How, and When, Our Smart Iron Outputs 100 Watts

We’re really excited about FixHub, our new smart soldering system. In a new series of posts, we’re going to take a deep dive into how it works and some of the design decisions that we made to create the most intuitive, portable soldering system possible.

One question that we’ve gotten a lot is about the power rating. 100 watts is a huge amount of power packed into a tiny package. How did we pull it off? Is it 100 continuous watts? Are there any secret ways to boost that power? Let’s dig into it.

Is the FixHub Smart Iron really 100W?

Here’s what happens when you connect the FixHub Smart Iron to a 100 W power supply:

1. Negotiation: The soldering iron requests a 20 volt, 5 amp supply from the power source. The power supply can choose to supply 5, 9, 15, or 20 volts. Our FixHub Power Station, GaN Fast Charger, or other USB Power Delivery (PD) device can provide the full 20 volts.
2. Internal Regulation: Inside the iron, the voltage is regulated to a little above 17 volts, which provides about 5.8 A of current to the downstream circuitry: a transistor and the heating element.
3. Heat-Up: When the iron heats up, this voltage is applied across a 3 Ω (ohm) resistance heater inside the soldering iron tip. 

With this 100 W going through the heater, the iron hits 350 °C, or 660 °F, in about 4 seconds. If the iron continued to apply power to the heater, the temperature would keep climbing—800 °C in 10 seconds, and a glowing-orange 1000 °C by 15 seconds. At this point, something inside the tip will melt.

Here’s a video of the iron with custom (and unsafe!) firmware doing exactly that. It maxes out the power draw for about 20 seconds, getting way too hot for its own good, before it cooks itself.

Now, getting this hot isn’t a good idea, for you or the iron. Instead, we designed the Smart Iron to reach a set temperature (you can choose anywhere between 200 and 420 °C), and maintain that temperature as closely as possible while you’re working. When you melt solder, the iron senses that it’s cooling down and the computer inside it quickly ramps the power up to compensate for the lost heat.

To prevent a meltdown and maintain a steady temperature, the Smart Iron uses a finely tuned control system to turn the heater on and off at precise intervals. The PWM, or Pulse Width Modulation, algorithm controls the duty cycle: the ratio of the amount of time that the iron is on and off. Each time the heater activates, it delivers the full 100 W to the tip. 

Thanks to this controlled modulation, the average power is actually much lower than the peak draw. For instance, once the soldering iron reaches 350 °C, the heater only has to be on 10% of the time to maintain that temperature, resulting in a power draw of ~10 W.

Our control system runs really fast (every 300 ms), so it’s difficult to measure the times that the heater is on or off with a standard power meter. Here’s an example of what the power demand curve looks like with a logging power meter:

What Happens During Soldering?

When you bring the hot tip of the soldering iron into contact with a colder piece of metal like a solder joint, stored heat in the soldering iron tip flows from the tip into the joint. This heat loss causes the tip’s temperature to drop. The Smart Iron responds by turning the heater on more frequently, delivering more power to bring the tip back up to the setpoint. This cycle of carefully calibrating the duty cycle for precise power adjustment is what makes the Smart Soldering Iron so effective for real-world soldering tasks.

Temperature Sensor Placement: The soldering iron relies on an internal sensor, or thermocouple, to measure temperature. This sensor is located between the heating element and the tip’s outer surface.

The Water Test: Why It’s Different

But! Dave over at EEVBlog put the FixHub in water and PROVED that it doesn’t draw a full 100 watts. Yes, Dave raises a good point! Let’s talk about that water test. The FixHub Soldering Iron wasn’t designed to boil water; it was built for soldering. When you put the tip in water, the iron detects the heat loss and ramps up the heater to about 50 watts. Here’s why the iron doesn’t draw the full 100 watts even though it’s submerged in water.

During Dave’s water test, heat is pulled away from the tip much faster than during typical soldering, resulting in temperature of the outer surface dropping considerably. This temperature drop is instantly observed by the thermocouple, and the control system responds to this drop by increasing the duty cycle of the heating element enough to keep the thermocouple at the setpoint, even though the tip is in a cup of 25 °C degree water. Thus, the soldering iron doesn’t draw the expected 100 W.

There is a thermal gradient between the portion of the tip right next to the thermocouple (350 °C) and the outer surface of the tip (25 °C). We could develop an algorithm for this water-cooled scenario that ran at a much higher duty cycle and gave a power output of 100 watts, but it wouldn’t be a good idea for regular, non-submerged use: the tip would get red hot very quickly and then melt, like we showed above.

Here’s the Smart Iron with our unsafe, super-hot firmware build on it. You can see it has no problem sizzling in water as long as you want. The water cooling prevents the tip from melting itself.

It’s not particularly relevant to day-to-day soldering, but the algorithm we’re using now happens to use about ~50 W of power to maintain the internal temperature of the soldering iron tip in water. The advantage of a software-based iron is that we can develop new algorithms for specific use cases, and this is something that we are continuing to tune. If a new application of the Smart Iron would benefit from a different power tuning algorithm, we’d like to hear about it.

We don’t advise performing this test at home, as the thermal contraction from rapid cooling by repeatedly dunking it in water can damage the tip. For future higher thermal mass tips, we are considering moving the thermocouple farther away from the heating element to more accurately reflect the external temperature in our PID algorithm.

How to Maximize Power Output for the Smart Iron

The Smart Iron negotiates with its power source to draw the maximum power available. When plugged into a USB Power Delivery (PD) device, the iron requests a 20 volt, 5 amp supply—a total of 100 watts. PD devices can offer power at different voltage levels depending on the capabilities of the power source. To get the most out of your iron, it’s ideal to use a power source that supports the full 20 volt output at high current levels.

As an example, here’s how power output changes at 3 amps for each common USB PD voltage:

• 20 volts: 60 watts (ideal for most soldering work)
• 15 volts: 45 watts (sufficient for most tasks, but slower)
• 9 volts: 27 watts (suitable for light tasks, but limited heat)
• 5 volts: 15 watts (this is the maximum output from USB-A, and not really enough for effective soldering)

If you’re using a USB PD power supply capable of delivering 20 volts and 3 amps, your iron can hit peak performance and heat up quickly for precision tasks. On the other hand, an older USB port on your computer might only provide 5 V or 9 V. While it’s technically possible to solder using your computer as a power source, the output power will result in slower heat-up times and less effective performance. For the best experience, we recommend using a dedicated USB PD charger with 20 V capability.

For reference, here are the power output capabilities of various Apple devices. Alas, you can’t solder with an iPhone as a power supply–yet. If you can get soldering going with 15 watts from an iPad Pro, please send us photos!

The FixHub Power Station outputs a shade over that, around 21 V, at full charge. If you plug in a USB power source into the back of a fully charged Power Station, you can actually output a bit more to the iron. Like all battery packs, as the charge in your batteries declines, the voltage drops. At 80% charge, the Power Station still outputs 20 V, but reduces current for a total power output of 90 W. By the time the charge gets down under 25%, the Power station can only output 18 W. It’ll still solder, but nearly as fast as when it’s fully charged.

The Bottom Line

The FixHub Smart Soldering Iron is fully capable of delivering 100 watts when needed, but it doesn’t brute-force its way through every task like your grandfather’s iron did. Instead, it intelligently manages power to balance performance, safety, and efficiency. The water test creates a unique scenario that highlights the thermal gradient within the tip, but this doesn’t reflect the iron’s actual soldering performance.

We designed the Soldering Iron to excel at the soldering tasks you’ll encounter in the real world—not to turn it into a tiny tea kettle. That said, we’re always working on updates to improve performance, and we’d love your feedback on its performance. Stay tuned for future firmware improvements that will further refine FixHub’s capabilities.

Keep those questions coming—we love nerding out with you!

Command Line Interface

One more thing: we’ve just posted documentation for FixHub’s serial interface. This interface opens up advanced settings and diagnostics for those of you who want to tinker under the hood of your iron: just connect via a USB TTY terminal and start exploring.

P.S. We’ve mostly finished shipping out Batch 1 preorders, and are hard at work on Batch 2 which will ship in April. If you need one right now, Microcenter has them in stock both online and in-store. And for the lucky people who have gotten their FixHubs already, check our web console for firmware updates. And stay tuned here, we’ll be posting more about FixHub over the next several days.