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G8MNY > TECH 09.12.24 13:37z 57 Lines 2449 Bytes #168 (0) @ WW
BID : 21459_GB7CIP
Read: GUEST
Subj: Thermostatic Iron Repair
Path: SR1BSZ<EA2RCF<LU9DCE<VK2RZ<VE3CGR<WW4BSA<N3HYM<IW2OHX<GB7CIP
Sent: 241209/0752Z @:GB7CIP.#32.GBR.EURO #:21459 [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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