Battery-modification to "smart" soldering iron

Thread Starter

Ephex

Joined Jul 4, 2021
83
Hi all

A couple of years back I got my hands on a used TS80P "Smart" soldering iron which I have used a bit, but I really dislike the whole "wire and adapter" situation, it makes it really clunky to use so I want to make it wireless.

It connects via USB-C and has a rating of max 12V, 30W. I first thought of using a single 3.7V Li-Ion with an 18650 battery holder. The holder would be soldered/mounted on to the PCB which also has the USB-C connector. This is how I imagined it being placed:

1699291769119.png

The battery has plenty of power (3.6V and most are rated for ~20A, so ~72W, right?) and it should survive long enough for my use cases.

Rough calculations (please correct me if I am wrong):

( 2600mAh * 3.7V ) / 30 W = 9.62Wh / 30W = 0.32 h = 19,24 minutes of continuous max power.

1. Do you suggest any other batteries? I would like to stay away from LiPo:s at least. I've seen some 12V power banks but most of them are really bulky and have unnecessarily large capacities.

2. How should I increase the voltage from 3.7V to 12V while also being able to stay at least 30W? The battery itself would have to pump out 8.1A @ 3.7V to reach 30V, and the output should be 3A @ 12 V (max). Are there any particular boost-converters that do this neatly? Any IC:s you recommend? I am comfortable designing PCBs but coming up with the whole circuit diagram and its components is not my strong side.

This is what I quickly sketched up just now:

1699293838309.png

3. I have never used relays, what are the specs of interest? Number of pins haha? What should I look for except for rated current and voltage? I just want to disconnect the battery to turn it off using a smaller switch that controls the relay.

Additionally, it would be nice to be able to have a separate 5V charge port to charge the battery, but that is not necessary. In the future, I would like to make a portable "kit" with a holder, brass "brush", etc but that is a future problem. I am also aware that the battery will add substantial weight further back, but I am fine with that.

Thoughts?

// Ephex
 

wayneh

Joined Sep 9, 2010
17,493
3. I have never used relays, what are the specs of interest? Number of pins haha? What should I look for except for rated current and voltage?
That's basically it. You're switching DC and so be sure to look at the DC ratings, not the AC. Every relay uses a small current through a coil to move the relay mechanicals. You want that coil to be rated to the voltage you want to apply to it. And everything else being equal, you might prefer a lower coil current to use less power.

Personally, I'd use a MOSFET for this application. The relay would be fine, but a MOSFET is cheaper, smaller, and no moving parts.
 

Thread Starter

Ephex

Joined Jul 4, 2021
83
@wayneh

It happens quite often that I walk to different parts of our workshop and solder small things here and there. Of course, wireless soldering irons are not necessary to be able to tinker with electronics, but it is the goal of this project and I think it would be a fun small-ish project.

Alright, MOSFETs sound good. I took a bit of a circuit from a project I worked on the last time I used MOSFETs and switched it out:

1699299160456.png

How does it look? All I need to make sure is that whatever MOSFET I use is N-channel, can handle the current, and that 3.7V is enough for the gate voltage.
 

wayneh

Joined Sep 9, 2010
17,493
I would omit R2 and C1. For the drawing I'd put Q1 between the PS1 and ground, which is battery ground. You can't really have a switch between battery ground and any other ground, since they are one and the same.
 

Thread Starter

Ephex

Joined Jul 4, 2021
83
Aren't N-channel MOSFETS supposed to be mounted this way? I mean, it decouples the negative pole of the battery from GND of the system. What would happen if this were to be implemented as it is right now?
 

wayneh

Joined Sep 9, 2010
17,493
Aren't N-channel MOSFETS supposed to be mounted this way? I mean, it decouples the negative pole of the battery from GND of the system. What would happen if this were to be implemented as it is right now?
No, that's not a normal way to use a MOSFET. It's common to use it to control the ground-return path of the load.

See that body diode in the MOSFET symbol? It'll allow current to pass from source to drain even when the gate is low.
 
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