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

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element14's The Ben Heck Show

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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).

Parents

  • 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.

Comment

  • 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.

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