What Wattage Rating Is Needed for a 33Ohm Resistor in a 9V, 1A Circuit?

[NascentOxygen]

You can connect the LED board directly to the +12 and 0V terminals of the supply, and connect the Arduino's plug to the two terminals of the supply also.

 

[HeyAwesomePeopl]

You can connect the LED board directly to the +12 and 0V terminals of the supply, and connect the Arduino's plug to the two terminals of the supply also.

Confused. Like this?

 

[NascentOxygen]

Two wires from the 12VDC supply have to go to the Arduino. You show only one, and it's +12V so should not be going to GND.

Separate question: you show something going to pin 5 of the board. You need the base resistors to each connect to a different pin on the board. Do you have this sorted? You seem to have the base resistors going to a constant +5V on the board---this would keep the LEDs lit all the time, not under software control.

 

[HeyAwesomePeopl]

Two wires from the 12VDC supply have to go to the Arduino. You show only one, and it's +12V so should not be going to GND.

That one line includes a postive and negative. It's kinda confusing, I cannot seem to split the line.

Separate question: you show something going to pin 5 of the board. You need the base resistors to each connect to a different pin on the board. Do you have this sorted? You seem to have the base resistors going to a constant +5V on the board---this would keep the LEDs lit all the time, not under software control.

Oh yes, I just forgot to remove that. Each "+5" sign resembles another pin used. In this case, I am using 3, one for each color.

 

[NascentOxygen]

 

[HeyAwesomePeopl]

And 56 replies later, I have a solution! Thanks for everything.

 

[NascentOxygen]

Better not close this thread 'till you have it working ...

 

[HeyAwesomePeopl]

Better not close this thread 'till you have it working ...

Haha I won't. It'll be a few weeks until I get the parts in.

Just to confirm, to calculate the wattage rating I should get for the resistors, I should use P = I^2 * R which is Watts = Current^2 * Resistance. In this case, I would just need a half watt for this circuit.

 

[NascentOxygen]

Just to confirm, to calculate the wattage rating I should get for the resistors, I should use P = I^2 * R which is Watts = Current^2 * Resistance. In this case, I would just need a half watt for this circuit.

No.  I suggest that you use resistors rated at at least 3 times the power you will require they dissipate. Some of your higher value resistors will dissipate close to 0.5 watts, so you should use nothing less than 1.5W. I guess this means going for 2W.

If you were to divide your LEDs up into strings of 3 LEDs and 2 LEDs you might need only 1W resistors. Do the math!

(An alternative to a single 2W resistor is to use 3 resistors, either: each 0.5W but 3 times the Ohms that you need all in parallel,
or
each 0.5W but one-third the Ohms you calculate, connected in series. Or something equivalent.)

 

[Windadct]

Kudos for your diagrams - they are the language of electronics! ( Edit - not that they are perfect or follow the best practices(still good) , but that you took the time to do them with a proper schematic tool...)

Now how does it turn on and off:)

 

[HeyAwesomePeopl]

Kudos for your diagrams - they are the language of electronics! ( Edit - not that they are perfect or follow the best practices(still good) , but that you took the time to do them with a proper schematic tool...)

Now how does it turn on and off:)

Thanks. The Arduino chip will send a 5volt, 40mA signal when I program it to, triggering the transistor.

No.  I suggest that you use resistors rated at at least 3 times the power you will require they dissipate. Some of your higher value resistors will dissipate close to 0.5 watts, so you should use nothing less than 1.5W. I guess this means going for 2W.

If you were to divide your LEDs up into strings of 3 LEDs and 2 LEDs you might need only 1W resistors. Do the math!

(An alternative to a single 2W resistor is to use 3 resistors, either: each 0.5W but 3 times the Ohms that you need all in parallel,
or
each 0.5W but one-third the Ohms you calculate, connected in series. Or something equivalent.)

Why do you suggest resistors wattage should be rated at least 3 times higher?

 

[Windadct]

Hap..

Not really to criticize -- the Arduino is a micro-controller(uC) platform ( easier to just say Arduin) - They are based on a chip - like ATmega328... just guidance...

The output is 5V, but the 40mA is based on the load think 40mA MAX ( it will not "force" 40mA) - the reason I make this distinction is when you look at the ATmega320 datasheet ( or which ever one is for your version of Arduino) the actual chip can put out a Max 40mA in one GPIO ( general purpose Input Output) - but the chip I think has a Max total of 200mA - so what does that mean? you can run 5 BPIO at 40mA - or 15 at 13mA etc.. -- but I would not advise it... as a general rule of thumb - if you are communicating (logic / signals) no problem - but almost any load ( think work..light, sound, motor etc) - use some type of a switch where arduino ( or any uC - including chips) is the control but not the source of the power to drive the load.

As for why should the resistor have 3x the power rating -- you generally can not predict everything the system will see ( experience) over time - one period of extreme conditions and the system will die. However for this case ( 1 pc build, LED load, non-critical application) I think 3 x is probably more then you need. It can also be considered what is the consequence if this resistor dies... LEDs and the Transitor-- worst case... I would be comfortable with 2 x or less. Above 1 W resistors tend to get expensive.

 

[HeyAwesomePeopl]

Hap..

Not really to criticize -- the Arduino is a micro-controller(uC) platform ( easier to just say Arduin) - They are based on a chip - like ATmega328... just guidance...

The output is 5V, but the 40mA is based on the load think 40mA MAX ( it will not "force" 40mA) - the reason I make this distinction is when you look at the ATmega320 datasheet ( or which ever one is for your version of Arduino) the actual chip can put out a Max 40mA in one GPIO ( general purpose Input Output) - but the chip I think has a Max total of 200mA - so what does that mean? you can run 5 BPIO at 40mA - or 15 at 13mA etc.. -- but I would not advise it... as a general rule of thumb - if you are communicating (logic / signals) no problem - but almost any load ( think work..light, sound, motor etc) - use some type of a switch where arduino ( or any uC - including chips) is the control but not the source of the power to drive the load.

Like I said, I am new to all this stuff. So this is really helping. But one thing I thought the transistor was to do was to act as a switch for the leds. The transistor would complete the circuit when it is provided a current. Isn't that what I did?

At least how I thought I designed the diagram, no power is coming from the board unless it is activating a switch. I know those LEDs take a good 30mA to power, meaning I could not power them all with the Arduino. This is why I have the LEDs on the main power source, to prevent using power from the GPIO pins.

As for why should the resistor have 3x the power rating -- you generally can not predict everything the system will see ( experience) over time - one period of extreme conditions and the system will die. However for this case ( 1 pc build, LED load, non-critical application) I think 3 x is probably more then you need. It can also be considered what is the consequence if this resistor dies... LEDs and the Transitor-- worst case... I would be comfortable with 2 x or less. Above 1 W resistors tend to get expensive.

That makes sense. I'll be sure to keep that in mind with later projects.

 

[NascentOxygen]

Presumably the 12V to the LEDs and to the Arduino will be present all the time (at least until this guidance system is no longer needed). This will be switched off at the wall power outlet, if ever it need be.

Do you intend having all LEDs off until a vehicle is detected entering by the ultrasonic sensor? Is that the plan?

The reason will for under-rating resistors is so that the board and nearby components are not troubled by heated surfaces.

 

[HeyAwesomePeopl]

Presumably the 12V to the LEDs and to the Arduino will be present all the time (at least until this guidance system is no longer needed). This will be switched off at the wall power outlet, if ever it need be.

Do you intend having all LEDs off until a vehicle is detected entering by the ultrasonic sensor? Is that the plan?

The reason will under-rating resistors is so that the board and nearby components are not troubled by heat.

Yes that is the plan, to use an ultrasonic sensor and PIR to watch for movement, then turn on LEDs.

 

[Windadct]

I was thinking to use a photo-transistor to power up the arduino. Basically the system sits near zero energy state until there is light - from garage light, headlights - daylight - etc.

 

[HeyAwesomePeopl]

I was thinking to use a photo-transistor to power up the arduino. Basically the system sits near zero energy state until there is light - from garage light, headlights - daylight - etc.

Oh I see, a photoTRANSISOR. Turn everything on when a light source is detected. Yes, I will do this then, it would be better.

Also, what's the status on my response to post #62(your post)? Post #63 is my response, I just wanted to make sure that I understood what you explained.

EDIT: Are you suggesting the the phototransistor goes between the positive wire for the arduino to activate the arduino when it senses light? I cannot seem to find a phototransisor that would accept the current the power supply outputs.

 

[HeyAwesomePeopl]

...

...

Okay I got some help from the forums over at the Arduino website. They helped me fix my diagrams and helped with more specific data. Here is my current diagram:

https://dl-web.dropbox.com/get/Stoplight-Project-2%20(4).png?_subject_uid=104089738&w=AAAuMEOU3iZPAZN4dS0Na0zjFg24NrpPvWF92cWvUavJ8g

So my setup now is to not use the full Arduino, but just the ATMega328P chip(saves space and money). This includes using a Switching Regulator to lower the voltage to 5 volts to power the chip, and a cemaric resonator clocked at 16Mhz. I have also just cleaned up my diagram a bit.

Please look over the diagram and confirm correctness. The only thing I am iffy on is the ground symbols. I know they mean to connect to ground, but what exactly is ground? One person says ground to Arduino and another says the negative wire on the power supply.

Thanks

Edit: Diagram doesn't want to show up. https://dl-web.dropbox.com/get/Stoplight-Project-2%20(4).png?_subject_uid=104089738&w=AAAuMEOU3iZPAZN4dS0Na0zjFg24NrpPvWF92cWvUavJ8g

 

[NascentOxygen]

Pic: ERROR 403

probably requires a login

 

[HeyAwesomePeopl]

Pic: ERROR 403

probably requires a login

Try this: https://www.dropbox.com/s/itfpuxfn903zw5p/Stoplight-Project-2 (4).png?dl=0

 

[jack476]

that.

The 1A capability of the supply must not be exceeded.

But doesn't the Arduino's board have its own voltage regulators and resistors to keep that from happening?

 

[HeyAwesomePeopl]

But doesn't the Arduino's board have its own voltage regulators and resistors to keep that from happening?

Yeah, it only uses what it needs and it has a maximum. The power supply I ended up using is a 1.5 amp, 12volt power supply, more than enough for everything.

 

[NascentOxygen]

 

[HeyAwesomePeopl]

You just posted the same thing I posted...

 

[NascentOxygen]

You just posted the same thing I posted...

An exact copy. I couldn't improve on perfection.

 

[HeyAwesomePeopl]

An exact copy. I couldn't improve on perfection.

;)
Thanks for all your help guys!

 

[NascentOxygen]

I just noticed your 4.7k base resistors should be lower resistance. They set a base current of less than 1mA.

Consider the yellow LEDs. You have two strings in parallel, each 30 mA, making 60 mA collector current. If the transistor is to function as a switch, you are requiring it to have a current gain (β) of at least 60. It is safest to bank on no more than a β of 10. So each 4.7k should be replaced by 560 Ω.

 

[HeyAwesomePeopl]

I just noticed your 4.7k base resistors should be lower resistance. They set a base current of less than 1mA.

Consider the yellow LEDs. You have two strings in parallel, each 30 mA, making 60 mA collector current. If the transistor is to function as a switch, you are requiring it to have a current gain (β) of at least 60. It is safest to bank on no more than a β of 10. So each 4.7k should be replaced by 560 Ω.

How did you come up with 560 ohms? Why no more than a β of 10? Is there some sort of calculation that says how much current must be put into the base to close the transistor?

Explain what would happen if I used a 4.7K vs if I used a 560 Ohm resistor?

 

[NascentOxygen]

β is not a well-controlled parameter. Typically, a manufacturer may quote a wide range, such as 20 <β< 300. So if you design the base drive for β less than 20, you can be confident of switching any transistor properly. If there is not enough base current, the LEDs won't get the current you intended, and the transistor may overheat.

So, if you need a collector current of 60 mA, I suggest aiming for a base current of around 6 or 7 mA.

base resistor = 4.3v / 6mA