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Touch switch for heating element

Discussion in 'General Electronics Discussion' started by Goog, Mar 21, 2016.

  1. Goog

    Goog

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    Mar 21, 2016
    Hi all, new to all thus stuff, so bare with me lol.

    I'm trying to find out if I can control a 2400w heating element using touch switches/capacitive switches.
    Eg: touch one switch to increase wattage, touch another switch to decrease wattage. Like a pot but using touch switches. Hope.im making sense!. I'm in nz, so it's 240v. Thanks for any help.
     
  2. cjdelphi

    cjdelphi

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    Oct 26, 2011
    What's it currently using? A switch? chances are the switch is not carrying the current it merely switches on a triac ir relay and that carries the high voltage/current

    If so then yes, a capacitive button could trigger it...
     
  3. cjdelphi

    cjdelphi

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    Oct 26, 2011
    As to vary the current, the switches would have to talk to a micro and then adjust the pwm to the triac...
     
  4. Goog

    Goog

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    Mar 21, 2016
    It's not setup. Starting from scratch. I like the sound of your last comment, just what im after. But, I have no idea what's what sorry, im not electrically minded lol. Appreciate your help, thanks.
     
  5. hevans1944

    hevans1944 Hop - AC8NS

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    There are basically two ways to control line power to a heater load: phase control or duty cycle. Phase control is a form of duty-cycle control applied to conduction in a triac over a complete AC cycle. You delay the trigger applied to the triac during each half-cycle to control its conduction from zero to 180 degrees in the first half-cycle and from 180 degrees to 360 degrees in the other half cycle. The delay is measured from the zero-crossings of the AC line voltage. Phase control has the disadvantage that turning on the triac in the middle of a cycle creates humongous amounts a radio-frequency interference (RFI) that must be suppressed, but it is usually quite simple to implement.

    Duty cycle control passes an integer number of complete cycles over a fixed period of time, say one second for example. Each cycle begins at the zero-crossing of the AC waveform and the number of complete cycles that are allowed to pass to the load in a fixed period of time determines the average power delivered to the load. So, if the line frequency is 50 Hz, there will be a maximum of fifty complete cycles passed to the load in a one second interval for a 100% duty cycle. If only every other cycle is passed, then a 50% duty cycle occurs. OTOH, if only one cycle is passed every second, then the duty cycle is 1:50 or 2%. Clearly the duty cycle can be varied from 0% to 100% by passing from none to fifty complete cycles every second. For heater loads with a lot of thermal inertia, duty cycle control has the advantage of virtually no RFI generation as the triacs are always triggered (when the are triggered) at the zero-crossings of the AC line voltage.

    There are commercial modules available that implement both phase control and duty-cycle control and they are typically used with a PID controller to maintain temperature at a particular set-point. Your up/down buttons would program the temperature, not the phase angle or duty cycle.

    Could you tell us what you are trying to DO and why a commercial off-the-shelf solution would not be less expensive?
     
  6. Goog

    Goog

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    Mar 21, 2016
    Thanks.for the explanation.hevans, I'll need to re read rant a few times to.get my head around it.
    I'm a distiller, it's legal where I am (new Zealand) , I want to build a controller for the element in my boiler. Most use ssr and a pot for element control. I want to eliminate the pot and use touch switches, just because I like things different and love tinkering and learning. I really struggle with understanding the electrical side if things though, it's my nemesis lol. Could you point me toward a off the shelf item please?, I have found nothing.
     
  7. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    I have been told that some of my ancestors (or least neighbors) operated 'stills in the hills of West Virginia in the previous century without benefit of government supervision or permission, which was (and still is) a risky business. They didn't use electricity to heat their boilers however.

    It is a good thing you like to tinker and learn because I wasn't able to find an inexpensive up/down power controller operated by push-button switches. Up/down temperature controllers, yes, some affordable and some not so affordable. So I am thinking a do-it-yourself approach with some off-the-shelf parts is a reasonable approach.

    There are a LOT of members here that struggle with understanding the electrical side of things. That's why this community exists: to help each other understand things. If you stick around, and start a project, you can count on at least someone here who has the expertise to help see you through to end.

    I don't suppose temperature control is an issue with a boiler running at atmospheric pressure, since the boiling temperature is more or less constant, so push-button up/down power control will be sufficient. The problem is somehow "remembering" the power setting between button pushes, which a potentiometer does nicely. That may be why most off-the-shelf controllers have gone digital, and you should too for this DIY project.

    For this sort of thing, I usually turn to Omega Engineering for the heavy lifting. Why re-invent the wheel when inexpensive wheels abound? Besides the solid-state relay (SSR), what you need is a pulse-proportional controller that mounts on the SSR and tells it when to conduct and for how long. Something like this. This spiffy gadget accepts a zero to 30 mA DC current-loop input with a loop compliance voltage of at least 6.4 V DC. It produces time-proportioned pulses to trigger a SSR. It is designed for use in 4 to 20 mA current-loop control systems, but it can also be voltage controlled by just wiring a fixed resistor across the control inputs and applying a variable voltage. So, for 20 mA full-scale input with, say, a 12 V DC supply you need R = V / I = 12 / 0.02 = 600 Ω resistor wired across the control terminals and a variable 0 to 12 V DC control voltage. Then all you need to add is a means to ramp the control voltage up and down with your push-button switches.

    The "easiest" way to do that is with either a Microchip PIC with an internal digital-to-analog converter (DAC) or an Arduino UNO which has a built-in 8-bit DAC with 0 to 5 V DC output. Both of these do require some programming, and may not be your cup of tea. There are people here who will help with that.

    The Arduino UNO (which is what I would recommend for you) has the advantage of coming pre-assembled and requires just a minimal amount of external circuitry to boost its 0 to 5 V DC DAC output to a 0 to 12 V DC output, while accepting two push-button control inputs. You can power an Arduino from a 9 V wall-wart or battery. To program it, you just connect a cable between the Arduino and a USB port on a personal computer or lap-top, download some free software from the Internet, write the program (called a sketch in Arduino-speak) on the PC using the free software, and download the program back to the Arduino. The program is stored in non-volatile memory on the Arduino, so once you get it working, the cable and USB connection to a personal computer is unnecessary. If you don't have much (or any) electronics experience, the Arduino approach is the way to go.

    A bit less complicated, but requiring some circuit construction is an up/down counter integrated circuit whose binary outputs drive a DAC. Either way you need to determine how much resolution you need, i.e., how much the power changes for each push of a button. I recommend using an 8-bit CMOS counter and a CMOS 8 bit DAC to give you one part in 256 resolution, or roughly 0.4% change in power per button push. Lower resolution makes for a more "jumpy" control and higher resolution means it takes more button pushes to change the power level.

    The least complicated, but the poorest performing, is an all-analog means of creating and "storing" an analog control voltage in the range of 0 to 12 V DC. This is very easily done with an integrator circuit (one op-amp, two resistors for up/down, and a capacitor) that charges or discharges a low-leakage capacitor depending on which button you press. Problem is, even with the best components, analog integrator outputs drift up or down over time and eventually the output is either full-on or full-off unless you are paying attention and make push-button corrections for drift. This could be a very steam-punk type of design, with a big analog meter to show the power, appropriately marked in red near the high-end maximum power on the scale. Not really recommended, but it would be fun. You could quickly build it on a breadboard to try it out, and then revert to a digital up/down counter and DAC after you find it too much bother to constantly push the buttons to bring the power back to where you want it. Score one more for the potentiometer approach: set it and forget it.

    So, please let us know if you want to make this a project and which approach you want to take.

    Hop
     
    Anon_LG likes this.
  8. Goog

    Goog

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    Mar 21, 2016
    Thanks mate, I really appreciate tournament the time to answer :). I like the sound of the arduino control, something I've been wanting to learn more about. I think I understand what you've described, I have a couple of ssrs already, so will just need the pulse controller and arduino. I'll get back to you with some.links later in the week, that I hope you can help.me with. Thanks again.mate.

    Forgot I had a question. In regards to the pulse module, why do I have to make.it voltage controlled?, and what do I use to control voltage?, bit confused about that bit. Just to be clear, I don't know what loop voltage is, or current loop, im very UN educated in this field!.
     
    Last edited: Mar 22, 2016
  9. hevans1944

    hevans1944 Hop - AC8NS

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    I should mention that if you are going to use an Arduino, you don't really need the add-on pulser controller. The Arduino is perfectly capable of sending pulses to your SSRs... it can even be programmed for either pulse-width modulation (PWM) of the SSRs, or variable duty-cycle, integral number of line cycles, if you don't want to, or cannot, switch your SSR on and off rapidly... depends on the SSR. Some SSRs are only for AC line operation and depend on the line current going through zero to switch themselves off, a action called commutation. If you already have some SSRs with enough current and voltage rating for you heater, would you please post a picture of them and possibly a manufacturer and part or model.
     
  10. hevans1944

    hevans1944 Hop - AC8NS

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    At that price, it's worth a try. It probably does NOT delay the turn-on until the zero-crossing of the AC line, instead just applying an "on" voltage to the gate of an internal TRIAC when the DC control input is at a high enough voltage. If it does use a TRIAC, commutation will occur automatically when the DC control input is reduced to a low enough voltage AND the AC line goes through the next zero crossing.

    One-off Time: ≤10ms is listed on the spec sheet, but this sounds like half the period of a 50 Hz power line, so it probably is a TRIAC that is self-commutative every half-cycle at the zero crossings, which occur every ten milli-seconds.

    Not fancy, but you should be able to control this SSR directly with a 5V logic-level output from your Arduino. Learn how to "read" the push-button switch states and use that information to turn the output on and off... one button for "on" and the other button for "off".

    Once you have that down pat, proceed to write the software that will produce a variable duty-cycle output over a period of one second. Use the switches to increase or decrease (in software of course) the duty cycle by incrementing and decrementing a software counter and comparing its count to a one-second timer that you allow to run. If the one-second timer count is less than the counter value produced by the action of the switches, then turn the output on. When the one-second timer count exceeds the counter value produced by the action of the switches, turn the output off but let the one-second timer continue until one second has elapsed. Then start the one-second timer again and do it all over... forever. The software that examines the switch states should be inside the loop that is testing the counter against the current value in the one-second timer. That will allow you to change the count (and the duty cycle) on the fly without waiting for the one-second timer to expire.

    If you need help with this, upload your attempts at coding it and someone here will give you suggestions. However, we would like for you to think this out for yourself. It's not rocket science, just some pretty basic Arduino coding. Look for examples on the web on how to code and compare time delays.
     
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