my consumption, green solar generation, the top line temp (but wont go over 39c and we get plenty of 40+ days) This is only an example, I did not run the still today, but its the same thing.

    So without being an electrician or electrical engineer, looks like power output controlled = consumption or usage there abouts.

    fadge

    the_fadge_controller.jpg
    800 x 600 - 64K
  • @Nitris Yes to everything you just said above. The controller may use a little bit more power than the element, if it includes things like fans and displays, but it should be minimal.

  • @Myles said: Nitris Yes to everything you just said above. The controller may use a little bit more power than the element, if it includes things like fans and displays, but it should be minimal.

    A+ Myles

  • @Nitris - re the POT - the SSR is more than just a triac in the pack, it also contains the other electronics for controlling the firing circuit. They just bring the pot leads out of the package as you need a "power pot" because of the current required to fire the Triac.

  • I haven't used the SD diy controller kit but I have a few controllers of my own. The phase angle and burst fired controllers that i bought as encapsulated units seem to work fairly conventionally.

    My own component built PAC has one slight quirk that some of you may find in other controllers.

    I have to turn the pot up past a threshold value to get it to switch on. However, once it is conducting I can back it off again so it stays on, but at a value lower than that threshold. I don't know if it is because of the components I used or if it is because the components inside the encapsulated controllers use a different circuit configuration.

    All the encapsulated ones I have seen don't have the threshold, they just switch on.

  • @Myles - that is because of the trigger level of the triac is higher than the sustain level - it is common. Once the semiconductor has fired it requires less current to maintain conduction.

  • edited February 2015

    Ok thanks guys for the responses.. Nothing but good blokes here with good info. So long story short this DIY controller will do what i want it to do by the sounds of it. I like the idea of modding it to have a lcd display. Cheers to all that has responded. If anyone one else has their 2cents to throw in Id also like to hear it :) I was wrong though in usa ud be doing a 60Hrz shuffle not a 50hrz, that was a typo.

  • Fadge Im assuming you just wire that lcd in the load side of it all?

  • Yep the voltage is wired on the output, the current is donut sensor you just pass one of the output wires through it. I will see if l have a pic anywhere later on. I also have a indicator light on the output, aka element on light.

    Fadge

  • :)) @dellae yes I know that. I used to teach basic electronics to military engineering technicians. I was just pointing it out in case some folks were scratch building controllers and noticed a difference to the encapsulated versions. ;)

  • edited February 2015

    @Nitris said: Just never used a POT to control a SCR so I'm having trouble getting my head around it.

    I can appreciate that comment, I was in the same camp. I'd only used SSRs for switching duty previously, and wasn't familiar with the Phase Angle Control variant (which DO use a pot). Seemed odd to me too, I remember reading through threads scratching my head until I realized it had nothing to do with a standard SSR.

  • edited February 2015

    After reading all of this, it confirms what I've been thinking all along: Propane mo bettah! :))

    “Do I have to explain everything? Can’t you just be amazed and move on?”

  • @Kapea said: After reading all of this, it confirms what I've been thinking all along: Propane mo bettah! :))

    No way! Gas flames bounce more heat off the boiler than what goes in the boiler. Electric is fine tunable, gas only gets you close, at best.
    You ever change a bottle of electricity in mid-run? No. Ever run out of a bottle of electricity and discover your spare bottle is empty?
    Ran propane for a long time, switched to electric and was hooked - never ran on gas again.

    But back on thread, I find the heatsink gets a little hotter at the higher setting rather than the lower setting. Not blazing hot but noticeable. Any heat to the heatsink is wasted energy but I'd think it is minimal.

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