556 used as a boiler controller

Discussion in 'The Projects Forum' started by sailorman9, Sep 15, 2010.

  1. sailorman9

    Thread Starter New Member

    Sep 11, 2010
    5
    0
    I need a control to be able to adjust the heat applied to a boiler. I have a 100 gallon boiler powered by 3 – 4500W, 240v immersion heaters. I need to be able to heat the boiler up quickly, but then back off the power to a yet undetermined level. The power level will change depending on several factors, so I didn’t want to use a thermostat. I’m using a 6475AXXSZS-DC3, 75A zero cross SSR and a heavy wall wart DC 12v 1A to power the circuit.

    I found a schematic on the internet and planned on using this when the boiler was completed, which it is now. I have spent the last week immersing myself into electronics and was able to build this board, but it only varies the duty cycle down to 50% and thus is not low enough. I did make one change to this schematic; I changed the 4.7uf to a 1uf, which lowered the pulse frequency to a lower and more acceptable level. Other than needing a big heat sink on the SSR and the 50% problem, the circuit works great on the boiler.

    I had read on one of the pages here that one could change the duty cycle down to below 50% by changing a resistor between pin 6 and 7, but I couldn’t figure that out even after switching out the 1K resistors with several others up to 33K.

    I’m also considering using all 3 heaters to initially heat the boiler and then using only one heater to keep it hot enough, but this does not solve my problem. 50% is still to hot using only one heater. I’m trying to figure out a way to safely switch from 3 heaters at full power to one heater through the SSR. Using a dedicated circuit breaker box with a 60A breaker and a 20A breaker might work well.

    My back is against the wall on this one. I’m opening a distillery and I should be spending my time perfecting my vodka and rum recipes instead of learning electronic theory. Everything is ready to go except this one small but crucial piece of the puzzle.

    If someone could provide me with an easy to understand schematic, it would be deeply appreciated; or even a way to modify the circuit that I have already built. I don’t understand all the talk about “saw tooth” and “ramp wave form”, but I guess these are the theories that influence what I need. I already have a 556 and two 555’s so a schematic that incorporated what I have would save me a week by not having to order different parts.

    Please help me. Thank you.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    Really? If you could use temperature to control the heaters, it might simplify things a lot for you. I've been using the LM35 as a thermometer; it's a 3-pin device (looks like a little black plastic transistor) that outputs a voltage equal to temp. (10 mV/°C). It accepts a wide range of power supply voltage.

    Anyway, comparing temperature (voltage) to some reference with a comparator (eg. LM339) then gives you an easy way to control any on/off devices. You can change the reference voltages in response to your other factors, whatever those are, and still retain a system that responds to temperature. I would think runaway could be a problem without temp. control.

    Just my 2¢
     
  3. sailorman9

    Thread Starter New Member

    Sep 11, 2010
    5
    0
    Thanks Wayney,
    The variable that I didn't discuss in an internal condenser in the distillation column and the need to increase or decrease coolant going to this condenser depending on what product I'm making: Vodka, Rum, and Whiskey all exit the still at different quantities of alcohol. So the boiler temperature needs to be different depending on what's being run. More controls = better product and quicker response times between different product runs.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I suggest that this is a case where you need a professional boiler tech to see what you have on site. They should be able to quickly select suitable off-the-shelf controls to maintain your boiler at the desired temperatures/pressures.

    I also suggest that boiler controls are not a suitable project for amateurs. Even though properly designed boilers have safety valves for overpressure conditions, etc. - if the safety valves DO actuate, you will probably have ruined a batch of product, and will have downtime on top of that. Just one such incident will cost you far more in lost production than the boiler tech's visit.
     
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,792
    948
    A 100 gallon boiler has enough energy in it to demolish your little factory

    [​IMG]

    PLEASE don't rely on a home'brewed' circuit for control of it. Depending on where you live, it might even be a felony to do so.

    Kermit
     
  6. campeck

    Active Member

    Sep 5, 2009
    194
    3
  7. sailorman9

    Thread Starter New Member

    Sep 11, 2010
    5
    0
    The boiler is not really a boiler as it does not attain 212 degrees. It is also open to the atmosphere so no pressure can build. Alcohol boils at 174 degrees. The boiler will go up to about 200 degrees max when in operation. It's actually like a big hot water heater that vents to the air.

    Thanks Campeck and Wayneh! I'll order that 339.

    Anyone know of another way using 555/556?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You did not previously indicate that it would be an atmospheric level boiler prior to my post. We have to err on the side of caution here.

    You need some kind of feedback loop to maintain the boiler at a constant temperature. Since your application is a fairly low temperature one, you might use (a) thermistor(s) in conjunction with pot(s) and (a) comparator(s) to keep the temperature relatively constant. The thermistor(s) would need to be physically attached to the boiler.

    J-B Weld (the original, not the 5-minute) would be a good candidate for a heat-conducting adhesive.
     
  9. sailorman9

    Thread Starter New Member

    Sep 11, 2010
    5
    0
    In the tests I did yesterday, using my previously attached schematic, the temperature was slowly climbing at 50% (2 sec. on 2 sec. off). If I could have the timing at say 40%, I'll bet that the temperature would stabilize. I only have to be close because I can compensate with the internal condenser.

    I can see that adding a thermistor to the boiler, column, and secondary condenser would allow me to better watch the variables.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Thermistors would at least give you feedback as to what the temp range is.

    If you simply attempt to PWM your heaters, anything will throw off the temperature - for example, the A/C system cycling, sun shining on the boiler or in shadows, adding more fluid to the boiler, etc.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You could also use something like LM34 or LM35 precision temperature sensors. These give outputs in 10mV/°F and 10mV/°C, respectively, if wired as shown in the datasheets.

    The LM35's are considerably less expensive than the LM34's, but that shouldn't be much of a consideration compared to what you're doing.
     
  12. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    First let me say that I endorse the "safety first" approach, even though you may not have pressurized vessels. Bad things could stll happen (hah, a pun) and the odds increase using home-brew electronics (hah, another one).

    FWIW, I don't care for thermistors because of their non-linearity. The LM35 and related ICs are crazy cheap and very accurate with just a multimeter needed to give a readout. I can use a $2.50 multimeter and $0.60 LM35 to achieve a digital thermometer for under 5 bucks! If you want to see an example of what I'm doing with them, let me know.
     
  13. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Use the diode trick in a 555 circuit to allow the duty cycle to approach 0. I'm unsure if this will work in the way you make a PWM circuit out of two if them or not thogh.

    Alternately have a look at this IC I'm planning to use on something else:
    http://ww1.microchip.com/downloads/en/DeviceDoc/21756b.pdf

    You'd just have to trick the SENSE circuit.
     
    SgtWookie likes this.
  14. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Interesting IC's, Marshallf3 - thanks. I'd been using LT1171 switching regulators in conjunction with NTC thermistors for controlling PC fan speeds, just because I had a very cheap source for them for awhile.

    But actually, I think that PWM is not really needed for this project - basically, just maintaining the temp within a few degrees should do it, and that can be handled with temp sensors like LM35's, some reasonably inexpensive comparators, and some kind of fail-safe circuitry.

    It would be very helpful if we could see some drawings or photos of the boiler with important temperature monitoring locations documented, positions of the heaters, etc.
     
  15. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Just ran into those, they've got a few others in the same series that have different features. I'm going to be using the 642 driving a 2.1 mOhms LL MOSFET to control a motorcycle fan but will obviously be getting my SENSE pulse from the drain end. I especially liked the startup feature as well as the /FAULT output.
     
  16. sailorman9

    Thread Starter New Member

    Sep 11, 2010
    5
    0
    What is the diode trick? I have 2 - 1N4001. Do I take one out before or after the pot?

    Thank you,
     
Loading...