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G8MNY  > TECHNI   13.10.21 08:50l 56 Lines 2443 Bytes #999 (0) @ WW
BID : 49933_GB7CIP
Subj: Thermostatic Iron Repair
Path: PI8DRE<PI8CDR<VE2PKT<PY2BIL<CT7AXA<GB7CIP
Sent: 211013/0634Z @:GB7CIP.#32.GBR.EURO #:49933 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                 (New Feb 21)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

I pickup a "Weller" type iron by "AdcolA" that has electronic temperature
control, using a negative temperature thermistor with the element. This is
rather than "Magnetic Curie Point" temperature switch of Weller system, or a
thermocouple & sensitive op amp system!

In the handle was a 14 pin control chip which was dud & did not trigger the
triac. So I developed this much simpler 1 transistor circuit as a replacement.

24V o컫컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커
AC   1K                                 
2A   __                                
     \_/                               12R     The thermistor is about 50K cold
      쳐컫컴컴컴컫컴컴컴컴컴컴컴커     48W     cold & less than 1K hot. The 3K9
                        +    2K   Element   & 150R set lowest & highest temp
       +      NTC    旼늘컴컫컴            with my thermistor.
  10V__===    Therm   47u /  __     
    '/_\ 100u   쳐컴쩡좔컴캑   \_/=LED 
              Temp  BC337\e     ______
           旼>pot               _\/_/\_
             2K             ____/ 3A
           읕컴  3K9                쿟riac
              150                   
0V o컴좔컨컴컴컴컨컴컨컴컴컴컴좔컴컴컴컴
                                     iron earth
E컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴
                                        ///

The traic is no longer phase fired, as I assume the original IC could do, but a
simple on off including an added status LED (used on some of their irons).

HOW IT WORKS
The 100uF is charged up via 1K & diode to 10V when the zener conducts. The
triac is turned on via the LED & 2K7, giving full power to the heater element.
When the thermistor is hot enough (lower resistance) to put 0.6V on the NPN
base with the preset pot load, the NPN conducts, & bypasses the current going
through the LED & gate, turning the triac off. The 47uF slows the action, so
the triac gets a chance to conduct on both has cycles & even a few on & off, as
shown on the flashing LED.

The circuit needs scoping to check the triac is firing OK on both + & - half
cycles, if not, damage can be done to very small 24V AC transformers!


Also see my Tech Bul on "Soldering Iron Economiser/Boost"


Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP


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