Originally written from Daniel
I always liked soldering tools with good quality. Though I currently have a nice ELV 50 Watts digital soldering station already, I decided to build my own soldering base station using an existing WPS80 soldering iron manufactured by Weller. After looking on the internet to see others DIY projects ,I decided to develop a special one, with several improvements in the design.
The main criteria for the design was that the device should be very compact, since I don't like big boxes on my crowdy workbench. Therefore I selected a small enclosure available from Hammond manufacturer and designed the PCB to fit inside the box without many adaptations. As you can see in the pictures bellow the main component is the big trafo (100VA toroid 2x24V); the electronics occupies a small fraction of the available space.
The electronic circuit was designed with several aspects in mind; first it should be fairly simple for anyone to build (as few components as posisible), even if it uses SMD components. Secondly, the reduced size of the electronic circuit is mainly achieved by using a power MOSFET transistor without the need of an additional heat-sink. Most of the DIY (and comercial) soldering stations that I found on the Internet used triac + driver topology for the heater but that solution needs extra heatsink for the power triac. These solutions have an advantage however, the commonly used driver chip is an opto-triac driver (eg. MOC3041) which is simple to drive and has additional 'zero-crossing' detection logic available.
The core component of the schematic is the ATMEGA164 type AVR micro-controller specially selected for PGA circuitry it has for ADC differential channels. This important feature avoids the need of an external OPAMP used to amplify the signal from temperature sensor (PTC in Weller's case). The temp-to-voltage conversion is handled by an constant current circuit implemented with LM1117 LDO chip, this ensures great linearity of the PTC readings. The logic power supply uses an SPMS circuit LM2674 which means we can directly use power trafo outputs (24VAC with full wave rectifier and capacitive filter). Of course the SMPS does not need a heatsink either ;).
The user interface is simple enough, I purposely selected an 3xDigits LED display since it has nice contrast ratio in almost every illumination conditions. For temperature setting I selected a cheap rotary encoder which greatly simplifies the operation modes.
As you can see from the pictures the display board directly plugs into the"main board"with no additional airwires, further more the main PCB is single side routed with very few airwires.
Here is a simplified list of important components (not so commonly available) of the project
Aluminium enclosure box
A:120mm; B:53mm; C:103mm
2x24V toroidal trafo, 100VA, 2x2.08A, 230VAC
Breve Tufvassons, 100VA type
24 pulses/rotation, 24 dents
2x8 pins connector, right angle
Used on LED display daugther board
LED display, 3 digits, common cathode
Circular connector, 7poles
AMPHENOL, 7-poles, circular connector