Moderators: Dave Moll, Andrew Davie, Steve Anderson
Klaas Robers wrote:Andrew, before you proceed: breadboard one string of 3 LEDs and one resistor of 47 ohm, apply power and measure voltages and currents. You need to reserve some voltage for the switching transistor.
Andrew Davie wrote:I'm currently thinking of alternatives. How about I have a higher voltage on my input supply. Say, 15V. I run the motor direct off that, and the LED matrix too. I get a DC-converter to convert down to 5V for powering the Arduino. 15V would give me (15-3*3.4)=4.8V left at 20 mA --> 24 ohms which seems much more workable for current limiting resistor + balancing resistors. Assumption is I can run the motor at 15V and that I can get a good DC converter.
Klaas Robers wrote:I agree with Steve: use a higher voltage for the LED cluster supply. If you look at your diagram in the link that you gave some posts up, you see that the voltage at the bottom of the LED cluster, also the collector of the video transistor, is quoted as 6 volt as a minimum. Now follow me step by step.
- The voltage over one of your LEDs is 3.5 volt at the max. current of 20 mA.
- If you place (just) 2 LEDs in series, the voltage over the both LEDs will be 7 volt.
- Add 1 volt for the current balancing resistor (47 ohm for 20 mA, thats correct),
- that will give 8 volts for the cluster chain.
- Place that on top of the 6 volts of the collector,
- and you need 14 volts minimum for the LED power supply, not 12 volts.!
Klaas Robers wrote:But is you use a laptop power supply of 19 volt, abundantly available from old laptops,
- You can place 10 parallel branches of 3 LEDs in series,
- with one balancing resistor in each branch,
- a total current of 200 mA
- 30 LEDs and 10 balancing resistors of 47 ohm.
Klaas Robers wrote:The advantage of a lower total current is that the transistor heats up less,
- for 600 mA it dissipates about 3 watt,
- for 300 mA it is about 2 watt,
- for 200 mA it is about 1 watt.
- In all cases the transistor needs some from of heat sink.
Steve Anderson wrote:Are you using PWM to drive the LEDs, or an analogue arrangement? If using PWM then a switched constant-current source for each 'string' is ideal. I'll elaborate if PWM is your chosen mode..
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