G8MNY  > TECH     01.05.25 09:15z 57 Lines 2449 Bytes #129 (0) @ WW
BID : 31608_GB7CIP
Read: GUEST
Subj: Thermostatic Iron Repair
Path: ED1ZAC<ED1ZAC<LU4ECL<VK2RZ<GB7CIP
Sent: 250501/0902Z @:GB7CIP.#32.GBR.EURO #:31608 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                 (Corrected Jul 22)
(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 oÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
AC   1K                                 ³
2A   _³_                                ³
     \_/                               12R     The thermistor is about 50K cold
      ÃÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿     48W     and less than 1K hot. The 3K9 &
      ³  ³       ³         +    2K   Element   150R set lowest & highest temp
      ³ +³      NTC    ÚÄ´ÃÄÄÄÂÄÄ´      ³      with my thermistor.
  10V_³_===    Therm   ³47u ³/  _³_     ³
    '/_\ ³100u   ÃÄÄÄÂÄÁÄÄÄÄ´   \_/=LED ³
      ³  ³      Temp ³ BC337³\e  ³   ___³___
      ³  ³   ÚÄ>pot  ³        ³  ³   _\/_/\_
      ³  ³   ³  2K   ³        ³  ³____/ ³3A
      ³  ³   ÀÄÄÄ´  3K9       ³         ³Triac
      ³  ³      150  ³        ³         ³
0V oÄÄÁÄÄÁÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÙ
                                     iron earth
EÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
                                        ///

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 & 2K, 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 and gate, keeping the triac off. The 47uF slows the action, so
the triac gets a chance to conduct on both half 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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