Adding a PID to a Gaggia Coffee espresso machine.

From thedomesticbarista.com

Adding a PID to an Espresso machine

It can be hard to achieve consistent shots with a domestic espresso machines because of the temperature instability. One of the most popular ways to overcome this problem is to fit a PID temperature controller the the machine to give a fine level of control over the brewing temperature. Here's a tutorial if you want to give it a try. The Gaggia Coffee really is an ideal machine for this as the small boiler means that any advantage you can gain in stability will help consistency considerably.

Warning - you will void the warranty on your espresso machine by doing this. Additionally you are working with high voltages that can cause harm or death. Do this at your own risk, take appropriate precautions and please check your wiring carefully

The PID.

The first thing I did was go online and look for a PID - RS components stock a wide selection but they're all over £100 - not exactly my price range. Ebay had some sellers and one Chinese made unit (from HBKJ) caught my eye. There are a few sellers with these units under various different model numbers but I settled on Auber Instruments in the USA. Shipping only took four days to the UK and the unit arrived with a K-type themocouple. The PID is a tiny unit, 2cm by 3cm by 7cm and seems to have all the fittings it should have. It's powered by anything from 26-240v AC or DC so I decided to wire it into the circuitry of the espresso machine itself. The only real gotchas are wiring the DC control for the SSR and the thermocouple up the correct way around. I used the auto-tuning function on the unit and it seems to work well.

The Thermocouple.

Thermocouple construction and placement is a subject of it's own but suffice to say you have to work hard to ensure that you are measuring a meaningful temperature from the boiler and that you make good thermal contact. The thermocouple was supplied welded but the join was messy and too big to pass through the bolt so I had to cut it off and solder a connection, this is sub-optimal compared to welding but if you're careful to make a decent connection the results are fine for this application. Most people seem to pinch their thermocouple between the existing thermostat and the boiler but I wasn't too happy with this approach. A few minutes with a wrench confirmed that the thermostat has a 0.7mm pitch thread and is 4mm in diameter. The recess it screws into does not go all the way into the boiler and so I decided to try to mount the thermocouple in this hole. I had a steel bolt of the appropriate size and a few minutes with a hand drill provided a suitable thermocouple holder. The hole is big enough for the inner insulation to run through it so the only uninsulated wire is the few mm at the junction itself. As you can see from the image below the soldered end of the thermocouple sits on end of the bolt and is simply forced against the boiler by the bolt. For those a little confused about the arrangement, here's a simplified diagram showing the way the thermocouple replaces the thermostat and is linked into the PID and SSR to the heater..

The SSR.

SSR stands for "Solid State Relay". If you're considering modding an Espresso machine I guess you have some experience of electronics and if you haven't come across an SSR suffice to say that it's like an old reed-relay but solid state so silent when switching. The thing to check is that your SSR can be switched on and off by the DC voltage supplied by your PID and that it will take suffucient current. I wanted an 10 amp unit but picked up a 40 amp unit from ebay for £5 inc P & P!

The Other Bits.

Your local hardware store can supply everything else you need. My complete parts list is as follows:

• PID
• SSR (10A +)
• 1 meter of 3-core cable with a rating above 10 amps
• Zip Ties
• 2 Sticky pads
• Project box for SSR
• Low-voltage cabling for DC control (I used speaker wire)
• Thermocouple
• Steel 0.7x4 nut
• 10 x male and female disconnects
• 2 x 10A+ connector blocks

Wiring the thing.

The wiring on the Gaggia Coffee is broadly similar to the wiring on the Gaggia Espresso and the boiler is identical (See image here). There are two thermostats - the coffee thermostat and the steam thermostat. The steam thermostat is the brass unit you see screwed into the top of the boiler, the coffee thermostat is the visually identical unit you can see screwed into the bottom of the boiler on the side facing you in the picture. It is the coffee unit that we have to replace. Here's the left side of the boiler with the thermostat removed.

Never having done this before I wanted to be able to return the Gaggia to it's original state easily and so decided to do all the wiring with disconnects and patches so that I didn't have cut any of the existing cables. In order to connect the PID to the AC supply I made this adapter to patch into the power connector..

The female connectors on the right go to the power connector, the males on the left connect to the existing internals of the machine and the shorter male connectors on the left supply the PID.

A moment with a multimeter, iced water and a kettle confirmed that the solder on the thermocouple was good and it screwed nicely into the threaded hole in the boiler. I'm really happy with this method for mounting the thermocouple and it gives good results - although it's worrying seeing how fast the temperature in the boiler drops when pulling a shot!

With the thermocouple taking the thermostats place we need wire the old thermostat connectors into the SSR to take current to the heating element. Again I made two short patch wires so that I could use the existing disconnects. Just remove the wiring from the coffee thermostat and connect it to the AC side of the SSR; Then connect the DC control side the the PID. I then connected the AC supply for the PID with disconnects and turned the thing on - worked first time. Here is a picture of the internals before cleaning-up and reassembly.

The Final Result

And here's the unit with everything in place - the SSR is secured with a sticky pad. NOTE: If you are in a country that uses 110v mains power, or you have purchased a low amp-rated SSR you will probably need to install the SSR with a heat-sink. A 40A rated SSR on 240v should be OK to attach with the method shown, but using a heat-sink is preferable and an important safety consideration.

Finally the upgraded Gaggia back in it's place.

If anyone is interested the cost breakdown for this project was £27 for the PID and thermocouple (inc P&P), £5 for the SSR and around £4 for the additional cable, ties and disconnects. The grand total therefore runs to £36, or $66. Not too bad at all.