MAXI FIX Professional Heavy Duty Hot Glue Gun Teardown and Help!

[Erwin Ried]

Something that you might want to do now that you know the circuit is 'hotwire' heater #1 and run your temperature test again just to see if the glue gun gets anywhere near the temperature you are trying to achieve.  If it doesn't then you'll at least know it won't be possible since you won't be able to get any more heat out of it.

So long as you're monitoring the temperature it should be safe to just short either the A1 and A2 leads of the TRIAC or R1 (in my schematic, I'm not sure what the board says.)  This will basically just take everything in the schematic to the right of Heater #1 out of the circuit and keep heater #1 continuously while you run your test.

Sounds logical, I can even remove the pcb and hotwire both heaters, I guess PTC heaters don't require any fancy, just like a light bulb. Anyway as the temperature I measure is in the nozzle and in the metal body, maybe it is designed to heat inside, so I measure 50ºC less because of that (supposedly the gun can stay 10 minutes without connection, like wireless).

[TerminalJack505]

I played around with the circuit and got it working in the simulator.  I'm guessing that I misread the value of the internal POT.  I read it as 502 (5k) but I'm guessing it's really 205 (2M.)

Here's the final circuit.  The internal POT is obviously used during manufacture to calibrate the temperature.  Lower values will keep the heater on longer.  The value of the pot can't go below about 10k, however.  Otherwise, too much current will flow through R4 D2 and the gate of D1.



Here's the output of the simulation showing the relevant waveforms.  Note that I just guessed at the resistive values of the heaters.



I've attached the simulator files if you want to play around with them.

[Erwin Ried]

I played around with the circuit and got it working in the simulator.  I'm guessing that I misread the value of the internal POT.  I read it as 502 (5k) but I'm guessing it's really 205 (2M.)

Here's the final circuit.  The internal POT is obviously used during manufacture to calibrate the temperature.  Lower values will keep the heater on longer.  The value of the pot can't go below about 10k, however.  Otherwise, too much current will flow through R4 D2 and the gate of D1.



Here's the output of the simulation showing the relevant waveforms.  Note that I just guessed at the resistive values of the heaters.



I've attached the simulator files if you want to play around with them.

HAHAHAH! coooool! you reversed the gun perfectly! nice!!! I still haven't tested hardwiring the heaters, mainly because the screws in the gun are a little "damaged" right now so I will check your simulation first to be sure what test to do over the gun and open it for the last time

[Erwin Ried]

Hey! I removed the internal pot to measure the factory default setting and it is 1.25 M ohms, what value do you thing is appropiate? I remember when I turned this pot to the RIGHT the circuit smoked and buzzed (and for the right the resistance increases), I have plenty of freedom to the left nothing seems to happen, can I set this pot to 100 kohm for example to  get neat the max temp? I am not quite sure about how this circuit works, AC things are confusing

I copied your circuit to multisim to try to understand it a bit more (I didn't find how to change things while the simulation is running in the simulator you said)

[TerminalJack505]

Yes, a setting like 100k should increase the time that the heater is on and, therefore increase the temperature.

The circuit basically works as follows... 

Capacitor C2 is charged through the two pots and R3 (which is in series with the external pot.)  The smaller the values of these two pots, the quicker the capacitor charges.  Until the capacitor's voltage is greater than the DIAC's breakover voltage the TRIAC is off.  When the TRIAC is off the heater is essentially off as very little current is flowing through it. 

When the voltage across the capacitor finally (if ever) reaches the DIAC's breakover voltage the TRIAC is triggered.  Once this happens there is a low impedance path for current to flow--through the heater and the TRIAC back to neutral.  The voltage across the TRIAC drops and the remaining voltage is dropped across the heating element.

The TRIAC will turn off when the current through it drops below a certain value.  This happens twice every cycle when the voltage crosses 0V.
 
You can see all of this in the screenshots of the waveforms that I attached.

So lower values for the two pots will cause the TRIAC to be triggered sooner, essentially.

You don't really need to dynamically change component values to understand how the circuit works.  Just choose some settings, run the simulator, make note of the behavior, repeat.

[Erwin Ried]

Yes, a setting like 100k should increase the time that the heater is on and, therefore increase the temperature.

The circuit basically works as follows... 

Capacitor C2 is charged through the two pots and R3 (which is in series with the external pot.)  The smaller the values of these two pots, the quicker the capacitor charges.  Until the capacitor's voltage is greater than the DIAC's breakover voltage the TRIAC is off.  When the TRIAC is off the heater is essentially off as very little current is flowing through it. 

When the voltage across the capacitor finally (if ever) reaches the DIAC's breakover voltage the TRIAC is triggered.  Once this happens there is a low impedance path for current to flow--through the heater and the TRIAC back to neutral.  The voltage across the TRIAC drops and the remaining voltage is dropped across the heating element.

The TRIAC will turn off when the current through it drops below a certain value.  This happens twice every cycle when the voltage crosses 0V.
 
You can see all of this in the screenshots of the waveforms that I attached.

So lower values for the two pots will cause the TRIAC to be triggered sooner, essentially.

Thanks a lot for the explanation and for your time! Lots of learning from a 25 usd glue gun! Will make more tests about temperature today with some fixed small resistor values!  

You don't really need to dynamically change component values to understand how the circuit works.  Just choose some settings, run the simulator, make note of the behavior, repeat.

Yes I know, but it is like a small detail that helps me a lot to understand things.

[Rufus]

In my glue gun research over the past few days, I read that 7 minutes is described as a "fast" warm up time. Huh? 

I have a glue gun from this place http://www.hotmeltgluegunsticks.co.uk/ which looks like a useful site for glue/gun information.

Had it for ages and the model is long obsolete. Never paid much attention to it before, it just works.

I checked and from cold it draws about 270W for 90 seconds then cycles about 90 seconds off and 15 seconds on. Obviously has on/off thermostatic control.

[Erwin Ried]

Hey TerminalJack505, can I ask you a final question?  :
http://screencast.com/t/wSkqtTfXaB

I noticed about the current over heater #2 when internal pot is very low, as you said before: will this damage something? (I mean, I used 100K recently in the real gun, and It does not seems to affect the heat a lot, maybe because the gun is just cheap the temperature I read now is actually the max) but just to do a final test to close the case, it is safe to replace the pot with just a wire? or it is safer to remove the pc and just hardwire the ptc's?

[TerminalJack505]

You can't replace the POT with a wire.  That will allow too much current to flow through R2, D1 and D2.

If you are wanting to keep the heater on permanently, you can replace R1 with a wire.

[Erwin Ried]

Finally I ordered a "JOER" chinese 150W glue gun, much cheaper (mainly because shipping to Chile from China is way cheaper than from UK) and way better!
heats in 2 mins to 200ºC and it has a pot from 140º to 220º

So, for those who need a quick heating glue gun on the cheap:
http://www.ebay.com/itm/250963772048

Some extra info: Starts using 122W, then drops to 90W and then to 0W and 70-90W

Ultra Cheapo things FTW!