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1  Development / Suggestions for the Arduino Project / Re: Arduino mega, using pins that are not mapped. on: Today at 10:24:13 am
If you think that's a problem the Due fails to bring out all the address bus and
data bus pins to headers - only 3 pins are omitted, but it makes the board
impossible to interface to parallel memory chips AFAICT.
2  Products / Arduino Due / Re: Arduino Due signal amplifier from DAC on: Today at 10:20:19 am
The LM358 will work up to 32V supply abs max - so OK at 25V.

That module isn't the thing to get though, since its decoupling capacitor voltage
is not known, it might fail.

Get a DIP LM358 and build the circuit yourself - you need a non-inverting amp
with a gain of +5.

Put 10k resistor between DAC output and +ve opamp input.
Put 10k resistor between -ve opamp input and GND.
Put 40k resistor (1k + 39k) between -ve opamp input and opamp output.
10uF 50+V decoupling cap on 25V opamp supply.
3  Products / Arduino Due / Re: DAC minimun output on: Today at 10:12:33 am
The range is 1/6th Vcc -- 5/6th Vcc.  You could put them through an opamp with
gain of 1.5 or -1.5 to get rail-to-rail.
4  Products / Arduino Due / Re: attachInterrupt get interrupted pin on: Today at 10:09:52 am
Use two handler functions.
5  Using Arduino / Motors, Mechanics, and Power / Re: Find AC current from DC Load on: Today at 10:07:36 am
I'm working on a project where I need to use a switch to control AC power to a laptop power supply (roughly 19VDC 3.5A). I need to make sure I'm not exceeding the max current on the switch, and I don't want to start cutting the power supply cable to measure it.

To figure out the current, I'd assume I find the DC power (Amps*Volts) then divide by 120 VAC. Something tells me the true conversion isn't so simple/ideal. I estimated that I'll be using about 40-45 watts DC (mostly at 12VDC after going through a buck converter). Can someone tell me if this is correct or point me to the right conversion if it is not? Thanks

Read the label on the laptop PSU, it's required to say the current draw of the device,
which gives you the answer without any work!

Mine says 19.5V 6.2A out, 100V -- 240V 1.2--0.7A in
6  Using Arduino / Motors, Mechanics, and Power / Re: Any ideas for a "water dripper" on: Today at 10:03:59 am
+1 to peristaltic pump + stepper, unless you want to conserve power, in which case
a DC gear motor + encoder driving the pump might be better - a CRS even.
7  Using Arduino / Motors, Mechanics, and Power / Re: ServoTimer2 works for a few seconds, then stops working. on: Today at 10:00:58 am
Wel another quick look at ServoTimer2.cpp doesn't suggest anything, where
did you get your copy from BTW?
8  Using Arduino / Motors, Mechanics, and Power / Re: Small stepper+Accelstepper- Q.1) rotation speed, Q.2 [solved]) absolute position on: Today at 09:53:23 am
I don't see any mention of microstepping in your post. Are you doing that ?

Its a unipolar stepper, 28YBJ-48, driven as described here:
so microstepping isn't relevant.
9  Using Arduino / Programming Questions / Re: 3D maths to sutract gravity from accelerometer data on: Today at 09:45:25 am
An object sat on the surface of the earth experiences a 9.8m/s^2
_upwards_ acceleration.
True, but it also experiences a 9.8m/s^2 acceleration downwards due to gravity. The two balance out and so the object doesn't move.
Lets be clear here, the accelerometer, in its own frame of reference, experiences
an acceleration upwards - that's all it knows (apart from the tidal forces, but they
are much much smaller).

In our frame of reference we also experience an acceleration upwards - we are in
the same frame, to a first approximation, so see the accelerometer as stationary.

Nothing is or appears to be accelerating downwards from this frame.  The force from
the floor is not cancelled out by anything, its a true upwards acceleration we feel
from our feet.

This is the key insight behind general relativity, that inertial and gravitational mass
are identical and apparent forces due to gravity are inertial forces.

The fact it doesn't appear to move due to this acceleration is because we are accelerating
with it.
No. We aren't accelerating and neither is the object. If we were accelerating we'd be moving.
see answer above.
There are two opposing forces on the object. The earth's gravity trying to pull it towards the centre of the earth and the force of the ground pushing it up by exactly the opposite amount. If the two forces aren't balanced, the object will be moving.
There is no force due to gravity, there is only curved space, described by the
gravitational tensor.

The fact that we stay the same distance from the centre of the earth is because
space is curved.
No. The curvature of space around earth is caused by the earth's mass and the result of the curvature is what we perceive as gravity.
Yes the curvature is caused by the mass, and YES that's why we don't appear to move
relative to the earth's centre.  We perceive the force from the floor, which is due to
the earth being an elastic body, resisting the deformation that the curvature gives it.

An object in free-fall has zero acceleration
That is a contradiction. An object in free fall is, by definition, accelerating.
An object in free fall is free of all forces, and therefore subject to zero acceleration
in its own frame - this makes it an inertial frame (apart from those pesky tidal forces,
themselves the signature that space is curved).

I recommend a course on general relativity.  Its been validated by several elegant
experiments since Einstein proposed it.  Without it you cannot explain why atomic clocks run
apparently faster at altitude - with it you realise you have clocks in different accelerating
frames hence the difference
10  Using Arduino / General Electronics / Re: MOSFET Safe Operating Area on: Today at 09:22:01 am

That means this MOSFET is handling a lot of power, and power makes heat.  So I find that there are graphs of Safe Operating Areas (SOA) for MOSFETs, and they seem to be premised on pulsed performance. 

Normally the graphs show SOA at various pulse lengths and DC.  You'll find the
curves are simply current, voltage and power limited (with the power limit
depending on pulse width / duty cycle.

You normally wouldn't go anywhere near the current limit.

Plot your system voltage on the I=0 axis, plot your max current on the V= 0
axis, both points should be inside the DC SOA.  Draw a straight line between
them (for a resistive load), and check that if the line strays outside the DC
SOA the shortest pulse time involved.  Ensure you switch the device faster
than that (in practice not a problem).

With a high voltage MOSFET you'll find the on-resistance is quite high and the
dissipation may be dominated by this.  Note that the edge of the SOA, where it
represents a power limit, assumes you have infinite heatsinking (which you
don't), so derate accordingly.

If you are PWM'ing you will have to calculate switching losses carefully.

BTW If I was handling voltages this high I would definitely use a MOSFET driver
chip to control the gate voltage against dV/dt from the drain.  Better still use an
IGBT which are much more robust for high voltage.
11  Using Arduino / Programming Questions / Re: Help,no matching function for call to 'Adafruit_NFCShield_I2C::readPassiveTarget on: September 16, 2014, 09:19:04 pm
Perhaps there's more than one version of the library?  Look at the .h file you are
actually #including.

byte goodCard=0x64,0x40,0xB1,0x76;
byte goodCard[] ={ 0x64,0x40,0xB1,0x76 };
12  Using Arduino / Programming Questions / Re: 3D maths to sutract gravity from accelerometer data on: September 16, 2014, 09:15:19 pm
You shouldn't need to use Euler angles at all in simple coordinate transforms - these
are performed on the DCM with matrix multiplication.   Remember matrix multiplication
is not commutative (neither are rotations in 3D) so you have to use the right matrix
in the right position to effect the wanted transformation.  Due to rotations being nice
and well behaved mathematically the inverse matrix is simply the transpose so its
trivial to calculate.

Pass some simple test vectors into the code to confirm its rotating the right
13  Products / Arduino Due / Re: Serial.print() Timer Routine - Arduino Due on: September 16, 2014, 09:04:22 pm
I think as things currently stand Serial output isn't interrupt driven on the Due
so you can use Serial.print in an ISR.

However I wouldn't rely on this state of affairs persisting, its likely to change at some point
I suspect.  Careful choice of interrupt priorities would then be advised.

Back to the real issue - you need a version of that timer setup routine that takes
a period in microseconds, not a frequency.
14  Products / Arduino Due / Re: Good parallel flash memory for the Arduino Due? on: September 16, 2014, 08:56:48 pm
Well you'll have a job, A6 (PC27) isn't pinned out on the Due board AFAICT.  Neither
are D8/D9
15  Using Arduino / General Electronics / Re: 0 Voltage from power supply when connected to peltier on: September 16, 2014, 08:40:39 pm
If the fan uses a brushless motor (and many of them do) then extra volts will not materially increase speed.  The 3phase module that converts the DC to an AC generally operates at fixed frequency ior at a frequency determined by an external temperature sensor.

Brushless fans run at a speed proportional to the supply.  They are 2-phase, not 3-phase,
normally, and have hall sensors so the frequency is locked to the fan rotation.  The back-EMF
of the windings roughly balances the supply voltage at equilibrium so that the speed is
nearly linearly related to the supply voltage.  (Most 12V fans work from below 6V up to
15+V if you don't mind taking the risk of damaging them).
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