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Believe me, there has been serious thoughts on my side building the power supply that way.
1. Isolation:
My first approach was to just take an off shelf switch mode power supply. That would have provided an isolation one could be sure of not to be killed by it someday.
BUT:
These switchers do have capacitive voltages to ground, the output isn't floating as freely as one would assume; A CMOS circuit could be offended by that and decide to go dying... 
Second approach: Isolation using a transformer. Choosing a transformer that is certified for SELV/PELV circuits on the secondary side would give me the isolation safely that I need without switch mode residual voltages to ground.
In fact, the older Weller station with two wire input, fully isolated case and a transformer do exactly that.
Conclusion: Building a fully isolated power supply with two wire mains connection built in an fully isolated case.
This is a concept of double isolation (like the Weller stations I mentioned above) and protects the user from touching any mains parts or parts that could carry mains if a failure occurs.
This means that I would build the station into a plastic case -and that can't be used a heat sink..
2. Regulation
I bought the KSGER T12 controller.
I researched the net a bit and found Information that this T12 controller stands ~25V ok.
The KSGER switch mode power supply doesn't exactly output 24V, indeed.
But above 26V it seems to quickly die. 
IIRC that was the result of someone trying to speed up heating by increasing the voltage.. 
This led to...
3. Voltage Regulation
A linear regulator is so inefficient that it would produce a massive amount of heat that I would have to cope with.
Since I build the station in a plastic case getting the heat out would be a major problem making it even more complex. 
The DC to DC switch mode converter has an efficiency of over 90% and it doesn't heat much when idle -unlike a linear supply. 
I do not need an isolated DC to DC switcher but when it comes that way, why not.
The most important feature of this DC to DC switch mode converter for me is that it doesn't need an earth connection when powered with an isolated voltage.
4. ESD-compatability:
For ESD-compatibility I need a proper isolation transformer. -check. 
Since the tip is then properly isolated and floating I will add a 4mm socket connected to the tip to be able to connect to my ESD hub. Or to earth. Or to whatever.
It's most simple and flexible to just add a connector.
Now, when it comes to German engineering:
My solution in every part is not optimized to size, weight, volume, price or power needs.
You could say it's oversized in every part, yes.
A manufacturer would have managed to put this in a case 1/4 of the volume exactly fitted power wise.
Since I used off-shelf parts I had to over-dimension the single stages:
The T12 controllers mostly come with a 24V/5A supply even though the controllers without supply have a DC input marked with 24V/3A.
I chose a 24V/5A DC to DC converter because I wanted to have the same power like the standard KSGER supply.
I couldn't find matching isolation transformers with 120VA rated power. So I went for one with 160VA rated power.
The two big caps are dimensioned to filter the rectified DC voltage under full load (5A) to under a volt ripple. 
I just like those big cans. 
mnem: This is the minimal count of parts using non-optimized off-shelf parts. It's not the cheapest and elegant way, though.
Dammitt! I should've known answering that T12 OLED post would drag me away from my very important Frustion360 modeling work!!!
You forget something very important here: the way the T12 tip works is NOT the same as others with iron-filament heaters. These cartridges are a ceramic element hermetically sealed against the outer SS tube and tip. The heater is wired in series with the temp sensor, so it is a true 2-wire system all the way back to the controller, and in conjunction with the resin handgrip, the tip is ALREADY double-insulated. All you have to do to be ESD-safe is to make sure that outer SS shell is connected to earth GND by at least 1MΩ of resistance. This was the core reason HAKKO designed the T12/T15/T18 cartridge system this way.
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Mine is also built from off the shelf parts; well, a
dead put out of its misery Radio Shank soldering station and OTS parts.
First: Here is the station complete. Still has that same
post-apocalyptic chic feel from when I built it almost 3 years ago; I never did get around to replacing the
"DOOMSDAY MACHINE ENABLE" power switch.
2) Here's the display up close: Note the input voltage in the lower right corner: At idle, the input voltage hovers right round 28.5V. Under full-load (heating up a cold tip) that'll briefly drop a volt or so. This station comes up to 320°C in 15 seconds cold; that includes the longer boot time of the STM32 processor/firmware.
3) Here's how I achieved it: The stock RS transformer puts out 28.8VRMS at idle. This is rectified by a 10-12A bridge salvaged from an old ATX PSU. Normally, I'd tag a 470-1000uF brute force cap right here; but the buck-boost converter underneath has 2000uF/50V of low-ESR caps right across the input only a few mm away, so I decided to skip it as space was pretty cramped right there anyways. From there the voltage is regulated to a hair over the nominal 28V which the T12 cartridges are rated for, and fed to the KSGER STM32 OLED T12 controller.
4) This is the KSGER STM32 OLED T12 controller. It has a much more sophisticated firmware with individual PID profiles for the most popular T12 cartridges; the FW also supports Hot-Air rework and desoldering handles right from the menu. A coin cell keeps the RTC up to date. This FW allows either clock or input voltage monitor on the front screen; it also shows safety temp, (inside the handgrip) and PWM duty cycle. Recommended input regulated 24-28VDC. Here you can see that I have the handle and power connector shell tied directly to earth GND; this is because I am using the metal-shelled iWeld handle, which is also connected directly to earth GND for user safety. Inside the handle there is a 1MΩ resistor from the barrel contact to earth GND encased in double heat-shrink tubing. Probably not the best, most assuredly ESD-safe arrangement; the original resin handgrip would be that.
But that was a concession I made to the smaller, better-balanced metal handgrip.
5) The buck-boost converter here is the real gem of this build; its specs are almost like they were MADE for this project. Input voltage 5-42V, output 5-35V, CV-CC from 0.1-10A.
eBay auction: #173983418047 It's an older model now; there are 20A equivalent models only a little larger footprint for the same ~US$10 all over fleabay now.
Now for why I went buck-boost: Voltage sag under load. Idle voltage from that 28.8VRMS transformer is approx 37.55V across the filter caps; pretty high because this a 70W-rated Pro-Line station made for them by Atten or Aoyue. I found I could easily load down the transformer to approx 26V at idle; but I was dissipating about 30W to do it, AND input voltage would sag to about 23V under full load. This put cold heatup times in a very meh 25-30 second range. Using this buck-boost converter allows me to turn that wasted wattage into fast-attack heating power while still keeping idle voltage well below the 32V absolute maximum of the 3.3V regulator on the OLED T12 controller.
Output voltage does drop a little under full load; not because of voltage sag, but because I have my current limiting set to 6A for safety's sake. When I ran it wide open, the duty cycle was high enough to make an audible whine; I expect that was a harmonic which was just low enough to audibly resonate in the coil. Whatever. Current limiting to 6A made it go away.
This station with a FX-9501 or FM-2028 resin handle would be exactly what you're trying to build. If in doubt, put the 1MΩ bleeder in the wiring in the base instead of the handle.
mnem
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Topic: Test Equipment Anonymous (TEA) group therapy thread (Read 15514088 times)