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Super Glue Gun 01: Hot Tool Teardown -- Episode 281

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element14's The Ben Heck Show

Join the Ben Heck team every week for amazing hacks! Watch them build and mod community-inspired projects using electronics!

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Super Glue Gun
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The Ben Heck Team re-imagines one of Ben Heck's favorite tools, the glue gun to being work on a prototype for a super glue gun. They get started by tearing apart a bunch of glue guns to find the one that is closest to the one they want to build. The plan is to find one that's closest to the one they want to use that as their inspiration.

 

Ben and Felix tear down a bunch of glue guns to look for a heating element that's close to what they want. The blue gun Felix takes apart is fixed temperature while the red gun that Ben takes apart is dual temperature.  They hook up a multimeter to get the resistance of the coil and figure out how much current it will draw using Ohm's law.  They're looking for one winding where they can pulse it to change the temperature.  Down the road they're hoping to find self-contained heating elements.

There are two coils in the glue gun.  If you pass the current through both of them there's more resistance which means it uses less current which means its cooler. If you flip the switch and bypass one of them current flows through just one which makes it hotter. Because AC voltage can vary a few volts by geographic location they hook it up to a Kill a Watt to double check the math on the voltage.  The Kill a Watt tells you how much energy whatever is plugged in is consuming.

Ben finds a glue gun close to the one they want to build.  There's only a single winding so one pair of wires that go to the heater block and then there's another pair of wires going into the thermistor. The temperature control is done with a potentiometer which is close to what they want to do. It contains a BT135-600 glass passivated triac.  The triac is the most commonly used semiconductor device for switching and power control of AC systems.  The triac can be switched on by either a positive or negative gate pulse regardless of the polarity of the AC supply at that time.

They go to work figuring out how to control it with a DC logic circuit.  Felix suggests that they use the optocoupler for that.  An optocoupler is an opto-emitter and opto-detector in a single package. The next step is to rig up a test circuit. They electrically isolate the DC from the AC to check if the circuit is correct.

They draw a Super Glue Gun Block Diagram to map out how everything is going to work. The diagram includes an MCU, 120 Volts coming into an AC transformer, a bridge rectifier, and a regulator. On the inside of the microcontroller Ben draws what kind of port they need: the IO, the motor control, and what they need for the zero crossing detector.  Zero crossings are points where the AC wave form hits zero volts.

During thermistor testing Ben logs glue gun test on high (300 F) then hits it with the fan and drops it down to 190 and then heats it back up to what is assumed to be the low setting (230 F). He gets thermistor readings for each setting.  He lets the glue cool down to room temperature before getting another reading from the Thermistor before figuring out what the range is.  A thermistor changes resistance based on temperature. By using smaller pull down resistors they get a wider range on the ADC (analog to digital convertor).

  • , , You guys have some great ideas! I'll be sure to bring them up and talk about them when we do more work on the Super Glue Gun project soon!

  • Hey, gang.  About thermistors... I created a very nifty Excel spreadsheet which you guys might find useful for the future.  Typically, my designs use 25C NTC thermistors as ambient temp sensors for oven controllers, while I use a K thermocouple for the thermal enclosure.  I was struck by the non-linearity of the thermistors and created the spreadsheet to help calculate correction factors for Arduino controllers converting the ADC value to a calculated ambient temp.  The map() function is not sufficient as it assumes a linear relationship between the measured voltage and ambient temperature.

     

    J.R. Stoner

     

    P.S.  If you email me with a sandbox email address I can send you what I have (there are no macros in it, so no worries image

  • I couldn't tell from just the show, but is it possible that the one glue gun had two coils in parallel? In low temp mode, one coil is energized while in high temp mode both are energized. In this way, the high temp setting will have half the resistance of the low temp setting as demonstrated on the show.

  • You might want to talk about different types of loads.  The glue gun heater coils have low inductance and are mostly resistive.  As they get hotter, their resistance changes.  So measuring the cold resistance is not the same as the resistance when it is hot.  An incandescent light bulb is another good example of a resistance type load.  You have to measure voltage drop across the coil and current to get the actual value.  If you want to get more interesting, try switching inductive loads like AC motors.  You can control the triggering of the on-cycles to correct power factor and reduce electric bills.  I think if you don't use a zero-crossing sensor, you have to add a cap so the triac is triggered at the right time due to phase shift of inductive loads.  Also, trigging an incandescent light bulb at zero crossing make it last longer--the worst time to turn on the wall switch and turn on the light is when voltage is at the peak of the sine waveform.

     

    Using a zero-crossing opto-coupler is good, but I didn't hear why.  I think triggering at zero reduces line noise and triac heat dissipation.  Line noise can cause false signals to X10 switches.  Some manufactures put a cap across the line to sink the noise, that also mutes X10 signals.  AC plug strips are notorious for this.  Monitor the temperature of the triac--you may need a heat sink.  The triac switching noise can be heard with AM radios.

     

    You may also want to add a dual color LED to indicate when the gun is switched on (or has power applied) and when the heater coil is energized.  You probably don't need an on/off switch--one should unplug it.  But you might want to add a safety feature that turns it off automatically after a certain amount of time or by sensing movement (gyro chip?).  To get it to turn back on, you could use a push switch.  That way you could leave it plugged in on the bench instead of always having to unplug/plug whenever the glue gun is used--that might be a selling point.  Then you have to make sure it draws virtually no current when it is off but still plugged in, or else it may be labeled an energy vampire.

  • in reply to cherring

    According to the article it sounds like he had the charger resting on his chest which probably meant he had the end of a 13A mains extension lead on his chest into which the charger was plugged. I think I'll go with Occam's razor on this one and the charger / extension socket simply went into the bath water...

  • in reply to beacon_dave

    Perfect example right there. That was not because of the phone battery, or the low charging voltage. Many of these cheap wall socket phone chargers are made to a very low quality and can pass some mains voltage over to the low voltage side. That is most likely the case in that article. Essentially, that poor guy touched mains through the charging cord. Unfortunately he was in water at the time so it was a million times worse as his muscles would have contracted, heart fibrillated and he had nowhere to fall. No escape.

     

    Realistically, if you touched Ben Heck's exposed mains. You would get a shock and jump away and be OK, maybe the RCD would trigger too. But it is equally likely that you would touch the mains, hear would fibrillate, cardiac arrest.

  • in reply to cherring

    It appears that even low voltage battery-powered consumer devices are still a risk to the general public judging by this recent news article:

    Man dies charging iPhone while in the bath - BBC News

    I guess it is the flip-side of trying to make things safer generally - 'risk compensation' kicks in where  people relax their behaviour in response the perceived risk as opposed to the actual underlying risk. 

     

    Perhaps the use of the 'CAUTION - Risk of electric shock' warning symbol could be used as a lower third in such video content to remind people that mains voltages are present.

  • As long as you have a full micro controller onboard and a tiny stepper driving the glue, I'd experiment with retraction the way 3d printers do. It might reduce or solve the ooz problem most glue guns have.

  • @9:30 You need to put a 'do not try this at home' disclaimer about how UNBELIEVABLY dangerous and irresponsible it is to hook mains up to a breadboard!! Sure you guys on TBHS know what you are doing and how to protect yourself, but there are so many beginner electronics hobbyists who otherwise think that the setup you have there is safe. IT IS A DEATH TRAP. And if anyone decides to replicate your project, it will take them 5 minutes to order a TRIAC, grab some alligator clips, wire up that circuit and KILL THEMSELVES!

     

    I am extremely disappointed in design practices used here. I am a longtime viewer and huge fan of your content - even the terrible acting. But you guys really dropped the ball on this one.