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5746  Using Arduino / General Electronics / Re: Power Supply Quirks on: July 02, 2013, 05:50:46 pm
It says on the data sheet double insulated. That means it is isolated. The transformers are not big because they do not carry much current and only isolate. That is they are one to one and wound on a torriode.
It also says leakage current at 0.25mA.

Which is quite a bit of leakage, hence you can feel it.  2mA is theshold of pain, things get dangerous quite
quickly above that, best to ground the output 0V line of the supply.
5747  Using Arduino / General Electronics / Re: Most common (single rail) Op-Amp used with Arduino? on: July 02, 2013, 05:47:26 pm
Thanks for the replies.

I'm looking at this circuit (my application has nothing to do with doors though) smiley
The op amp in this case is used as comparator, I could probably do away with this and feed the output of the transistor to the arduino and do the same in software?

Or use the analog comparator on the ATmega itself?
5748  Using Arduino / General Electronics / Re: Most common (single rail) Op-Amp used with Arduino? on: July 02, 2013, 12:11:02 pm
I've seen LM358's mentioned, have a few in fact, but they don't have a particularly
great spec (you won't get to +ve rail and it can't pull down to ground with more than a few microamps).

I did some research for an audio project and found the (rather expensive) AD8656 which has
some excellent specs for a rail-to-rail 5V device, in particular very low offset voltage, ultra low noise,
good bandwidth and settling time, and can drive extremely close to the rails at 1mA current level.
5749  Using Arduino / General Electronics / Re: Can someone confirm n and p mosfets for me please on: July 02, 2013, 11:50:42 am
Hi, just my 50cents worth, I agree with lemmings solution.
But have you measured how much current is consumed by the glowplugs, this can be quite high.
So choice of MOSFET and its physical connections will have to be also considered.
There are a lot of MOSFETs are out there that will supply 10 or more amps, but when you look at their physical construction their leads
would be lucky to survive half that.
(I work in the industrial electronics repair field and I've seen problems caused because of this.)
So choose your mosfet considering the physical attributes as well.

WIll do thanks - each of the glow plugs will pull 8 amps when cold.

TO220 package would be fine for that current level, assuming soldered to adequate
area of copper to dissipate the heat.  Standard circuit board is not really adequate though,
the copper is too thin.
5750  Using Arduino / General Electronics / Re: Pull-up resistors on: July 02, 2013, 11:40:40 am
In other words you have to treat the input as "active-low".

Just found that the relevant wikipedia entry is wrong (a rare occurrence for technical entires):

Active low represents asserted/true/yes/1 by the lower of two voltages...
5751  Using Arduino / General Electronics / Re: H-Bridge on: July 02, 2013, 11:34:57 am
You've got the PNP transistor pin outs wrong and labelled NPN/PNP incorrectly on the RHS. 
PNP are the high side switches, NPN low-side.  collectors to the motor, emitters to the rails.

You also have a circuit that will burn out if you press both buttons at the same time!
5752  Using Arduino / General Electronics / Re: Power supply for arduino 78xx on: July 02, 2013, 11:28:39 am
A voltage regulator in a car or motorcycle is a good idea for extra safety.
A safe value would be 7.5 or 9V.
So a 7809 would be a good choice.

And add plenty of filtering. The power will be connected to an alternator so the 12V line will be awful.

To give you an idea of how awful:  "automotive" MOSFETs are typically rated at 55V for use on 12V car
electrical systems.
5753  Using Arduino / Motors, Mechanics, and Power / Re: Motor Controller Current Limiting on: July 02, 2013, 11:25:56 am
At those power levels you'll want a hardware over-current fault detection circuit, it only takes a fraction
of a millisecond to turn MOSFETs to vapour at kilowatt dissipations...  You can output the current
sensor output to a comparator for fault detection as well as ADC for control and feedback purposes.

You'll need good layout practices to reduce the amount of electromagnetic interference from your
high-current paths.

375A H-bridge is no mean feat BTW....
5754  Using Arduino / Motors, Mechanics, and Power / Re: Motor Shield Power HELP!! on: July 02, 2013, 11:18:40 am
The chip used loses about 2 to 3V due to darlington output stages at nominal rated current, so a 4.5V motor
and 7.2V battery (6AA NiMH's or 2S LiPo) pack would be a good combination.  6V is perhaps OK, but I'd
recommend having a little more, remember that battery voltage drops with load and discharge...

If the battery can handle the max (stall) current from all motors simultaneously with ease (ie its a LiPo),
then you could just connect it to Vin as well as the motors.  For LiPo batteries you should have
a fuse or over-current protection circuit (they can push hundreds of amps into a short).

If the motor battery pack can't withstand 4 lots of stall current then use a separate supply for the
Arduino or it'll probably reset every time you load the motors sharply...
5755  Using Arduino / Motors, Mechanics, and Power / Re: L298 non zero output voltage on: July 02, 2013, 11:03:43 am
Since the device has darlington output stages it will have quite a large leakage current
(compared to single transistor output stage), but still effectively zero when we talk
about driving a motor.

The chip designers didn't care about the unladen output voltage of a motor driver,
its not a useful parameter, you might even be the first to try measuring it!
5756  Using Arduino / Motors, Mechanics, and Power / Re: distance sensor for motor rotation on: July 02, 2013, 10:57:07 am
What kind of distance sensor ?

What kind of motor?

Do you have a motor controller board or shield?

In general vague questions will only get a vague answer - anything is possible with electronics
(subject to the laws of physics) after all!
5757  Using Arduino / Motors, Mechanics, and Power / Re: Managed to damage a power supply but can I fix it? on: July 02, 2013, 10:54:38 am
Frankly any semiconductor involved in the damaged supply rail is a candidate,
replacing them one-by-one till it works (starting with the cheapest) is a possible

Might be easier to get another supply that has current limiting outputs (bomb-proof).
5758  Using Arduino / Motors, Mechanics, and Power / Re: Motor solution. on: July 02, 2013, 10:52:02 am
Certainly a big servo like that cannot perform on an inadequate power supply - fix this first.

But secondly you have given it a large torque load to handle - your robot arm would be far far easier
to drive if each section was counter-balanced (even partially).  The current setup needs a lot of torque
just to raise the arm, let alone any load.

The servo motor is a 10kg torque, should I change it into 15kg torque so that it can raise up easily?

kg is not a unit of torque.  I suspect you mean 10kgf-cm or some such unit - without knowing for sure I can't
tell if the change would be useful.  You should always try to work in S.I. units for torque, its so much more
convenient and simple.

You need to work out the torque needed to drive the arm comfortably (a rough estimate in N-m is
10 x M x L, where M is the total mass of the moving arm in kg and L is the length in m.)

For instance a 0.4m long arm weghing 0.5kg would work well from a 2N-m servo.
5759  Using Arduino / Motors, Mechanics, and Power / Re: Driving 4 small geared motors on: July 02, 2013, 10:40:28 am

How would I go about using a 7.4v 2200Mah battery with this?
An off-topic note about units - the case for symbols matters:  volt is "V", not "v", milli is "m" not "M" (which means mega), ampere is "A" not "a".

Also 2200mAh would normally be written 2.2Ah (except by manufacturers of lithium and NiMH batteries
who seem to fetishize the use of mAh!   lead-acid batteries and cells are marked in Ah though.

This industry-wide oddness occurs in at least one other area -  capacitors are rarely marked
in nF and never marked in mF (pF and uF are used even when this makes the numbers huge or

I suspect its something to do with marketing departments obsession with using larger numbers than the
competition, assuming a customer will be drawn to "10000mAh D-cell" in preference to "10Ah D-cell".

The capacitor thing is more mysterious though, possibly related to the US's backwardness in adopting
S.I. (I believe pF used to be called uuF originally).  nanofarads have sort of become mainstream now
but millifarads haven't, which is even odder (milliohms and millihenries are commonplace)

[ When I write u as in uF I really mean a greek lowercase letter mu, of course ]
5760  Using Arduino / Motors, Mechanics, and Power / Re: Driving 4 small geared motors on: July 02, 2013, 10:21:09 am
The output current is rated up to 1.2A per channel (or up to 3.2A for a short, single pulse).
A fuse is useful.

I would say for LiPo and lead-acid batteries (both capable of currents high enough to set fire to
wiring) a fuse (or over-current protection circuit) is essential.   Place fuse on +ve wire right next to
the battery, then any wiring downstream is protected should there be an accidental short.
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