Comparator soldering station : Controlling the heating element

Discussion in 'General Electronics Chat' started by Voidugu, Jun 25, 2013.

  1. Voidugu

    Thread Starter New Member

    Jun 25, 2013
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    I want to drive a 60 watt 24volt heating element of a 24volt 80 watt transformer. The trigger pin will be connected on a comparator that is capable of sinking up to 18 mA of current. I know that the ideal circuit for the job would be circuit 4 yet the MOC30xx family of triac drivers is incapable of handling 90 mA of current in order to latch the main triac. Moreover, i got 4 optocouplers and again none of them can handle 90mA of current.The triac i am using is the BTA16 600B
    Datasheet: www.haopin.com/PDF/BTA16-600B.pdf

    > Which circuit do you suggest i use? (i was going to go with circuit 1 to be honest)
    > Is there a better way of getting the job done?
    > Do you know any other triac drivers capable of handling such currents?

    Last question is a bit off topic:
    Which would be the best way to switch ac current?
    >Triac
    > Full wave rectifier and a MOSFET (Tried it and the power wasted in the diodes of the full wave rectifier is huge)
    > 2 MOSFETs connected source to source and the drain terminals connected to the AC(Tried it and destroyed 2 MOSFETs one after the other by unknown cause {note: the MOSFETs were at all times cool, no magic smoke was released in any case} Still haven't figured out why they got destroyed as the circuit was correct at all times. After disconnecting the MOSFETs and measuring them out, some of their pins would be shorted, in one case the gate with drain showing 0 resistance or the drain with source showing 0 resistance. Do you have any idea why this might have happened? {The circuit is shown in one of the pictures. The MOSFETs are IRF630MFP with a max gate voltage of +- 20V})

    I am really sorry for the crappy pictures. I know i need to change my mobilephone.


    Thank you in advance
    Regards
    Void
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I have an iron that uses a triac and a phase controller that is no longer in production. If I were to rebuild it today I would probably rectify the 24Vac and use a low side mosfet to control it. And if you want low losses you could use schottky diodes in the rectifier.
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I second that. It's easier to rectify the low voltage side of the transformer, and use a single FET to switch the element on and off.
     
  4. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    What's the end goal of all this???
     
  5. Voidugu

    Thread Starter New Member

    Jun 25, 2013
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    The goal is to make a simple yet reliable soldering station
     
  6. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Use the triac and opto combination, with pwm pulses,but how do you monitor the temperature,you need a thermocouple to give precice feedback.


    Similar to this circuit i have modified, the led lights solid upto temp, then pulses at the set temp.
     
    Last edited: Jun 26, 2013
  7. Voidugu

    Thread Starter New Member

    Jun 25, 2013
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    Is it okay and reliable to use a simple pnp transistor to trigger the power triac? or should i really go for a triac driver? Is there a chance of the AC messing up the rest of the circuit in case i use the pnp transistor?
     
  8. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Any pnp or npn can switch on Triac, but use it to pulse on an Opto triac as they have built in zero crossing detector.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    i still think that since the heater is 24v, it's just easier (and safer) to run a 24v DC rectifier, and one big FET using PWM with all the control circuitry safely on the 24v DC side.
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    You could thave the triac on the secondary side as well, but still I think that a mosfet will be easier and cheaper solution.
     
  11. Voidugu

    Thread Starter New Member

    Jun 25, 2013
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    I've decided to go with the triac and triac driver solution since its cheaper. (the starting resistance of the heating element of the iron is about 3 ohms and therefore diodes with a very low voltage drop able to handle such currents are expensive)

    Do you guys know of a standard triac able to handle at least 10 amps with a trigger current of no more than 50 mA and a suitable standard triac driver without zero crossing detection for it?

    Thanks loads people. Cheers
     
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Actually with DC PWM the bridge rect diodes only handle the average current, the PWM peak currents are supplied from the 24v DC filter cap.

    In TO-220 NFETs you can easily get one for 30A 60v, which will handle peak currents of 300A or more.

    If you are determined to use a TRIAC, most common TO-220 TRIACS would do that. Try a BT139.
     
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