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### Author Topic: Can You Help Me With This Circuit Plz  (Read 6443 times)

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#### Plecc

• Guest
##### Can You Help Me With This Circuit Plz
« on: August 26, 2010, 12:09:30 PM »
Hi all,
I would like to make a simple 10x LED circuit with a potentiometer to control the brightness.
I want to use some 10mm blue LED's,
Spec:
Forward Voltage 3.2v
Power Dissipation 70mW
Peak Forward Current 100mA
Continuous Forward Current 20mA

I was thinking a circuit like this would be best:

^etc up to x10^
|                                                |
|----(resistor?)------(+LED-)------|
|                                                |
|----(resistor?)------(+LED-)------|
|                                                |
|----(resistor?)------(+LED-)------|
|                                                |
|----(Potentiometer?)---|            |
|           |
(+)         (-)
(Power Supply?)

Sry i'm a total noob at this, never rly understood how to calculate resistance.
Can someone advise me what resistors, potentiometer and power supply to use?
Is this the best way to rig up this circuit?

Thanks.
Plecc.

#### kam

• Hero Member
• Posts: 1849
##### Re: Can You Help Me With This Circuit Plz
« Reply #1 on: August 26, 2010, 20:58:59 PM »
Hi,
not easy to answer, for LEDs do not have same characteristics and same behavior in current change. The circuit you posted will work. You can use resistors of 20ohms 1/4 watts (the 100mA is correct??). For the potentiometer, you will need to experiment. I would start with a 200 Ohms potentiometer... Above calculations stands for 5V power supply.

Should i suggest rather a simple PWM circuit??? Better and more stable control, with high power efficiency. Here is one : http://pcbheaven.com/circuitpages/High_Frequency_PWM_Fan_Controller/ which can be very easily converted to rive LEDs, with only a few resistors.
« Last Edit: August 26, 2010, 21:08:39 PM by kam »

#### Plecc

• Guest
##### Re: Can You Help Me With This Circuit Plz
« Reply #2 on: August 27, 2010, 21:54:15 PM »
Thanks for the quick feedback,
Sooo much more complex than i thought to dim an array of LEDs.

A friend of mine found one based around one of the 555 timers.
Very similar to the one you suggested.

http://www.reuk.co.uk/LED-Dimmer-Circuit.htm

I can get hold of all these bits really easily, would this be suitable for running 5 strings of two, each pair joined to a 1/2W 330Ohm resistor?

Oh and the BC547 i have is 100mA rather than 200mA is this a problem?

Thanks.
Plecc

#### kam

• Hero Member
• Posts: 1849
##### Re: Can You Help Me With This Circuit Plz
« Reply #3 on: August 28, 2010, 08:38:59 AM »
What voltage will you use for power supply?

#### Plecc

• Guest
##### Re: Can You Help Me With This Circuit Plz
« Reply #4 on: August 29, 2010, 02:15:11 AM »
12v hopefully, but it doesn't have to be.