FreshRoast SR500 Teardown – Part 2

Welcome to Part 2 of the FreshRoast SR500 Teardown. In Part 1 you saw the basic steps to open the SR500 properly. This article will explain how to continue a teardown a FreshRoast SR500 roaster into the various components and probably give insight to similar pieces in a SR300 as well. If you are looking to modify an SR500 the following content will be the best starting point for understanding what makes your roaster work as it sits. It will include technical information about the components that make the SR500 work. Part three will begin to make suggestions of where modifications will need to take place if you wish to split fan and heater control to external dimmers, VARIAC devices, or otherwise control your SR500 with a Microchip PIC, Arduino, or other microcontroller or PID controller device.

This article was made possible by having purchased a spare SR500 base that can be completely broken down and tested upon. I will be using this base to interface to my roasting computer.

You should refer to Part 1 if you need assistance opening the roaster. Most people should be able to open the roaster without the first guide but it is a good idea to review it briefly to become familiar with what you are getting yourself into and deciding if you are up to it getting inside. Most of the connections are very “coarse” and use through hole parts.

For my project I am going to be using surface mount parts which requires a bit more skill. There are many options out there so if you are looking to simply use something like a 20×4 character LCD you can probably find a way to do this without the surface mount parts. If you want to continue with a graphical LCD like I am you will likely need to learn about more advanced methods of soldering.

If you are unsure you should research and contact a “Maker” or “HackerSpace” club in your area. Examples of such include NoiseBridge in San Francisco. There are plenty of other groups throughout the USA as well as in many European countries. The closest one for me is about 3 hours away unfortunately so I’ve had to resort to figuring most of these things out on my own. Coffee roaster computers are a pretty popular thing out there so you can probably find other reference material out there.

Most of these groups offer classes in soldering as well as often having capabilities to help design/build enclosures, mill parts, create circuit boards, and have shared equipment for laser cutting, CNC milling, 3D printing and many other systems. Not all groups have these amenities and many require you to demonstrate a level of mastery, take classes, or otherwise “wait for time slots” on the items of interest. Hackspaces normally charge a monthly membership to participate and use the facilities but usually have forums, IRC channels, and other such things where you can find out more information before you commit to driving for a visit.

Removing the inner parts of a FreshRoast SR500 Coffee Roaster

You will need:

• A small phillips screwdriver.
• A SR500 Base with the bottom removed (SR300 may be similar except for changes to the microcontroller board)
• A baggie or small tray to hold screws
• A clean area to work
• Optional: Items to label the wires removed. This should be done as you unplug each wire so there is no confusion.
• Recommended: Needle-nose pliers to grasp some of the flat connectors and pulling them from the circuit board.

Step 1: Unplug your SR500 from the wall and remove bottom as described in Part 1.

Step 2: Identify the high voltage power wires inside the case leading from the main black wall power cable and detach them (N and L1)

It is absolutely critical that you have unplugged the power before performing this step or you will be electrocuted.

The wires you need to remove are labeled N in the middle of the board and L1 on the JP2 side lower down on the board. Normally this would be Neutral (N) and Load (L1). Normally in most North American electrical devices you would not label things L1 and L2 unless you intended to have an L3 for three phase electric or were enumerating your loads. Since the wall plugs into one of the L’s and the other goes to the actual load this seems a little odd but I can follow the reason. Regardless, both will need to be removed to lift the circuit board and heater system up out of the enclosure. You should use the needle nose pliers to grasp the connector and pull it up. Grasp it by the metal and not the wire. Pulling by the wire will rip it out of the connector requiring it to be replaced with crimpers and appropriate ends. You should try to support the board so that pulling the ends do not put additional stress than is necessary.

It is ok to slide the circuit board up some as shown in the L1 photo. It will be difficult to lift the board up very far prior to removal of the N and L1 wires. Again, support the board as you use the needle-nose to pull the wires off.

Step 2a: If desired during step 2 above or 3 below you may wish to remove the power circuit board from the PCB guides in the enclosure.

Gently slide the power board upward on both sides trying to clear the top edges. This may be difficult to do and is generally not necessary until you wish to remove L2, 100W and 1000W.

Step 3: Remove JP1 and JP2 low voltage cables.

Both of these cables are low voltage and will come off easily when pulled. JP1 connects to the main logic board with the Atmel CPU and front control panel. JP2 connects to the Fan Speed Control potentiometer.

Step 4: Slide heater/fan assembly out while looking for the NTC sensor cable.

Do so slowly because the NTC sensor is still attached to the main logic board. Once the heater top layer begins to slide out of the enclosure you should try to find the wiring coming from the side of the heater. If you removed the circuit board from the guides pay attention to it as well so that it does not catch on anything.

Identify the connection on the main logic board and unplug.

Step 5: Inspect the removed heater and power control board.

You should be left with a loose middle portion of the enclosure with the main logic board still attached.

Put it to the side and continue with the power board removal.

Step 6: Remove the 1000W, 100W and L1 cables to separate the Power Control Board.

Using the needle-nose pliers pull off the 1000W, 100W and L1 cables from the Power Control Board. This will allow replacement, modification, or other inspection to occur more easily.

As mentioned above L1 and L2 are a little odd in North American wiring but regardless the white wire is the Neutral and Black is the Load typically. Since we have 1000W and 100W and one has black and one has a white wire this continues in not complying with North American wiring standards and there are other reasons this roaster is abnormal with wiring so we will disregard this in thinking about the roaster. The 1000W connection white wire comes from a large outside heater coil and gets fed power from the L2 connection. The 100W side requires voltage to be applied to L2 and will operate both the fan and the small center heating element.

The 100W side heater to fan wire connects to the bridge rectifier on the base of the fan motor and is wired this way to use the heater coil in the middle to provide resistance to drop the voltage to a level acceptable to run the fan. The black wire side of the bridge rectifier is connected to the 100W connection on the power board creating a second complete circuit. The fan itself (after the rectifier) picks up its power on the other 2 wires once the internal mechanisms “do their thing” in that bridge rectifier. There is a capacitor jumped across the rectifier and I’m not very familiar with rectifiers and using a capacitor but I would guess this has something to do with the zero crossing and trying not to “sputter” when the power cuts out momentarily since this is DC for the motor and AC for the power source.

JP1 is labeled with J1 through J5 positions. J4 and J5 go to pins 4 and 5 on the MOC3043 chip. This is for microprocessor control of the attached devices. The board appears to have spots of solder placed on each of these that look like a “via” that someone tried to solder over but there does no appear to be a real via on this board since it appears to be a single layer board when viewed in front of a bright light. I’m not really sure why those spots are there. If someone has any ideas please pass them on and I’ll add them here.

J3 connects to the Neutral connection. J1 appears to go around the edge of the board linking to the fan potentiometer circuit and the BTA08 (Q6) trigger. J2 leads directly to the BTA16 (Q5) trigger.

Electronics and Connections Analysis

Normally in home electrical work you are required to switch the load side on or off prior to whatever object gets the power. For example you have power going to a light switch. The wire that comes out of the switch then leads to the light and then the light connects to neutral. When you flip the switch you supply load power to the switch, the light then illuminates, and then it passes the electric to neutral completing the circuit. This is typically a safety measure so that you can change light bulbs and particularly remove broken ones with the power off and not get electrocuted as well as generally being good practice in case of almost any other malfunction. This is not how the roaster is wired. If you had the ability to touch either the fan or the heater coil while it was plugged in but not running you would get electrocuted because they appear to be always live. Since they are physically enclosed it is not as important but will affect how you control the roaster.

The slide switch on the front and the cool/up/down adjustments only control the (right) gate sides of the two triacs. This gate is like a light switch. Normally this is a high voltage that is triggered by an opto-isolator chip. The MOC3043 inboard does this for one of the sides while other circuits trigger the other side. The other pins of the triac are the high voltage switched side connections. Both the 100W and 1000W neutral wire connections lead to the middle triac pins of their respective sides and are then gate switched to the wall Neutral pin in the middle of the board on the left side of the triac.

The 100W side’s use of the heater coils is used to drop the voltage by resistance to the fans rather than using transformers or other devices to provide lower voltage to the fan. It is more important as a voltage control than it is as a heater. The fan being connected this way results in some heat being generated whether you are in the cool cycle or not and would vary with the fan speed. Obviously changes in heat are slightly mitigated by the air flow. To fully control the fan separately from all parts of the heater you would need to separate the white wire side and route it to neutral while supplying power separately to the fan by a transformer to control the voltage or else completely remove the bridge rectifier and control it by DC power directly. 100W of heat is not very much and is likely not much of a concern and certainly not really useful either as a reserved heat source. Watching a wattage / amp meter in real time when the fan (with small coil) is running shows about 125-150 watts of power use at full fan. The 1000W side full heater bumps wattage into the 1450 to 1520+ range when supplied a full 120VAC.

The 1000W connection white wire comes from a large outside heater coil and gets fed power from the L2 connection. If you were to separate both of these wires and plug them into wall power you would get A LOT of heat being generated. The 100W side requires voltage to be applied to L2 and will operate both the fan and the small center heating element. Both of these when plugged in separately operate normally. Together I had some issues keeping the fan running once the fan side circuit is set to full power and the heater exceeds half power but this may be due to a faulty potentiometer I was dealing with. Ultimately I wanted heat totally separate from fan so I bypassed this with a transformer entirely and have a different set of potentiometers to install soon. I also switched from neutral gate switching to load side switching.

The fan does not appear labeled with any part numbers or any indication of voltage requirements. Based upon the common construction of many other air roasters and specifically a DIY favorite, the Poppery, being almost all similar it is likely to be a 20 volt fan and runs higher than 1.5 amps at the top.

As shown in Jim West’s blog entry about modifying a poppery http://popperyii.blogspot.com/2011/01/completing-hiros-journey-poppery-ii-mod.html (not affiliated with this site nor endorsing my modifications) the use of a transformer is probably required on the fan if we separate things fully and wish to control fan alone without a lot of complicated electronics. Use of a transformer is more expensive than some of these other options but it is quick and easy. Jim’s diagram of the heat system with the labels for each connection point is identical to the FreshRoast including the safety mechanisms. Those mechanisms on the FreshRoast, however, are calibrated to higher temperatures than what is allowed on a Poppery so the FreshRoast is much like a coffee calibrated poppery.

As a result I would hesitate to agree with anyone removing any of these devices like happens with a Poppery without being absolutely certain of their choice and without building in a lot of additional mechanisms. Pay attention to the old and new schematic areas for my point about the way this is wired. Running the fan alone on a variac at lower voltages works fine. As it exceeds the 20 volts area it begins to show the constant glow of an arc spark in the motor and may arc outside (dangerously) at higher voltages. A transformer is likely to be the easiest and safest solution.

Possible SR300 Fan Speed Modification

On the rear of the “power control board” above it has a silk screen label P/N:306171. I believe this is probably going to be the same board found in the SR300. On the SR500 the socket labeled JP2 leads to wiring on the rear marked R29, R30, and D5 on the “left” side. The position for R29 has no resistor installed on a SR500 but it appears to still have jagged solder and scrape marks on mine.

I believe this is a pre-assembled board made for the SR300 that is tested this way and then forwarded to the roaster manufacturer. My guess then is it is allocated to a SR500 where the manufacturer manually removes R29 and installs it in the SR500 case connecting a potentiometer that is installed on a SR500 panel assembly.

If JP2 exists and is unused on an SR300 the R29 might be able to be removed and replaced with a potentiometer using the JP2 (or holes for it). The front panel might then be drilled and the potentiometer then gets “creatively” installed. I’m going to guess that the plastic is probably already shaped for it and possibly drilled too because it’s probably easier to just put a different gray “sticker” on top depending on the roaster being made.

No impression yet on the “brain” bits of the roaster low voltage board and if the heat switch can be adapted. I would expect this part to be unique for each roaster model unless the installed switch only gets wired to the “High” and off positions of the board and excludes the middle and low position being connected to anything.