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Topic: managing current (Read 1 time) previous topic - next topic

katybradford

I am trying to control led strips with my arduino using only the pwm capable pins. The diagram shows how they are wired... three LEDs in series, wired in parallel with other groups of three. The LEDs are bright white 3528 smds, 20mA and 3.4 V. Therefore each strand requires 12V.

My question is how do I go about wiring this together? The 6 pins will have as many groups of three LEDs as follows:

1: 168 (3360mA)
2: 132 (2640 mA)
3: 96 (1920 mA)
4: 60 (1200 mA)
5: 24 (480 mA)
6: 6 (120 mA)

CrossRoads

Have the arduino control an NPN transistor, or an N-channel MOSFET, to sink the LED current to Gnd.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

winner10920

Especially for the higher current led strings, the easiest way is a logic level n-channel power mosfet with a 220 ohm resistor on the gate

katybradford

so there would be a MOSFET on each pin I use? or one MOSFET for the whole setup?

CrossRoads

One MOSFET for every string/group that you want to control.
If you are only controlling them as one huge group, then you could use a single Mosfet.
About 10 amps there?
http://www.digikey.com/product-detail/en/NTD5867NL-1G/NTD5867NL-1GOS-ND/2401422
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

winner10920

@10 ur gonna have atleast 5W heat so definetly need a heatsink,

winner10920

#6
Dec 20, 2012, 05:40 am Last Edit: Dec 20, 2012, 05:44 am by winner10920 Reason: 1
power dissapated is current squared multiplied by resistance and rds on at 4.5v is .05 so 10*10*.05 is 5w
And I get that because P=IV, V=IR so P=I(IR) or P=I^2*R

CrossRoads

You are correct, I withdraw my erroneous calculation.
P=IV=I*I*R = V*V/R, in this case I & R are known.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

katybradford

so is this what it will look like? A strand of lights to a resistor to the gate and then the arduino to the source??

I think I'm going to utilize the other 7 pins as well so I guess I will need a heat sink. They will be connected to more strands of LEDs.

I really appreciate the help! I am just shy of physics 2, therefore a noob.

MarkT

No, much more like the circuit in reply #1, but with MOSFET instead of NPN transistor.
[ I won't respond to messages, use the forum please ]

winner10920

Id maybe look around for a better fet, maybe get away without a heatsink if it matters, unless u have heatsinks laying aeound it'll be cheaper just to get a better mosfet

CrossRoads

The Arduino drives the gate.
The source connects to Gnd.
The drain connects to the LEDs.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

katybradford

Thanks everyone!

So how do I know what mosfet to choose? Like I said I have six strands of LEDs, 3.6 amps down to .12 amps. Would I use the same mosfet for each strand?

katybradford

If anyone would be willing to just help me design a little circuit completely I would be so appreciative...

CrossRoads

See Reply #1, replace the NPN with the Mosfet in Reply #4.
Can use the same Mosfet for all of them, or browse digiey for a similar part:
N-channel mosfet
single FET
in stock
logic level
thru hole (or SMD, your call)
I usually sort by price at that point, looking for a Low Rds/Low Input Capacitance part that can be purchased as single items (vs 1,000).
You can sort by Rds also if you'd prefer.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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