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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