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How to get rid of fluctuating voltage on soldering tip (T12 clone station)?

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Haggis McHaggis:
Hello,

i bought a T12 Quicko Mini soldering station and measured some voltage (0.5-3V) fluctuating (leakage current??) on the tip with my cheap DMM. Measured against the negative pole of stations DC input.

The station is a 24V DC version, which is powered via an old Lenovo laptop psu. The psu has PE on the primary, but only +/- on secondary side.

I want to solder some parts like a MCP1541. The respective datasheet says it's a CMOS. Thus i assume ESD safety is needed.

I read about earthing the tip could help, but i'm not sure how to do it correctly.

Any tips are welcome.

Thank you!

sn4k3:

--- Quote from: Haggis McHaggis on April 29, 2019, 11:07:45 pm ---Hello,

i bought a T12 Quicko Mini soldering station and measured some voltage (0.5-3V) fluctuating (leakage current??) on the tip with my cheap DMM. Measured against the negative pole of stations DC input.

The station is a 24V DC version, which is powered via an old Lenovo laptop psu. The psu has PE on the primary, but only +/- on secondary side.

I want to solder some parts like a MCP1541. The respective datasheet says it's a CMOS. Thus i assume ESD safety is needed.

I read about earthing the tip could help, but i'm not sure how to do it correctly.

Any tips are welcome.

Thank you!

--- End quote ---

The best thing is open and check what inside the device, but since you connect it with a DC charger it means no earthing on tip

Ian.M:
Odds are, because it has three pin power in with PE, the laptop PSU has the 0V (-) side of its DC out grounded. You can check that by checking for continuity, PE to 0V with  the PSU totally disconnected.

If the tip's floating you can ground it, so with the iron cold and power off, check resistance between the tip and the 0V side of its DC in jack.  If it reads open or very high resistance, its OK to ground the tip (via the barrel).   If it  shows as direct continuity, its already supposedly grounded so something else is going on e.g. your PSU doesn't have a grounded output so you'll need a seperate PE to 0V ground connection.   As with all Chinese clone stuff, you'll probably have to do some resistance measurements with a DMM or even a teardown to figure out if the handpiece already has a ground wire fitted separate from the heater connections, so you can simply mod the station.

Once you've I.D'ed the tip ground wire at the connector, try a 1K resistor between there and 0V.   Nearly all the stray voltage on the tip should go away.  If so you can link the tip ground wire directly to 0V.  If it doesn't, further investigation is required + possibly rewiring the handset.

N.B. After successfully grounding the tip, be *VERY* careful when working on circuits with charged capacitors or batteries.  If you touch the grounded tip to a circuit node that's got a significant voltage on it, shorting it out, if the discharge current is high enough, you'll damage the bit and possibly also damage the controller in the station.  If you've got no choice but to solder a battery terminal with the battery present, disconnect *ALL* other lead to the board or device to ensure there is no other ground connection to complete the circuit, and work on an ESD mat, *NOT* a grounded metal surface.

Haggis McHaggis:
Thank you very much.



--- Quote from: Ian.M on April 29, 2019, 11:53:48 pm ---Odds are, because it has three pin power in with PE, the laptop PSU has the 0V (-) side of its DC out grounded. You can check that by checking for continuity, PE to 0V with  the PSU totally disconnected.
--- End quote ---
I just measured that. There is no connection between PE on the input to the DC output.



--- Quote from: Ian.M on April 29, 2019, 11:53:48 pm ---If the tip's floating you can ground it, so with the iron cold and power off, check resistance between the tip and the 0V side of its DC in jack.  If it reads open or very high resistance, its OK to ground the tip (via the barrel).   If it  shows as direct continuity, its already supposedly grounded so something else is going on..
--- End quote ---
The resistance between DC in (-) and the tip is around 13Mohm, no direct continuity.

The handle is a T12-9501 and has a 4 pin plug. One of the pins is connected to the tip and is connected to some pin on the controller pcb. I'm not sure how to check if this is the ground wire or there for other purposes.

Haggis McHaggis:
I've attached a photo that shows a similar controller board, but with the same handle. On my unit there is no wire going to Earth (DC IN) on the board. It makes sense because there is no Earth input or terminal.

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