Weller WHA 900 hot air station - teardown

[mahi]

I had not seen teardown pictures of the Weller WHA 900 hot air station before and since I happen to have one here right now I decided to take a quick peek inside.

Weller WHA 900 specifications

• Temperature control: 50 - 550 °C (closed-loop control with sensor near nozzle)
• Airflow control: 5 - 50 l/min
• Hot air pencil output: 612 W
• Power consumption: 650 W
• Noise: 56 dB(A) at 1 m with maximum airflow level
• Dimensions: 210 x 170 x 110 mm (l x w x h)
• Weight: 2.9 kg

Temperature and airflow level are set with traditional rotary knobs. The potentiometers do not directly control the outputs but are interpret by a microcontroller. This allows the station to be programmed with a standby temperature and airflow level independent from the currently set values. In other words, it's just as digital as any other modern hot air station but lacking the push buttons and display.

The power switch is on the mains. When the switch is off the unit is really off. You will not find the hot air pencil burning like a torch while the unit is switched off - like some of its Chinese competitors occasionally do. The drawback is that there's no automatic cooldown period after switching off the unit. To prolong the life of the heating element it's probably best to run cold air for a couple of minutes before switching off the unit (although the owner's manual makes not mention of this being necessary).

A stand is included for the hot air pencil that can be mounted on either left side or right side of the station. The stand does not have a switch or sensor to detect the pencil.

The hot air pencil has a painted metal grip. The stand tends to scratch and chip it very easily. As you may be able to see in the attached pictures I tried to prevent this with heat shrink tubing on the stand.

Hot air nozzles are secured to the pencil with a single screw. Simple and easy. Sadly no nozzles are included with the hot air station. Weller offers a wide variety of nozzles but obtaining them is a different story. At the moment of writing all popular nozzles appear to be on backorder with lead times of up to 8 weeks!

Since neither pencil or stand have a switch, the only way to activate the pencil is through the green push button on the front of the station. This is rather inconvenient. The Weller WHA 900 is also available in a version with additional foot switch but the standard version does not have the connector for it on the back.

In comparison to modern competitors the Weller WHA 900 clearly lacks in features. At the current price point in Europe (~1000 EUR including VAT) it's probably not the first choice.

I had been looking for a hot air station for some time. I wanted to steer clear of the Chinese potential fire hazards but since I would need the device only every once in a while I did not want to spend too much on it. The Weller WHA 900 was not even on my shortlist but I was made an offer I could not refuse. I guess the sentiment of having used Weller soldering gear all my life won over common sense. Nevertheless I'm sure that for my relatively simple purposes the WHA 900 will serve me well.

[mahi]

Lifted the lid... There's not that much inside. A transformer, air pump, circuit board and some switches and potentiometers.

[mahi]

The transformer is manufactured by Weiss. Primary 220 V, secondary 18 V. No information on the VA rating. Obviously the US models will have a different transformer.



The air pump is a turbine fan powered by a 3-phase brushless DC motor made by ebm-papst. The part number is 939 3020 003. I was unable to locate a datasheet for it but apparently this model/series is nick named "turtle blower" and has a blog entry on the ebm-papst website. Judging from that article the motor is most likely an ECI 30.20. Nice little fella.



The spade connectors connectors carrying mains voltage have nice heat shrink tubing. What I did not like is that the bottom spade connector on the main power switch is not fully seated. It made firm contact but the security latch is not engaged and in theory vibrations could make it come off.

The circuit board is nicely laid out. (As seen from my annotated picture) the power supply is on the right side, center bottom is the triac drive of the heating element, top left the fan drive, and bottom left the microcontroller and some connectors. I was unable to dislodge the mains spade connectors from the board so I didn't take the board out for bottom view pictures but there are no components on the bottom side and nothing of interest underneath.



I'm not too impressed by the large Samwha electrolytic capacitor. You'd expect slightly better brands at this price point.

The fan is controlled by a TB6551FG 3-phase full-wave/sine-wave PWM controller and 3 TPC8407 P/N-channel MOSFETs.

The heating element is controlled with a BTA12-600C triac and KTLP160J triac driver.

What surprised me about the heating element is that there are only 3 wires: Both ends of the element and ground. Where's the temperature sensor? The Weller documentation clearly states it's a closed-loop system with a temperature sensor near the nozzle. I did not attempt to reverse engineer the diagram but there seem to be more electronics than strictly needed above the triac drive so I guess Weller uses some trickery to read the sensor over the heater wires.

The brain of the unit is an unknown microcontroller labeled "Icwha_3" - I did not lift the label, sorry, but it's probably some PIC device. The unused red connector right above the microcontroller is most likely the programming interface. There's a mystery jumper directly left from the microcontroller - I have no idea what it's for. When shorted it's pulling one of the µC pins to 5 V but I did not have the guts to try it - for all I know it's to reset the calibration data.

Between the programming interface and the switch connector are a few unoccupied islands suggesting an optional second input for the microcontroller. Could this be for the version with foot switch?

There are more unoccupied islands between the power section and the fan drive - purpose unknown.

In the bottom left corner are two connectors. Apart from some extra diode protection both connect to the same pin on the µC through the CNY75B optocoupler. Given the orientation of the optocoupler both connectors must be inputs? Maybe this is for the foot switch? Certainly makes more sense to have exactly the same PCB for both versions but why 2 different connectors?

I might take a closer look at this later. Maybe the foot switch input can be used to make the stand "smarter"...



That's it! I hope you liked this mini-teardown!