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1036  Using Arduino / Motors, Mechanics, and Power / Re: How do i measure the maximum current of an AC variable power supply with resetib on: February 12, 2011, 05:11:01 pm
Puting it simply, there's no way you will get 5 amps out of this without also releasing the smoke genie

If you want 5 amps at 8 volts then you need 40 volt-amp

If you need 5 amps at 15 volts then you need 75 volt-amps


The reason that ac power units are specified as volt-amps is because the supplier has no way of knowing what power factor your load will be.  At unity PF volt-amps is equal to watts.  However at reducing power factor the available watts is also reduced since watts is specified at the equivalent unity power factor.

Hence if the unit is specified as 40 volt amps with an 8 volt output then you will always get 5 amps out of it, even if you have a low power factor (inductive or capacitive load) only represent say 20 watts


1037  Using Arduino / Motors, Mechanics, and Power / Re: How do i measure the maximum current of an AC variable power supply with resetib on: February 12, 2011, 03:35:47 pm
Do you mean "how do you measure the current it is being loaded at"  If so the simplest way is to buy a cheap clamp-on test meter.

If you mean, what can you load it to without causing it to trip, then simply read the rating plate.

The resettable trip is probably a simple thermal device, in which case it will have a non-linear current x time relationship, so the maximum load current for say 1 second is probably twice that for 5 seconds

1038  Using Arduino / Motors, Mechanics, and Power / Re: Problem interfacing arduino and relay on: February 12, 2011, 11:59:56 am
If you have a meter I suggest you check the arduino output port to confirm it is going both Hi and Lo on demand.

Is your arduino a 5 volt unit.  If not then a 3.3 volt output is not within the specification of your relay board and will not be switching it.

If it is a 5 volt unit and you are getting 5 volts output then there is a possibility that the relay board isn't being switched - even though its specification states it should switch at 4 volts.

1039  Using Arduino / General Electronics / Re: Zener diode??? on: February 12, 2011, 04:57:31 am
I fail to understand your circuit.  Where is your 9 volts coming from to drive the milliamps and where is the P/D resistor you mentioned in your opening query.

You really do need to provide the full information if you want informed and correct answers, rather than best guesses.

1040  Using Arduino / Motors, Mechanics, and Power / Re: Motor Selection on: February 11, 2011, 02:40:14 pm
Not the answer to your question but for what it's worth :-
For that sort of application you really need an instantaneous release of energy.  "Unwinding" a motor will not give this as the motor inertia will effectively act as a brake, resulting in a slow release.  I'd advise you look at a trigger mechanism to release the "elastic" once it's been wound up.  The motor can then be run back to pick up and restretch the elastic.

1041  Using Arduino / General Electronics / Re: Zener diode??? on: February 11, 2011, 07:34:30 am
You should be asking yourself where the 9 volts comes from and you could then solve the problem.

A 4-20 signal should feed into a 250ohm resistor (R in your drawing) to produce a 1-5 volt signal for feeding into your arduino.

If the unstated value of R is greater than 250 ohms then the sensor will drive the voltage higher in an endeavour to ensure that the current range remains within the 4 to 20 scale.

I suspect your value of R is much too large.

Having said that, the zener should have conducted to clamp the voltage so I suspect you've blown it and it no longer works.

1042  Using Arduino / General Electronics / Re: separate grounds? on: February 10, 2011, 09:56:58 am
IF and it's a big IF, if the output side is truly isolated from the input side of the 48/12 convertor then you should be able to form a common ground between the 48 volts, 12 volts and the arduino.  Similarly you should be able to monitor the 48 volts supply provided you use say a 15:1 voltage divider chain across the 48 volt system.  15:1 since you will drop 15/16 of 48 volts (45 volts) leaving 1/16 (3 volts) to measure on the arduino.  You will need a bit of leaway since a 48 volt charged battery will be measuring somewhere in the order of 56 volts so the 3 volts could rise as high as 4.6 volts.

1043  Using Arduino / Motors, Mechanics, and Power / Re: Protecting Arduino connected to a power supply? on: February 10, 2011, 07:34:12 am
What you have looks perfectly OK, with power for the Arduino coming via the USB.  However, why not also connect your +12 volt supply into the Arduino power system and let the Arduino self-select between USB or main power.

From a colour semantics point of view I'd suggest you do not use red for the ground connections.  Red to me (and no doubt a few others) suggest a positive voltage (such as +12v).  I'd also suggest black for the ground connection.

1044  Using Arduino / Motors, Mechanics, and Power / Re: speed contraller on: February 06, 2011, 04:55:42 pm
More like you require a speed controller if the motor is capable of going faster than you require.   If the motor speed is suitable and you want to control its acceleration you will require a speed controller.   If the in-rush current during uncontrolled acceleration (the motor running up to full speed from a stopped condition) is greater than your power source can supply, you will require a speed controller.  If you want to reverse the motor whilst in motion you will require a speed controller.

In fact for any reason other than the most basic system you will find a speed controller a desirable feature

1045  Using Arduino / Motors, Mechanics, and Power / Re: How do flyback diodes work in H-bridge configuration? on: February 03, 2011, 09:08:44 am
Using your diagram of the H-bridge, you have to consider the battery as an effective short circuit (the battery internal resistance being extremely low).  The voltage generated by the motor then has a circuit route via the first series diode , through the battery and back to the motor via the second series battery.  It works irrespective of which direction the motor may be turning.

In effect this current flowing through the battery in a reverse direction actually performs a recharge of the battery and so provides regenerative braking to the motor.  However as the motor slows, its voltage output falls and hence the regenerative braking effect diminishes.

1046  Using Arduino / General Electronics / Re: Solar, Capacitors, Amp Hours and Discharge Time on: February 01, 2011, 07:04:14 pm
What do you mean by "a small solar panel"

The solar generated power during daylight hours has to be greater than what you extract at night.  Say you want 75mA for 10 hours, then you are consuming 750mAH during the night.  Will this be a 24 hour load, if so then you are looking at 1800mAH/day.  Say your solar panel is seeing sunlight for 10 hours then it needs to be rated for 1800mAH, or 180mA constant recharge current.   However the sun doesn't shine constantly, so allowing for cloud, rain, fog and general overcast you might want to assume getting one third of the panel rating.  So you're  actually looking for a panel that can provide a recharge current of say 500mA.

1047  Forum 2005-2010 (read only) / Troubleshooting / Re: Arduino skiing recorder/GPS on: November 21, 2010, 01:11:38 pm
On the basis that body temperature is +30ish and environment is -30ish, there will be a temperature gradient across your clothing.  Suggest you wear the equipment inside your outer thermal layers (ski jacket etc) but outside your body thermals.  Perhaps you can rig up all the equipment into a bum-belt fitted inside your salopettes.
1048  Forum 2005-2010 (read only) / Troubleshooting / Re: how close to 16 mhz? on: September 25, 2010, 02:09:29 pm
and here's a quote from wikipedia
"With any particular CPU, replacing the crystal with another crystal that oscillates half the frequency ("underclocking") will generally make the CPU run at half the performance. It will also make the CPU produce roughly half as much waste heat. Conversely, some people try to increase performance of a CPU by replacing the oscillator crystal with a higher frequency crystal ("overclocking").[1]  However, the amount of overclocking is limited by the time for the CPU to settle after each pulse, and by the extra heat created."
1049  Forum 2005-2010 (read only) / Troubleshooting / Re: how close to 16 mhz? on: September 25, 2010, 02:05:53 pm
Within reason I'd have thought that any resonator frequency (within the micro capability) would be OK.   Many micros are clocked at lower frequency to cut down on power consumption.

1050  Forum 2005-2010 (read only) / Troubleshooting / Re: power supply on: September 26, 2010, 01:24:33 pm
Maplin do a very neat polarity box which via a couple of LEDs shows the polarity of flying lead wall-warts.
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