Show Posts
Pages: 1 ... 716 717 [718] 719 720 ... 1169
10756  Using Arduino / Installation & Troubleshooting / Re: Small negative voltage on analog input on: April 21, 2011, 03:00:34 pm
Hello.
I have just one easy question - will a small negative voltage on the analog input damage the Arduino board (Mega 2560)?
My project involves reading some volages from a measuring circuit, and sometimes (when the measured device operates at a higher frequency than the op-amp in the circuit can handle accurately), about -10mV to -15mV appears on the analog input. Is this a problem for the board?


Shouldn't be a problem. The negative spec is -.5 below ground I believe. That's when the internal negative clamping diode will start to conduct and it can only handle a small amount of current. So -10 - -15 mv should not be a problem in my opinion.

Lefty
10757  Using Arduino / Motors, Mechanics, and Power / Re: Why do I need a diode over the coil contacts on a relay ? on: April 21, 2011, 02:50:11 pm
An interesting aspect of this is that one can often hear a difference in the relay's action when switched off between if there is a diode installed or not. When there is no diode the relay's off action is faster and on larger relays one can hear the sharp click sound, but with a diode installed the off action sound is more muted and the speed is a little slower.

 I noticed this when checking out a larger industrial 4 pole double throw 24vdc relay at work, using a scope to try and quantify the size and duration of the generated transient pulse. It was quite a short pulse, maybe just a microsecond or so, but easily 100+ volt peak could be seen on the scope.

Lefty
10758  Using Arduino / Programming Questions / Re: Encoder Output on: April 21, 2011, 11:12:22 am
Quote
I have the encoders going straight from pin 6 and 7 to the encoder. And the middle pin of the encoder going to ground.

That will work but only if you either wire external pull-up resistors from pins 6 & 7 to +5vdc, or enable the internal pull-ups for both those pins.

Lefty
10759  Using Arduino / Motors, Mechanics, and Power / Re: Why do I need a diode over the coil contacts on a relay ? on: April 21, 2011, 10:58:39 am
I'm sure the reply to this is common knowledge for those with experience, but I can't understand the purpose of a diode over the coil of a relay.

Don't get me wrong - I have been told I need it, and I don't dispute that, but I would like to understand why.

Any explanation or links in newbie terms (that means small words)?


When you apply a voltage to a coil is creates a magnetic field. When you remove the voltage the magnetic field collapses and creates a reverse polarity voltage and can be many times the value of the original applied voltage (X4 in my o-scope experiments). This creates a transient voltage pulse that can damage other components in the circuit that are not rated for this polarity or the higher voltage created, things like semiconductors and caps have a maximum voltage limit and breakdown if exceeded. Having a reversed biased diode across the coil allows the diode to conduct for reverse polarity voltages and creates a 'short circuit' across the coil that allows the pulse to be dissipated in the resistance of the coil wiring.

Lefty
10760  Community / Bar Sport / Re: Presented without comment on: April 20, 2011, 10:02:14 pm
Great, opens up the Arduino platform to a whole new user class age group of people.


Lefty

10761  Using Arduino / General Electronics / Re: Arduino Uno not providing power? on: April 20, 2011, 05:12:56 pm
Quote
the led only need 20ma to power up; a exemple: if you plug a 500ma usb on a arduino that need 40ma alone, the arduino will take only 40ma from usb. this is basic eletrical enegering. 


I'm afraid your basic electrical engineering needs a refresher course. 20ma is a pretty normal recommended continous current for a LED. However the LED does not determine this value, the current must be controlled external to the LED, normally with a series current limiting resistor. Once a LED has greater then it's forward voltage drop applied it will draw excessive current unless controlled externally.

I really don't care if you get this or not, what I'm trying to prevent is other newcomers reading your post and thinking they don't require the use of series resistors with LED because that will lead to damaged arduino boards and burned out LEDs.

Lefty
10762  Using Arduino / General Electronics / Re: Arduino Uno not providing power? on: April 20, 2011, 03:56:58 pm
20ma that should not damage the chip.

How is the led current being limited to 20ma?

Lefty
10763  Using Arduino / Installation & Troubleshooting / Re: Voltage regulator getting hot on: April 20, 2011, 03:55:39 pm
 I already answered that.
10764  Using Arduino / General Electronics / Re: Arduino Uno not providing power? on: April 20, 2011, 03:52:50 pm
Quote
this not short or damage atmega328.

Yes it will. To prove it to yourself measure the current you are allowing to flow through that led. Then note that the absolute maximum safe output pin current is 40ma.

Lefty

10765  Using Arduino / Project Guidance / Re: Live Action Scenario Bomb Prop on: April 20, 2011, 03:41:44 pm
Can't you just be happy with a blinking LED like the rest of us?  smiley-grin

By the way if you build one, don't take it to an airport.

Lefty
10766  Using Arduino / Motors, Mechanics, and Power / Re: Driving a solenoid on: April 20, 2011, 03:37:03 pm
Quote
Could it be that without the pull-down resistor to ground (gate to source) when I power up the system, the mosfet could momentarily turn the 12v water solenoid on?  Kinda like the H-bridge without pull-downs on the control pins let a motor 'jump' when powering up?
I don't have pull-downs on my mosfets, but can put them on if this scenario happens.  The mosfet and Arduino will always be on at the same time, other than powering up.

That is the purpose of the pull-down resistor. Recall that an Arduino starts up at power on with all pins as input pins and until the bootloader times out and your sketch sets the pin as an output pin and you set it to low, the mosfet's gate is floating without a pull-down resistor. So adding such a resistor should prevent the momentary activation at first turn on.

Lefty

10767  Using Arduino / Microcontrollers / Re: problem programming attiny45 on: April 20, 2011, 03:31:53 pm
No I'm not, I guess I should do that right? what size cap and where should it be placed?

Jb

You will have to search the posts for verification because I use a hardware programmer and haven't tried to use ArduinoISP as a programmer. I think it's a 10mfd wired between the reset pin and a ground pin, positive lead of cap to reset pin.

Lefty
10768  Using Arduino / Installation & Troubleshooting / Re: Voltage regulator getting hot on: April 20, 2011, 03:28:34 pm
How hot the on board regulators is a function of the following:

External voltage value, 7-12vdc. The closer to 7 volts the better for less heat dissapation.

How much external current any wired components are using of the board's +5vdc power.


Note that voltage regulators like this will automatically shut down if they get too hot or you draw more the it's rated maximum output current. So while hot is not good and should be avoided, it's not likely to damage the board.

Lefty



10769  Using Arduino / Microcontrollers / Re: problem programming attiny45 on: April 20, 2011, 03:20:50 pm
After loading the ArduinoISP sketch are you defeating the board's autoreset feature by using a cap from reset to ground?

Lefty

10770  Using Arduino / General Electronics / Re: Arduino Uno not providing power? on: April 20, 2011, 03:16:55 pm
Quote
Another possibly important detail:
When I connect a wire to the 5V, the green power-LED turns off (and disconnecting the wire turns it on again).

What is the other end of the wire going to? You are somehow shorting the power on the board. Are you sure you are following the wiring correctly. Are you using a resistor in series with your diode?  What value resistor?

Lefty

Pages: 1 ... 716 717 [718] 719 720 ... 1169