What can possibly go wrong with this setup? - Page 1
Forum section: electronics / projects

author: Connoiseur
author_rank: Regular Contributor
timestamp: July 18, 2017, 02:37:30 pm
content: Hello everyone!I've been thinking of converting my old trusty soldering iron into a soldering station using the attached scheme. However before designing the actual circuit, I thought of having a look around the web to see if there are any reference designs similar to this; surprisingly I couldn't find one. (My question ends here if someone has already done it. Related links/pointers are welcome)Although I'm confident enough that I can design and build the controller, I'm wondering what can possibly go wrong in the long run (or even immediately like too much capacitive leakage current to the tip due to high frequency components in chopped DC etc.). Why do reputed manufacturers include a bulky transformer when one can easily get away with such a setup? I just wanted to have temperature control with small LCD screen in this project.

author: alm
author_rank: Super Contributor
timestamp: July 18, 2017, 03:13:01 pm
content: I imagine it would not pass EMC due to spraying RFI everywhere (high dI/dt) unless you include a (bulky) inductor to smooth out the edges. Obviously you should limit the duty cycle so you do not exceed the nominal mains RMS voltage. If the iron has any kind of thermostat built in, then it might fight the PID controller. Would traditional phase angle control (like old-style dimmers) on the AC not work better? Simpler and probably less (though not zero) problems with EMC.A low voltage station is obviously safer, especially if the tip is not grounded.

author: abraxa
author_rank: Frequent Contributor
timestamp: July 18, 2017, 03:16:02 pm
content: I'd assume that they use low-voltage DC because a) the heating element can be smaller as there's less insulation needed, allowing it to be moved closer to the tip and b) because the feeback element will be a low-voltage part, running both line-level AC and low-voltage DC in the same cable will require more wires, good insulation and a good earth connection as well. Using low-voltage DC removes a lot of that hassle while also allowing advanced features to be put into the handle as well (e.g. an accelerometer for detection of the "standby" state).The mere fact that the Hakko Red 503 doesn't seem to have a built-in feedback element would make this project rather pointless to me - if I were you, I'd just get a standard low-voltage iron and build my own PID regulator for it. Heck, there are already finished (and proven working) designs.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 18, 2017, 03:38:02 pm
content: NO. You are going to kill yourself with this. If you don't see why, that's very alarming.Don't mess with the mains voltage. Tip: the transformer is there for a reason.

author: ajb
author_rank: Super Contributor
timestamp: July 18, 2017, 05:26:02 pm
content: Also, the choke you'd need to get acceptable ripple at any reasonable switching frequency is going to be about the same size and weight as the transformer. So you've just traded one type of bulky inductor for another type of bulky inductor.

author: MosherIV
author_rank: Super Contributor
timestamp: July 18, 2017, 06:12:21 pm
content: There seems to be a fundamental missunderstanding of the type of soldering iron you have and why it is and therefore is not worth turning into a 'soldering station'.The Hakko red 503 is a thermally balanced soldering iron. It relies on the fact that there is thermal mass between the heater and the tip. The thermal mass holds a certain amount of heat which is sucked out when you solder, it then takes time for the heat to be replaced.This type of iron is not suitable to turn into a soldering station. Soldering stations irons minimise the thermal mass in favour of a tight closed loop thermal control loop, in simple terms, when the temperature of the tip drops, the station rapidly pumps heat in to bring the tip back up to temperature.Watch bigclive vlog about it

author: Connoiseur
author_rank: Regular Contributor
timestamp: July 19, 2017, 02:58:08 am
content: Thank you guys for your feedback/comments.Quote from: alm on July 18, 2017, 03:13:01 pmYeah that could really screw up a nearby equipment or even the PCB that you're soldering if it's powered ON maybe not directly but by RTC battery etc.Quote from: abraxa on July 18, 2017, 03:16:02 pmI know that, was thinking of somehow attaching a thermocouple near the tip by kapton tape and later by a machined copper part.Quote from: Siwastaja on July 18, 2017, 03:38:02 pmThank you for your concern for my safety. I'm an electrical engineer and have worked in the past with electrical utilities. I have experience with HV equipments upto 220kV and LV (415V) of upto 6300A. I know how not to kill myself.Quote from: ajb on July 18, 2017, 05:26:02 pmI was not including the inductor anyway. I wonder how HAKKO FX601 achieves temperature control. It has everything built into the handle.Quote from: MosherIV on July 18, 2017, 06:12:21 pmThank you very much for helping me reach the root of all confusion. Indeed there will be considerable lag between the application of voltage and consequent rise in temperature due to the thermal mass. How about using a simple relay cutoff to prevent overheating when the iron is left idle? The temperature control was intended only to prevent hard rosin flux scales that form when it's cooked for long at high temperatures. Exact temperature control is not critical as long as it remains within the working range or 60/40 solder.

author: ludzinc
author_rank: Unknown
timestamp: July 19, 2017, 03:12:55 am
content: Quote from: Connoiseur on July 19, 2017, 02:58:08 amQuick question - how do you know that the heater element will cope with 325V DC applied to it? Will it's insulation break down, making the tip live?325V DC is in the one mistake and you are dead category....

author: rs20
author_rank: Super Contributor
timestamp: July 19, 2017, 03:19:37 am
content: Quote from: Connoiseur on July 19, 2017, 02:58:08 amSo where in your diagram is the galvanic isolation between neutral/live and the soldering iron tip?

author: DBecker
author_rank: Frequent Contributor
timestamp: July 19, 2017, 06:24:03 am
content: Using high voltage or line-referenced voltage on the heater has a major drawback. It requires bulky insulation on the cord.I once bought a Vellerman "Soldering Station" that turned out to be a lamp dimmer controlling a line voltage iron (thus three wires with ground, not even with additional sensor wires you are planning). It is the worst soldering iron I have ever used. The thick springy cord is either pulling the iron, hitting the work, or trying to lay itself on the hot tip.You are far better off using low voltage, if only because you can use a flexible cord to the iron. Selecting a high voltage, high slew rate, unipolar drive is a combination of the worst choices.

author: KL27x
author_rank: Super Contributor
timestamp: July 19, 2017, 07:24:54 am
content: Well, you are going to need to reduce the voltage quite a bit to get the "and related signal processing" part. That screws up the elegance, a bit.I don't know why you would bother with a thermocouple. They are used in high end stations because they can be made very tiny and they can withstand higher temps than thermistors. Thus, they can be put directly inside the tip, right on the heater. Other than that, they are a pain. I suppose you would be tacking this sensor as close to the tip as possible, internally or externally. If a thermistor fits, I would use one over a thermocouple.I am not sure why you would do that route with the BJT. You could use a TRIAC and just run right from mains. The iron already is designed to be run that way, so you know it will handle it. And I think it would be safer than DC if something went wrong.I would probably use an isolation transformer at least for the development phase. Before boxing it all up and calling it good.Mosher makes a good point. The plug in iron probably takes over a minute to heat up. The wattage of the element is not made for fast response. It's chosen so that the tip doesn't glow when you leave it on too long, unused. But that said, people use LAMP DIMMERS with these firesticks. So any thermostatic control will be a big improvement.As for the question you put right in the title. What could possibly go wrong?QuoteWe should be asking you.

author: Zero999
author_rank: Super Contributor
timestamp: July 19, 2017, 08:49:31 am
content: Quote from: ludzinc on July 19, 2017, 03:12:55 am1) If the soldering iron's insulation can take 230VAC, it can take 325VDC.2) 325VDC alone is no more dangerous than 230VAC. Any additional danger will be due to filter capacitors which remain charged, even when the power is removed. The higher voltage is offset by the fact that DC is less likely to disrupt the heart's rhythm than AC.3) The greatest risk from powering the soldering iron from 325VDC, rather than 230VDC is not electric shock but fire. At 325VDC, the power dissipation in the element will be double what it would be at 230VAC. If there isn't a thermal fuse and the element can't take it, there's a huge risk of overheating and flames.

author: 3db
author_rank: Frequent Contributor
timestamp: July 19, 2017, 11:03:03 am
content: IMHO the whole idea is pointless and stupid.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 19, 2017, 11:18:39 am
content: Quote from: 3db on July 19, 2017, 11:03:03 amYeah no kidding, the whole thing is like Gordon Ramsay wanting to make his own pots and pans.

author: MosherIV
author_rank: Super Contributor
timestamp: July 19, 2017, 11:26:18 am
content: QuoteA simple relay will not stop temperature over shoot.The only way to control the temperature with some degree of accuracy with the thermal mass is to use a PID control loop.Even with the PID control loop, you would still have much charactisation to do to map the exact transfer function of the system.It is possible, I did a job where there was a huge thermal mass to account for but the system had some limitation, eg a masive overshoot on first power up. The temp contrl was also very crude, could not even manage +/-2'C that is how bad it was due to the thermal mass.QuoteA thermally balanced soldering iron, such as your Hakko Red 503, should always stay within the working range of 60/40 solder - that is their sole purpose.One trick which I have heard about is to put a switch in line with the mains, where the swicth adds a diode in line with the mains. This has the effect of only allowing half the power to the iron, this keeps the iron warm but not at full temp. Turn the switch back and the it does not take long for the iron to reach full temp apparently.If you are after Hakko quality, you may want to look at T12 bits from Hakko, apparently they are cheapish at $15-20 each. The bits have the thermoucouple and heater built into the bit. There are many chinese knock off soldering stations build around them. I mention it because you can pick up a cheap chinese handle and build your own controller for it. This will be a far better option than trying to adapt a thermally balanced iron into a soldering station. There are many DIY project on the web covering this.

author: Connoiseur
author_rank: Regular Contributor
timestamp: July 19, 2017, 02:07:43 pm
content: Quote from: ludzinc on July 19, 2017, 03:12:55 amThe insulation will cope well because 230 Vrms does peak at 325V. Only concern is the steep rising/falling edge of the PWM. I always use variac and isolation transformer for anything mains related.Quote from: rs20 on July 19, 2017, 03:19:37 amIt is all there. This is called a block diagram for a reason and I do have the privilege to choose some level of abstraction.Quote from: KL27x on July 19, 2017, 07:24:54 am*I had planned to use tiny switch(TNY276PN) based flyback converter or just a linear supply with transformer+LDO for the signal processing and MCU. The driver would provide isolation from the BJT switch.*Lamp dimmers would not have a feedback. Making a PID controlled TRIAC driver (phase angle controlled) is somewhat tricky. I need zero crossing as well for that.*Well I fully know the bang, pop and zap part of it but I wanted to know more about other issuesQuote from: Alex Eisenhut on July 19, 2017, 11:18:39 amSo what's wrong with that at least I've tried something new.Quote from: MosherIV on July 19, 2017, 11:26:18 amTemperature overshoot upto a few tens of seconds is not an issue, the major issue is the crud that forms due to cooking of flux and solder for long.Yeah characterising PID parameters is really difficult and time consuming, if you don't know the exact transfer function. Arduino using Matlab hardware support package might be useful for such a task.The diode option does sound good. Maybe I'll add a relay to short out the diode when the iron is lifted. the relay coil will be wired through a microswitch installed at a suitable point on the stand.

author: KL27x
author_rank: Super Contributor
timestamp: July 19, 2017, 09:03:17 pm
content: You dont need phase angle control. Your iron already has a relatively anemic power plant. Full power is slow enough.I just discovered things like this myself:Optotriac with zcd. These are made to drive power triacs. So u got isolation. Just give it a signal, and it will turn on the power triac at the next zero crossing. This will keep emi to a minimum. cheap, too.Even high end ztations do not need phase angle or pwm control. Power is 100% or 0% for entire period. Pid can be added, and even that does not need to modhlate power finer than that. 120 periods per second is more than fine enough resolution. Why would u want to chop that up into even smaller squarer bits? My first diy battery powered (power control not temp) soldering station, i intentionally made pwm period about quarter of a second. Slow as molasses. Why waste power switching and heating up a large package fet by switching superfast, when i get the same end result with sot23 part and higher efficiency ? This is still 1000x faster than manually flipping the power switch to try to keep the tip between cold and burning itself up. And that is good enough to sell these things. For temp control, u want faster response, but still no need for anything finer than full triac switch on/off 60(balanced) or 120 Hz. The thermal mass of the tip is a "heat capacitor" averaging out the lumps.Even without zcd, randomly switching a triac will give no worse emi than 320vdc fet switching.and better than phase control.

author: 3db
author_rank: Frequent Contributor
timestamp: July 19, 2017, 09:26:36 pm
content: I think the diode and micro-switch is perfect for your needs.Back in the day we used to do this or use a switch similar to the ones used in the line cord for a lamp.It's very effective.

author: bson
author_rank: Unknown
timestamp: July 19, 2017, 09:56:53 pm
content: Failure modes:- Fire- Massive project destruction- Electrocution- DeathI'll pass.

author: KL27x
author_rank: Super Contributor
timestamp: July 19, 2017, 10:07:59 pm
content: QuoteThis is essentially how I made a battery powered iron work. It was one of those pocket thing that uses 3AA batteries, and it heats very fast and hot (but tiny thermal mass). I used PWM to create a standby temp (which I calibrated to about 200C). And a press of a button bypassed to full power, which brings it to soldering temp in about 1-2 seconds. This worked pretty good. The diode and micro-switch would probably work very good, here, for the average mains iron.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 20, 2017, 03:29:36 am
content: Quote from: KL27x on July 19, 2017, 09:03:17 pmThe really high end irons (Metcal and a few others) are induction heated.