A couple weeks ago I received a really nice Tenma Soldering iron from a friend. The other night while I was looking through the Banggood Electronics I came across a very inexpensive solder iron that reminded me of the handpiece of my new iron. The one my friend sent was a bench station and this one was a direct plug in with a small dial on the handle. It also came with 6 tips so I decided to take a chance and ordered one of them.
They were on sale so my total cost including shipping came to a little over $10.00. I figured that at the very minimum the tips could perhaps be used on the higher quality bench station. Here is the link to the Banggood Iron.
The iron was advertised as being adjustable from 200 C to 450 C. When mine arrived however my tests showed that the tip temperature would go no lower than 350 C. I thought that perhaps I got a defective unit so I wrote to the company. After posting the complaint I realized that for $10.00 nothing was going to come of my inquiry and with so little to loose I decided to crack it open and see what made it tick.
The unit came apart very easily. The small knob popped out of a trimmer mounted on the board. The front heater housing unscrewed and the heater and circuit board pulled out to the front. It looked like there was no temperature sensing involved but only a triac dimmer circuit powering the heater. I reverse engineered the circuit and made a schematic.
The circuit board and heater.
In the original design the output control trimmer had a 200K Ohm resistor (R1) in parallel. This brought the maximum resistance across the trimmer to about 137K Ohms. 137K Ohms would cause the current through the triac to settle around 150 mA which meant about 18 Watts to the heater element. I removed the 200K resistor and began experimenting with the 500K Ohm trimmer alone. By setting a resistance greater than 137K Ohms and then measuring the stabilized temperature of the tip I was able to determine that what I needed was a minimum resistance across the 500K trimmer of 175K Ohms. This produced a current of 50 mA and about 6 Watts of power to the heater. After an extended warm up period the temperature of the tip would stabilize around 200 C. The formula for parallel resistors said that I would need approximately 300K Ohms for R1. Reality forced me to use a 294K Ohm instead as that is as close I could get with supplies on hand. After reassembly of the iron I again tested it and found that I could now control the temperature from 200 C and up. Caution is needed if the trimmer is turned all the way up. At 60 Watts the little iron quickly streaks past 500 C and the nice iron clad tip turns into a piece of junk. Here is a closeup of the iron reassembled and the control knob which will probably need to be protected with a cover to keep it from being moved by normal handling.
If you don't mind a little fun modifying it this is a fun project. However, if you want to use it out of the box the high minimum temperatures from the factory will make it short lived and difficult to use. The heater element is a 110 Volt unit as opposed to a 28 Volt one in my bench unit. The spare tips that came with it are compatible with the Tenma Bench unit.