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

In summary, a 33Ohm resistor in a 9V, 1A circuit would require a wattage rating of at least 9 watts, as calculated by the formula P = VI, where P is power, V is voltage, and I is current. This ensures that the resistor can handle the amount of power passing through it and prevents it from overheating.
  • #36
Only one resistor. It's R1 that soaks up the spare voltage (and in the process sets the circuit current). Don't put a resistor where you show R2.
 
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  • #37
NascentOxygen said:
Only one resistor. It's R1 that soaks up the spare voltage (and in the process sets the circuit current). Don't put a resistor where you show R2.
Other than that, it is fine? What about the output of the arduino. Should I limit the milliamps or voltage going into the transistor? And does the voltage going into the base affect the current/voltage going through.
 
  • #38
HeyAwesomePeopl said:
Other than that, it is fine? What about the output of the arduino. Should I limit the milliamps or voltage going into the transistor? And does the voltage going into the base affect the current/voltage going through.
Oh, I see what you were up to. Yes, you'll probably need a resistor in series with the base to limit the current from the Arduino. So you had just shown it in the wrong place. How many volts will the port deliver for output HIGH? Anyway, choose R to limit the base current to about 3 or 4mA.

What is the maximum current the Arduino port can supply?
 
  • #39
Are you definitely going to power the Arduino board from the same voltage source as the LED array? If so, as well as connecting the +12V to the Arduino, you need to connect the 0V of the supply to the 0V of the Arduino board. It might be labelled GND or (-).
 
  • #40
NascentOxygen said:
Oh, I see what you were up to. Yes, you'll probably need a resistor in series with the base to limit the current from the Arduino. So you had just shown it in the wrong place. How many volts will the port deliver for output HIGH? Anyway, choose R to limit the base current to about 3 or 4mA.

What is the maximum current the Arduino port can supply?

The arduino can supply 3.3 or 5 volts and a max of 40mA per pin. Meaning I would need a

NascentOxygen said:
Are you definitely going to power the Arduino board from the same voltage source as the LED array? If so, as well as connecting the +12V to the Arduino, you need to connect the 0V of the supply to the 0V of the Arduino board. It might be labelled GND or (-).
0V of supply, the output of the transistor?
 
  • #41
HeyAwesomePeopl said:
0V of supply, the output of the transistor?
The emitter of the transistor, which is also the 0V of the power supply, connects to the 0V of the Arduino.

The output is 3.3V or 5V ... how does it choose?
 
  • #42
Using the 3.3 volt output of the arduino, a 100Ω would make it 0.033 amps
 
  • #43
HeyAwesomePeopl said:
Using the 3.3 volt output of the arduino, a 100Ω would make it 0.033 amps
The transistor base won't need that much current. Around 4mA will do.

The base uses 0.6V, so that leaves 2.7V across the base resistor. You can calculate the Ohms needed from that.
 
  • #44
NascentOxygen said:
The emitter of the transistor, which is also the 0V of the power supply, connects to the 0V of the Arduino.

The output is 3.3V or 5V ... how does it choose?
There is a 3.3 and 5 volt version boards. In this case its 5 volts
NascentOxygen said:
The transistor base won't need that much current. Around 4mA will do.

The base uses 0.6V, so that leaves 2.7V across the base resistor. You can calculate the Ohms needed from that.
So using a 5 volt board, it would come out 4.4 volts, and to get it around 4 milliamps (3.8 milliamps) it needs a 1200Ω resistor.
 
  • #45
HeyAwesomePeopl said:
So using a 5 volt board, it would come out 4.4 volts, and to get it around 4 milliamps (3.8 milliamps) it needs a 1200Ω resistor.
Sounds good. Are you going to update your circuit diagram now?

With the reds figured out, have you researched how many volts you'll see across your green and yellow LEDs? When you do this, you may decide to go with just 3 LEDs of each colour. :)
 
  • #46
NascentOxygen said:
Sounds good. Are you going to update your circuit diagram now?

With the reds figured out, have you researched how many volts you'll see across your green and yellow LEDs? When you do this, you may decide to go with just 3 LEDs of each colour. :)
Updated diagram with 3 colors of LEDs and correct resistors:
hdKcKel.png

Does everything look right? I'm going to double check the calculations right now to make sure everything is right.

NascentOxygen said:
When you do this, you may decide to go with just 3 LEDs of each colour. :)
Why 3? 5 too many? They are pretty cheap at 5 for a buck.
 
  • #47
I was thinking over 3V drop for one colour, but just checked and that's for whites.

You might end up tweaking the LED resistors, anyway, if it seems one colour is too bright in comparison with the others.

Don't forget to connect the power supply gnd to the circuit's gnd.

Good luck!

Thanks to Windadct for Arduino insight.
 
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  • #48
NascentOxygen said:
I was thinking over 3V drop for one colour, but just checked and that's for whites.

You might end up tweaking the LED resistors, anyway, if it seems one colour is too bright in comparison with the others.

Don't forget to connect the power supply gnd to the circuit's gnd.

Good luck!
Thanks. I used the data sheet to provide the recommended voltage and amperage per LED type. The Luminous Intensity varies for the LED colour, and would have to overamp/volt the LEDs which could significantly reduce life expectancy.

Don't forget the connect the power supply ground to the circuits ground? The power supply doesn't have a ground build in, no third pin(American Style). The ground is digital and is maintained by the Arduino. Does the whole ground to board still apply now?EDIT: Also, this transistor should work, correct? https://www.futurlec.com/Transistors/PN2222pr.shtml
 
  • #49
Instead of making the dimmer LEDs brighter, you could make the over-bright ones dimmer. :oldwink:

I wasn't talking about mains ground. I meant the "negative" terminal of the 12V supply should be connected to the negative or ground on the Arduino board and your LED board.

The 2222 transistor will be okay. It's always a good idea to place a finger on these switching transistors when you first switch the power on, they should not get even warm. If they get noticeably warm, you've made a wiring or design blunder.
 
  • #50
NascentOxygen said:
Instead of making the dimmer LEDs brighter, you could make the over-bright ones dimmer. :oldwink:

I wasn't talking about mains ground. I meant the "negative" terminal of the 12V supply should be connected to the negative or ground on the Arduino board and your LED board.

The 2222 transistor will be okay. It's always a good idea to place a finger on these switching transistors when you first switch the power on, they should not get even warm. If they get noticeably warm, you've made a wiring or design blunder.
The power to the board is a 2.1mm plug. That is why I question you plugging everything into the boards ground. The boards ground is usually only used if you use the boards 5V power pins.

And thanks for the warning!
 
  • #51
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.
 
  • #52
NascentOxygen said:
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?

AbRMWrd.png
 
  • #53
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.
 
  • #54
NascentOxygen said:
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.

NascentOxygen said:
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.
 
  • #55
thumbsup.png
 
  • Like
Likes HeyAwesomePeopl
  • #56
NascentOxygen said:
thumbsup.png
And 56 replies later, I have a solution! Thanks for everything.
 
  • #57
Better not close this thread 'till you have it working ... :nb) :nb)
 
  • #58
NascentOxygen said:
Better not close this thread 'till you have it working ... :nb) :nb)
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.
 
  • #59
HeyAwesomePeopl said:
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.)

 
  • #60
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:)
 
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  • #61
Windadct said:
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.

NascentOxygen said:
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?
 
  • #62
Hap..

Not really to criticize -- the Arduino is a micro-controller(uC) platform ( easier to just say Arduino:)) - 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.
 
  • #63
Windadct said:
Hap..

Not really to criticize -- the Arduino is a micro-controller(uC) platform ( easier to just say Arduino:)) - 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.

Windadct said:
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.
 
  • #64
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.
 
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  • #65
NascentOxygen said:
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.
 
  • #66
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.
 
  • #67
Windadct said:
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.
 
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  • #68
NascentOxygen said:
...
Windadct said:
...
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
 
  • #69
Pic: ERROR 403

probably requires a login
 

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