Last year, I bought a temperature controlled soldering iron from local supplier. I bought a hot air rework station soon and I seldom used this iron because new rework station also has a temperature controlled iron. I like the idea of this iron because of its design. It is not like others temp-controlled soldering stations. See the photo.
Here is close up. You can see inside circuitry.
It is just an iron and temp-control circuit is inside it’s handle. No dedicated controlled station like famous Weller gun. There is a knob at handle to adjust temperature from 100~400 C. Yes, it is made in china. The price is very cheap, about 5$ (do you believe it or not). But the quality and performance is quite good.
One day, I used this iron and it was not working. So, I opened fixed it. Fixing is quite simple and I checked the fault within minute.
– Removed the screw and knob first.
– I checked circuit.You can see an IC, components and a Triac. There is no SMD devices or expensive, complex thing. I noticed that there are 4 wires output to heat element tube. Interesting, I first thought there will be no feedback circuit or sensor in such cheap tool. I was wrong. The red-blue pair is a nichrome coil and the white pair is a thermo-couple. It is a closed loop control circuit, unbelievable.
I started to examine the circuit.
The fault was nothing. The soldering joint of heat coil was dry. Soldering directly to a nichrome coil is not a good idea in such circuit. It should be crimped. But this is no room for this and I just do re-soldering the joint with acid paste flux (which can help to solder metal joint like nichrome, tin sheet ect.. Don’t forget to clean the joint after soldering, acid from flux may conduct your circuit lines). Then, the iron worked again.
I started checking the circuit detailed. The IC is simple common Op-Amp, LM358, low power dual operational amplifier. The Triac is MAC97A6, logic level triac. This means that this Triac can be directly switched with logic level, 5V.
The close up,
As usual, I tried to trace the circuit with the help of back light.
This is final circuit.
# Updated 21 July 2015 : Added the value of C and corrected polarity according to Nguyen Ngoc comment, Thanks.
– The principle of circuit operation is simple. The Op-Amps are configured as comparator. The Triac drive directly by Op-Amp2 via C and LED1. The thermo couple is connected to U1, non-inverting input. The preset VR1 is used to adjust the comparator reference voltage. When temperature is lower than preset, the output of U1 will be lowered than U2 non-inverting input, The output of U2 will be high state and switch on Triac to heat up iron. When temperature is reached to set-point and U1 output will changed to high and U2 output will set to low to switch of Triac.
– The C1,ZD1, D2 and R7 are a simple transformeless zener regulator supply.
-Although it is not a linear feedback controlled circuit, it works, at least for this type of iron. Quite a simple and working design. You cannot defeat china 🙂 .
– I haven’t construct this circuit yet. If you want to construct, I cannot grantee it will work. Do it at your own. If any one have experience on this, pls let me know.
#Updated August, 2015.
“who care”(may be someone 🙂 ) shared this schematic with some useful calibration tips. Thank a lot. If someone has experience in these tips, pls share your experience also.