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Rancilio Rocky doserless MD40 grinder mod by Avi

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Modding Rocky

I chose to add a digital timer to Rocky and have selected to use the Watlow 935 controller which is 1/32 size with built-in timer and relatively low cost.

Rocky’s housing is pretty tight, so there is no room in it for a controller and other large components.
I had to use an external box.
I picked a small project box for the controller and placed it on Rocky’s rear side with its front above the hopper lid.
This allows easy access to operate it. It is not the ideal location because the panel is not facing to the front, but out of the other alternatives, this seems to be the most convenient and space saving one.

Here is the schematic layout of my design:

I used Watlow model 935A-1CD0-00R.
This model has a mechanical relay on its 2nd output which is the one with the timer function.
Timer accuracy is 1 Sec. which is enough for this purpose. Dealing with more accurate timer will not lead to better results based on my tastings.

Since it is triggering Rocky’s motor, it is recommended to add an arc arrestor / snubber / Quencharc to this relay to protect it.
Watlow recommend the use of Quencharc model 0804-0147-0000.

What is a Quencharc anyway?
This is a component to filter out noise generated by relays, relay contacts, solenoids, motors, etc.
A snubber is a simple filter device using a 0.1µf, 250 volt AC, non-polarized capacitor in series with a 100 oHm, 1/2 watt resistor. 
The device can be used on AC or DC circuits to effectively dampen noise at its source.

With a Quencharc, this relay can be safely connected directly to the motor.
Rocky’s motor is 50W (NOT 1400W as mistakenly written on Rancilio site and in some printed manuals and brochures.
(1400 is the motor RPM, see motor’s label image )
Watlow relay can handle 2A@250V so we are good with it.


Using a standard project box with dimensions of ~73x47x110 mm left enough space on the front panel for adding a clone momentary grind switch and a light indicator.
This switch allows improved and safer access in case I need to override the automatic timer to perform manual grinding without getting hot water drops over my hand.

The light bulb is an indicator for the power switch, a feature that Rancilio seemed to neglect.


tipTip: Please note the Watlow 935 controller requires the presence of a sensor in its input, placing a 100 Ohm resistor across its input terminals will simulate 0.1 RDT sensor and allow it to work as timer only.

tipTip: For the light I used a 220V neon indicator that made installation easier.
You can use a low voltage led as well but it need additional components to be able to work on 220VAC mains.
Alternatively, the power switch can be replaced with one that has a built-in light bulb.

tipTip: Since 3 wires are going from Rocky to the control box, I used a standard appliance cable with 3 wires.
Instead of drilling the stainless housing for this cable, I used the base plate and drilled one of the existing holes for the cable diameter.
With this installation I also replaced Rocky bumper feet with higher bumpers to allow enough space under the base plate for the cable that goes to the rear side.



 

 

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