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Significantly improving the performance of a very cheap machine is possible
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Prune
Senior Member


Joined: 15 Sep 2002
Posts: 29
Location: Vancouver
Expertise: I love coffee

Espresso: Heavily modified old machine
Grinder: Solis Maestro
Roaster: iRoast2
Posted Tue Jun 7, 2005, 9:36pm
Subject: Significantly improving the performance of a very cheap machine is possible
 

I've had a very low end machine for about three years.  It was a Gran Gaggia I bought refurbished from an electronics repair shop for a mere $87 Canadian.  Taking apart the boiler there was half an inch of scaling which I cleaned out.  The machine worked OK, but now with Caffe Artigiano in Vancouver I felt my own coffee wanting.  Yet I did not want to spend lots of money on a good machine, so I decided to modify mine for the cheapest amount possible.

One thing I like about my machine is that the group head forms the bottom of the boiler itself, so the grouphead temperature is the same as the boiler temperature.  Thus, the main source of temperature instability is the incoming cold water from the pump, especially given the small boiler size.  I tried hooking up a PID controller I had borrowed, but the controller-heater-boiler system doesn't respond fast enough to the change due to incoming cold water.  The solution was simple and, as many people had done, I went for preheating.  Not by adding a second heater, but by making the incoming cold water pass near the boiler so it gets warm.  In my machine the pump is connected to the boiler by a short bit of teflon tubing.  I got some similar size tubing and going from the pump, I wrapped a turn around the boiler, near the top above the external heating element (to maximize preheating) before connecting to the boiler.  The improvements were noticeable immediately.

An analog PID controller can be built for about $5, because you only need a single opamp (email me if you want more info).  I had previously built an electronic thermometer for my chemistry stuff by using a thermocouple that is cold-junction compensated by an LM335 (see National Semiconductor Application Note 225), and it cost me about $10 in parts (except the analog Voltmeter display I used for temperature display), it works great here.  For controlling the power, most people use a triac-based solid state relay.  The problem with that is that it only switches off at zero crossings of the AC waveform, which limits its PWM frequency to the line frequency, and power is not linear with PWM width because the AC waveform is a sine.  This makes the control not very precise and fast.  A better alternative is to use an optically coupled MOSFET solid state relay, but these are expensive for any reasonable power rating.  Instead, I built my own for about $10 in parts, along with an analog-to-PWM converter to drive it using a signal from the analog PID.  Overall, for under $30 in parts I have built a good PID setup.  I've done limited testing this far as I've not properly secured the thermocouple head, as that would involve drilling the side of the grouphead.

The results of the combination of the two modifications are hard to believe, given the cheap machine.  I've pulled shots comparable to some of the better ones at Caffe Artigiano, with their customized monster of a machine.  Another suggestion is that if your water is very soft, you can harden it with aquarium water hardener + baking soda, and a bit of that actually makes an improvement.  You can measure the hardness of your filtered tap water with a testing kit from an aquarium store.  This will only increase scaling if you steam a lot.  Caffe Artigiano hardens their water, and is where I got the idea from.

Other modifications are possible as well, and I'm planning two: add a pressure gauge to see if the pump needs some adjustment (easy to decrease pressure by putting a resistance in series with the pump), and a manual three-way valve (I have a solenoid valve from a surplus sale that I could put, but it's too much trouble when adding a manual valve from the hardware store is easy (making sure it's rated for say 190 psi or whatever your pump may reach)).

DIY is fun!
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ChicagoSandy
Senior Member
ChicagoSandy
Joined: 29 Jan 2005
Posts: 1,192
Location: SW Coast of Lake Michigan
Expertise: I love coffee

Espresso: Quickmill "La Cora,"  Silvia
Grinder: Mazzer Mini, Rocky DL
Vac Pot: Presses, Aeropress
Drip: postnasal, Technivorm
Roaster: Behmor, I-Roast2, SC/TO
Posted Tue Jun 7, 2005, 10:01pm
Subject: Re: Significantly improving the performance of a very cheap machine is possible
 

ow. my brain hurts.

 
Sandy
www.sandyandina.com
-------------------
Life's too short to drink lousy coffee, play crummy guitars and write with ballpoint pens.
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snoboy
Senior Member


Joined: 4 Jun 2004
Posts: 452
Location: Rossland, BC
Expertise: I love coffee

Espresso: famous Silvia... now PID
Grinder: modded Rocky SD
Roaster: SC/CO
Posted Tue Jun 7, 2005, 10:28pm
Subject: Re: Significantly improving the performance of a very cheap machine is possible
 

I would email you, but it bounced from the address on your profile... :(

You can email me if you don't want to post your address here, my posted email works once you translate it...

edit - I tried the same alias @yahoo.com as that was what Google returned, hope you get it one way or another. /edit
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Prune
Senior Member


Joined: 15 Sep 2002
Posts: 29
Location: Vancouver
Expertise: I love coffee

Espresso: Heavily modified old machine
Grinder: Solis Maestro
Roaster: iRoast2
Posted Tue Jun 7, 2005, 10:46pm
Subject: Re: Significantly improving the performance of a very cheap machine is possible
 

Sorry about that.  I've updated the email to the yahoo one.
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Prune
Senior Member


Joined: 15 Sep 2002
Posts: 29
Location: Vancouver
Expertise: I love coffee

Espresso: Heavily modified old machine
Grinder: Solis Maestro
Roaster: iRoast2
Posted Tue Jun 7, 2005, 11:50pm
Subject: Re: Significantly improving the performance of a very cheap machine is possible
 

snoboy, using the SSR is a bit extreme.  Kind of like silver/teflon interconnects for one's audiosystem (though I made those as well :O).  I would recommend you use a simple triac, but if you want to try this, read on.

Looking at commercial optically coupled MOSFET SSRs, it's really hard to find ones that can handle the current needed for an espresso machine heater (usually on the order of 10 A) and have low resistance so they are not dissipating a lot of heat themselves.

I built a discrete one.  If you use the new MOSFETs with overvoltage/overcurrent protection built in, it's less parts.
Here's an example output (current through load) at 50% setting, switching at 625 Hz: http://www.cs.ubc.ca/~trifonov/dss/switched.png

Referring to the schematic: http://www.cs.ubc.ca/~trifonov/dss/AC_SSR.png
Note that everything is referenced to the AC voltage here, so it has to be enclosed safely.  The MOSFETs I chose are IRF640, because they can handle the voltage, have low resistance and gate charge, and are cheap.  The two inductors L1 and L2 represent a small transformer -- you really do need a very small one, as this doesn't draw much power.  This power supply is necessary because it has to be floating, referenced not to ground but to the AC signal.  Use a transformer that when bridge rectified with four diodes gives out 14 to 17 VDC under load (the usual cheap ones such as 1N4007, or a one-piece bridge rectifier, ignore the ones in the schematic as they are just what I threw in for the simulation).  The capacitors must be rated for at least that voltage.  D1 is a zener diode that regulates the output; use something a couple of volts less than the voltage at C1 under load -- 12 to 15 V.  The driver transistors Q1 and Q2 are standard and very cheap small ones, anything with the voltage and current handling can do it.  R3 represents the load (in this case, espresso machine heater), and V1 the house electricity outlet.  The optocoupler on the right is anything that's rated for connecting to mains power, such as ones used for isolating triacs from the electronics, etc.  R6 depends on the voltage your controller outputs, and the current rating of the optocoupler's internal LED (from datasheet).  For example, if your controller puts out 6 V and the optimal LED current is 20 mA, with the LED dropping a volt, R6 is (6 V - 1 V) / 0.02 A = 250 Ohms.  The other resistor is in case there's a base pin on the chip, use any resistor of several Megohms.
Now, R7/8 and Q3/4 can be omitted, and the driving signal connected simply to the MOSFET gates.  That simplifies the circuit, but removes overcurrent protection.  This means that you might fry a MOSFET (and chain into frying more things) when turning on a load with lots of reactance.  I don't think a heater would be an issue, but I recommend the protection.  If current gets too high, the voltage drop over R7 and R8 turns on Q3 and Q4, which shut off the MOSFETs.  One thing not shown in the schematic is overvoltage protection.  Voltage spikes from the mains, or switching a reactive load such as a transformer, can fry a MOSFET.  Put a 150 V metal oxide varisor across each MOSFET (drain and source).  Since MOVs are pretty slow, it's also a good idea to parallel them with silicon avalanche diodes or gas discharge tube arrestors of similar ratings (though these increase the price).
As you can see, the circuit is an order of magnitude more complex than a simple optocoupler-triac combo, but can switch at any time during the AC phase and at several kHz.
The control signal is represented by V2.  If your PID has PWM output, it can be connected straight to the optocoupler (with a proper R6), and some PIDs have optocoupler isolation already, so that makes it even simpler.  In case you need to invert the output w.r.t. the control, you can swap R1 and the optocoupler.
Since I built a simple analog PID, I made an analog-to-PWM converter to control things through.  There are various ways to do that.

Edit: The IRF640s should be mounted on a small heatsink (with silpads for electrical insulation of course, as the device's metal tab is not isolated), and ground the heatsink for safety.  They could dissipate as much as 15 W each with a very high power heater.  Also, have a 15 A fuse on the hot line.
My analog-to-PWM converter can be controlled either by an analog signal such as from a PID controller, or a potentiometer, so I added a knob and put the thing in a small box with an AC power outlet, so I could plug in various things besides the espresso machine heater, such as some of my lab hardware, lamps where I want linear control that the cheap dimmer doesn't give, etc.  For the latter, if high frequencies are not needed, there are some linear dimmer controller chips that will drive a triac to get output directly proportional to the control signal.
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