Hi Craig,
I've reviewed your updated design in KiCad, and here are some more comments:
+ D1 should be removed completely. It's not doing anything for you.
+ D2-D5 are also not sensible, I'm afraid. D4 and D3 would pass all of
the motor's operating current, wasting power and robbing the motor of
approximately 1.4 Volts due to the diodes' forward voltage drop. D2 and
D5 would then be forward-biased by the motor's back-EMF and conduct the
spike directly into your circuit.
+ What you need is a single diode connected directly across the motor
windings such that it is reverse-biased when the motor is powered. That
is, the anode should be connected to the motor's "-" pin, and the
cathode to the motor's "+" pin. When a DC motor shuts off, it suddenly
acts like a battery connected to the circuit in the opposite direction.
You want your snubber diode to act like a short circuit to that battery,
dissipating the power that would otherwise attempt to hammer its way
through the rest of your circuit.
+ 100 Ohms may be too small for R4. The gate of an FET that is off
looks like a discharged capacitor (that's exactly what it is, to some
degree). R4 limits the charging current that your arduino has to supply
in order to get the FET into its "ON" state (or sink to turn it off).
If the digital "high" voltage is 5 volts, a 100 ohm resistor would allow
50 mA of current to flow for a split second while the FET gate capacitor
charges. According to the datasheet, the sink/source rating for the Uno
R3's GPIO pins is 20 mA. Use a 250-ohm resistor.
+ It may be wise to have a series resistance between the Arduino and the
SSR's control pin. 250 ohms would be a good value to use here as well.
+ Remove the PWR_FLAG node. AFAIK, PWR_FLAG is used internally by power
components, and isn't typically used directly in a schematic. It's only
there to help the ERC warn you when you have power pins connected to
logic pins.
+ The arduino's ground is not connected to your global GND. A1 pin 7
should connect to a GND component instead of the PWR_FLAG to which it is
currently connected.
+ Where is the actual +5V coming from? That isn't represented in the
schematic.
+ When you lay out the PCB (if you make a PCB), loop us in for another
review.
Other notes:
+ I don't know how you plan to prototype this, but a solderless
breadboard is going to melt if you run your heater power through it.
+ I'm assuming you're using the 2-pin headers (J1-J5) to represent
connections to off-board components. This will be bad if you decide to
lay out a PCB. Use pairs of 1-pin headers instead. Better yet, plan
ahead for the correct number of off-board connections so that you can
use screw terminal blocks in your layout.
+ Since K1 is (probably) insufficient anyway, if you're determined to
use an SSR, get a high-current-rated one with its own screw terminals
instead of PCB mount.
+ A value of 10 kOhms for R5 is fine, but here's why, if you're curious
and don't already know: R5 is there to make sure the FET is "off" while
the Arduino boots up, because D13 will likely be tri-state (open
circuit) until the Arduino code configures it as an output. R5 also
forms a voltage divider with R4. The gate voltage the FET sees will be
5 * (R5 / (R4 + R5)). For an IRF510, the Gate-Source threshold voltage
(the voltage at which the FET turns on) might be as high as 4 volts, so
R5 needs to be chosen such that the divider gives at least 4 volts on
its output:
5 * (R5 / (250 + R5)) >= 4
R5 / (250 + R5) >= 0.8
R5 >= 200 + (0.8 * R5)
(0.2 * R5) >= 200
R5 >= 1000
Since 10000 is definitely >= 1000, you're in good shape. Vgs will be
4.88 volts when D13 is driven high.
Note that the same situation applies if you put a series resistance on
the SSR's control line (between A1:21 and R2). It will form a divider
with R2, and the control voltage seen by the SSR will be 5 * (R2 /
(Rseries + R2)). R2 and Rseries together must be picked so that the SSR
sees an appropriate control voltage.
Is this the same project I vaguely remember having some e-mails about a
while back? IIRC, we were never sure about the voltage rating for the
fan. This is a hot-air popcorn popper conversion, right? Am I
remembering correctly that originally, the heater was in series with the
fan motor, acting as a current limiter? Did you ever (very carefully)
measure the voltage across the motor while the popper was powered?
Keep us in the loop. I for one think it's great to watch you work on
your design and share helpful knowledge when needed.
Cheers,
-Brian
On 12/29/19 12:03 PM, Pete Soper via TriEmbed wrote:
Hi Craig,
Very few people on this list are going to have KiCad at their
fingertips, so I transmogrified your schematic into a PDF. Folks may
have to turn their heads sideways to look at it depending on their PDF
viewer:
http://triembed.org/doc/craig-cook-coffee-roaster.pdf
In my opinion the stuff on the left side of the schematic should be
inside an all metal box with the box connected to earth ground by a good
power cord with standard three prong AC plug properly wired. You don't
specify R1 (heater), so it isn't possible to judge whether the current
capacity of the SSR is up to the task. Two amps seems marginal for a
heater designed to heat more than a few beans at a time.
R4 is too large, in my opinion, and the FET turn on time might be
very slow, but that may be a non-issue. It would be harmless are more
likely to work well if it was 100 instead of 10k ohms. D1 should be
swapped end for end and have 20V on the cathode end, not ground, to act
as a snubber (i.e. it's in parallel with the motor but only conducts in
the opposite direction). The 20V of course has to match the voltage
rating of the actual motor.
I'm not in the position to judge the display connections. I'm also
not up on KiCad enough to judge whether you've got the Arduino's five
volts shorted to ground with the "PWR_FLAG" net, but it seems that way.
The thermistor circuit seems reasonable.
-Pete
On 12/29/19 10:36 AM, Craig Cook via TriEmbed wrote:
Would someone be willing to review my kicad drawing of a popcorn
coffee roaster?
https://drive.google.com/open?id=141Y2Q9tdyt8iiL_LouNO4CarcEIShBtb
I have the Arduino Uno working. Tested the thermistor, display and fan
control. Have not tried connecting the solid state relay yet.
I am basing my work from this instructable:
https://www.instructables.com/id/A-fully-automatic-coffee-bean-roaster-Arduino/
Thanks
Craig
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