Author: james

I made a schoolboy error last night when measuring current through the Ulka pump on my modified Gaggia Classic, which resulted in a very loud explosion of the SSR (pictured below).

The machine was working perfectly well, but I just wanted to take a few measurements…. which ended badly due to a very simple mistake. I thought this would be an interesting example to share, to demonstrate the risks involved in this kind of experimentation!

The intention was to measure the AC current through the pump when using the IGBT versus the current with a direct connection. First of all, I started by putting my multimeter on the AC current range in series with the IGBT (between the IGBT collector and the pump). That went without incident.

The next step was to measure the current without the IGBT, and here’s where I made the error. I disconnected the IGBT collector lead from the pump, inserted the meter between the pump and Neutral, and powered on. There was a tremendous BANG like a gunshot, a cloud of smoke rose from the machine, and all the lights went out. Oops.

I didn’t immediately spot the problem, until I examined the circuit diagram, at which point it became obvious (see below). When I disconnected the wiring, I had accidentally removed the series diode D7 which prevents AC flowing back through diode D6. Placing the meter across points (A) and (B) and setting on the AC 10A current range created an effective short-circuit from the anode of D6 to Neutral:

This allowed AC to flow from Neutral, through D6 and through the SSR K3 directly back to Live with no load in the circuit, as shown by the bold red line above. The poor, unsuspecting SSR (rated 8A) and diode D6 wouldn’t know what hit them! They both died a quick death, before the 13A fuse in the plug blew. The SSR blew with a loud explosion and a cloud of smoke. The diode failed silently, but went short-circuit.

After this, I was pretty annoyed with myself for not double-checking the circuit diagram before powering on. Concerned about the possibility of toxic chemicals from the burnt SSR, I also had to strip and clean the machine, which wasn’t a whole lot of fun. On the positive side, I did have a spare SSR, diode and fuse, so I was able to fix it that same evening, and the machine is working again now!

Next time, I will check my wiring much more carefully before powering on…

Some time ago I fitted both a mechanical pressure gauge (shown in the photo below) and an analogue pressure sensor (Danfoss AKS 32R) to my Gaggia Classic. The analogue sensor is connected via an ADC board to a Raspberry Pi, which displays the output on an LCD display. Calibration is needed to make the sensor reading match the gauge exactly.

When I first installed the sensor, I used an approximate calibration as shown in the pseudo-code below:

minVoltage = 0.5V
maxVoltage = 4.5V
maxPressure = 20.6842719 bar (300 psi)
voltage = pressure sensor voltage (measured by ADC)

voltageRange = maxVoltage - minVoltage
bar = maxPressure * (voltage - minVoltage) / voltageRange

Although this was only based on the sensor specifications, it actually gave surprisingly good results. There was only about 0.5 bar difference between the gauge and the pressure sensor (the sensor always read slightly higher than the gauge). Nevertheless, I decided to calibrate the sensor against the gauge (on the assumption that the gauge was more likely to be correct). Absolute accuracy isn’t really critical, but I wanted them both to show the same values.

To calibrate the sensor, I took about 20 simultaneous measurements for the gauge and the sensor and plotted them in Excel as an X-Y chart. This isn’t as easy as it sounds, because the readings need to be taken when the pressure is fairly stable. After running the pump, it takes a while for the pressure to slowly reach equilibrium, and that’s when you need to very quickly take a note of both values! It’s also difficult to read figures very accurately off the gauge.

These measurements gave a reassuringly good straight line fit (below), which suggests that both the gauge and sensor have good linearity. Using Excel, I then added a trend line to the chart and read off the scale and offset values directly (Excel has an option to display the equation), as shown below.

To correct the pressure sensor reading, the value is scaled by the reciprocal of 0.9385 and the offset 0.829 is subtracted. These parameters are stored in a configuration file, so they can be easily changed if required.

This simple calibration process worked out really well and means that the pressure sensor readings displayed on the LCD (and stored in the log files) now match the mechanical gauge exactly!

I always seem to have Portafilters lying around on the kitchen worktop, partly because I have a couple of them (the original Gaggia filter, and a naked PF), and partly because I tend to leave them off the machine after cleaning. Some kind of wall mount seemed like the solution, but I did a quick search and didn’t find anything suitable, so decided to have a go at making one.

Wood seemed like a natural choice, to avoid damaging the Portafilter, and being easy to work with. I started with some 45x45mm scrap soft wood, and routed out two 70mm diameter holes (I made a temporary MDF jig for the router to cut this radius). Then I routed slots for the Portafilter handles (about 19mm for the standard PF, and 16mm for the naked PF) as shown below:

The next stage was to chisel out some slots to accommodate the “ears” on the Portafilters. This was pretty easy for the naked PF, but it took a bit more work to shape it to suit the original Gaggia PF, mainly due to the big flared handle on the PF. I put a 45 degree chamfer on the lower front edge of the block, to make it easy to insert/remove the Gaggia PF.

Having finished the wood work, I sanded it smooth and treated it. After searching for a suitable “food safe” coating, I settled on pure Tung Oil. I didn’t thin the first coat (to avoid mixing in any toxic thinners/solvents!) and it worked out fine. I applied three coats at 24 hour intervals, sanding lightly between coats. Each coat was applied with a brush, left for 20~30 minutes, then the surface was wiped clean with a rag.

Here’s a view after the final coat. It seems to give a nice smooth finish, and is apparently resistant to water, alcohol etc. Time will tell how well it lasts in daily use!

Here’s a top view showing the recesses needed to accept the “ears” on the Portafilters. The left side is for the original Gaggia PF, which needs a very large area removed to allow it to be lifted in/out easily. The right side is for the naked PF and needs a much smaller recess:

Here’s a view of the two Portafilters slotted into the wall mount bracket. Now I just need to drill a couple of holes and mount it! I’m planning to leave this for a week or so to cure properly before using it in earnest.

Here’s a final update, showing it mounted on the wall with a couple of 8×2½” screws:

In the process of updating the IGBT driver in my coffee machine with the new design using the TC426, I decided to replace the strip-board with dedicated PCBs. I’ve separated out the high voltage section which interfaces to the pump IGBT, making a standalone board (which could potentially be useful for other projects).

Here’s the circuit diagram for this new board, which I’m calling the “IGBT driver” board (imaginative name, eh?):

Since I had one driver spare in the TC426, I decided to expose this by adding another opto-isolator. So there are two opto-isolated IGBT drivers available (initially, I only need one of these to drive the pump).

There’s a 4-pin connector JP4 which takes two 3V3 logic inputs IGBT1_IN and IGBT2_IN. When driven high, these will enable IGBT1 and IGBT2 respectively (driving the IGBT Gate high). This connector also has an open-collector output PWR_SENSE_OC which will be pulled low when the mains power is on. This can be used by the controller to safely detect when the pump driver is powered on.

The mains power input is by screw terminals on JP1. In my machine, this is connected to the front panel pump switch. This powers up the AC-DC converter, and provides the non-isolated 5V rail (referenced to Neutral) used for the IGBTs. The IGBTs are not fitted to the board, as they need high current wiring, and potentially heat-sinks also, so this design gives more flexibility with installation. There are two headers JP2 and JP3 which provide Gate connections for the IGBTs. In my setup, the IGBT Emitter is connected to Neutral through the wiring loom.

For the PCB design, I opted for through hole rather than SMD for ease of assembly. It’s crammed onto a 60x50mm board, to save space, and reduce manufacture costs. The PCB was manually routed in Eagle:

The low voltage section on the right has a small copper plane, but the rest of the board is naked tracks (having big filled areas at mains Neutral seems undesirable…) There are slots under the opto-couplers OC1, OC2 and OC3 for isolation purposes. Hopefully these will manufacture OK as this is a bit of an unknown for me!

Here’s a rendering of the board, created using GRBV:

I’ve just sent this off to ragworm.eu for manufacture, and I’m now waiting to see how that turns out!