any idea what's going on in this test then when SDG tries soldering to a grounded pad?
Yes, I do. I spent some time investigating, I thought others might appreciate my learnings and here is what I have learnt:
- The T320 does NOT have a grounded tip (despite me telling others it did, using my DMM). Similar to the T3A, the tip is low resistance to GND, but not tied directly to GND.
- Using a calibrated low ohms meter (not a DMM), the TIP measures 0.15 Ohms to GND (Ground is the IEC GND pin on the rear of the unit)
- Using a calibrated low ohms meter (not a DMM), the GND port on rear measures 0.03 Ohms to GND (Ground is the IEC GND pin on the rear of the unit)
Soldering Test:
- I cut a 100mm length of 0.7mm tinned copper wire, and soldered it with the T320 in an insulated position, and a ground position.
- In the insulated position, the wire soldered normally, and the T320 behaved normally, that is to say,
- upon touching the wire with the iron tip, the reported temperature on the screen did not deviate, and
- upon removing the iron tip from the wire, the reported temperature on the screen did not deviate
- In the ground position, the wire soldered normally, but the T320 showed a variation in temperature on the screen, that is to say,
- upon touching the wire with the iron tip, the reported temperature on the screen dropped about 15 deg, and
- upon removing the iron tip from the wire, the reported temperature on the screen jumped about 30 deg C
- As a result of the temperature variation in the grounded wire position, I connected my Siglent SDS1202 to the wire in the INSULATED position and observed the trace shown in the screen capture.
- @tony359 notes the same problem in the AiXun T3A in this thread
https://www.eevblog.com/forum/reviews/any-opinions-on-the-aixun-t3a/- According to my SDS1202 capture, there is up to 3.1volts that appears on the tip when soldering, that is to say,
- When soldering an ungrounded item, the current flowing in the handle, to GND, via the 0.15 Ohms resistance, causes a voltage of about 3 volts to appear (you don't see this when the item is grounded, but clearly there will be current flowing, as observed by @tony359 in his T3A review video)
- It is also important to note that unexpected rise in temperature (as reported by the iron), occurs only when you disconnected. However, some people have noted the problem is not significant. Actually, this depends. If you observe carefully in @tony359s videos, he is rapidly connecting and disconnecting the iron, and that rapid intermittent connection is what causes the temperature to sky rocket.
The wider question is : is this a problem? Personally, I would say yes. I blew up some STM32 micros while using this iron. I didn't understand why, but now I do. I had unintentionally grounded the pcbs I was soldering. If you are careful and only solder pcbs that are isolated from ground, or for example, if you solder on a grounded surface, you make sure you ground the surface via a 1 Meg resistor, you should be fine.
However, as noted by @tony359, this is risky. A few beers, and I could easily make that mistake, destroying countless hours of work, or something expensive. So for me, I am declaring the Aixun T320 a no go. Yep, I spent my own good personal money on mine, but I am now going to look for another iron.
Is the problem a root cause of a SMPS in the T320/T3A? I dont know. I have seen references to the problem being the handle not having an additional wire for the tip GND, and sharing it with the heater? Apparently the Aixun T420 does not have this issue. So why it is done like in the T320 (and T3A), I am not sure. Disappointing : yes. Is Aixun misleading when they claim the iron is ESD protected? Well, probably no, your not getting kilovolts of ESD on the tip, but 3.1volts is not great either when soldering a uC with an internal diode to GND and no current limiting resistor!
But I am glad I know, and it was a good example of why you sometimes need a low ohms meter because a DMM can be misleading. I bought this iron while surfing the net after a few beers, and if anything, its a great lesson in the dangers of you get what you pay for. It is definitely better than my Hakko 907 in terms of temperature regulation, but the issue with voltage and current leakage to ground is a deal breaker. Make your own mind up, but caveat emptor.
Home
Help
Search
Login
Register
