Controller for Hakko FM-2023 tweezers - Page 1
Forum section: electronics / projects

author: johnwa
author_rank: Frequent Contributor
timestamp: May 23, 2018, 10:59:16 am
content: Hi all,I saw a few threads recently about home made soldering stations, which has prompted me to relate my own experience. I have had the opportunity to use the Hakko FM-2023 SMD soldering/desoldering tweezers, and was impressed with the lightweight handset. I would have liked a a set for myself, but unfortunately they are quite expensive, especially considering that you must also buy the FM-203 station to power them. And, while the tweezers are undoubtedly good quality, I was less impressed with the FM-203 station that I had used - although it is a dual channel station, it cannot power the tweezers and a separate iron at the same time. And the four-button user interface for parameter setting is fairly horrible.So, I decided on a compromise: I would shell out for a genuine handpiece, and build my own power supply. After all, how hard could it be: a transformer, a couple of op amps for the temperature control, and a triac or something? Little did I know...When the handpiece arrived, I measured the resistance across various pin combinations of the 8-pin connector (this is different to the 5-pin connector used on other Hakko products), and identified two pairs of pins that measured about 8 ohms, and another two pairs that measured about 10k. Heating the tips with another soldering iron caused the 10k readings to decrease, so I assumed that I had identified the elements, and some NTC thermistors for temperature feedback. Based on this, I built up a simple controller, with the feedback loop implemented using a simple op-amp circuit.However, performance of the circuit was disappointing, with severe overshoot, and the temperature setpoint drifting around considerably. Adjusting the controller gain did not seem to help. Graphing the response showed that the temperature indicated by the sensor lagged the actual tip temperature (measured with a thermocouple) considerably.I then made the discovery that the temperature sensor was not actually part of the tips at all - the 10k resistance was still present, even with the tips removed! So how did the Hakko station control the temperature accurately then? I thought that they must have used some sort of elaborate PID algorithm, with an accurate thermal model of the tips and some sort of feedforward compensation. So, I rebuilt my controller to use an ATTiny85 microcontroller, and implemented a PID control loop in software.But after many adjustments of the control loop coefficients, I still couldn’t achieve acceptable performance. I therefore decided a different approach was required. I remembered seeing a circuit some time ago that measured the element resistance in a bridge circuit, which varies according to its temperature. I reconnected the element to a multimeter, and measured the resistance as I again heated the tip with another iron.The resistance dropped quite rapidly from 8 ohms to about 5 - almost too good to be true! In a moment of suspicion, I reversed the connections to the meter, only to find that the resistance had now increased to about 12 ohms! Switching the meter to the millivolt range told the true story - the tip was generating a thermocouple voltage of about 5mV. So, could this voltage be used to provide the temperature feedback?To cut a long story short, yes, this does appear to be feasible. The voltage cannot be measured when power is applied, so a short dwell interval is provided after every mains zero crossing to measure the temperature. Power is then set by a conventional phase control circuit. Fortunately, I was able to use the x20 differential inputs on the ATtiny85 I was using to measure the thermocouple voltage. There still appears to be a little bit of temperature variation with this new control method, but the tweezers are now quite usable.I don’t know how the FM-203 stations actually control the temperature - does anyone have any ideas? The thermocouple in series with the element may be a deliberate addition, or it may just be a consequence of the metals used for the element.I can provide construction details if anyone is interested in duplicating the circuit, though it may take me a little while to put everything together.

author: Aeromanuk
author_rank: Newbie
timestamp: January 04, 2019, 01:11:23 pm
content: Hi im interested in you soldering iron station to power a set of Hakko fm2023 tweezers. Can you forward me your wiring diagrams / schematics you've been working on cheers

author: johnwa
author_rank: Frequent Contributor
timestamp: February 03, 2019, 04:14:09 am
content: Hi Aeromanuk,I have posted a description of this project, with schematic and firmware, athttp://loopgain.net/solderingtweezers/

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 03, 2019, 09:56:34 am
content: Thumbs up on the 'Brand name'

author: fossil
author_rank: Contributor
timestamp: February 03, 2019, 02:58:44 pm
content: Quote from: johnwa on May 23, 2018, 10:59:16 amCheckout "Really universal soldering controller" athttp://dangerousprototypes.com/forum/viewtopic.php?t=7218.Probably one of the most comprehensive soldering station controller one can build on his own.

author: johnwa
author_rank: Frequent Contributor
timestamp: February 12, 2019, 07:09:01 am
content: Quote from: TERRA Operative on February 03, 2019, 09:56:34 amYes, I thought I had better continue the tradition of front panel labelling on homemade equipment not being done in an entirely serious manner...Quote from: fossil on February 03, 2019, 02:58:44 pmThanks, if only I had found that before I started designing my own! For reference, the thermocouple does appear to be a deliberate inclusion, and the thermistor is for cold junction compensation.