Simple circuit. Can you help?

Hi,
I'm building a ligth system for a video camera. Basically its 36 diodes aranged in a circle and placed over the lens. I can manage that part alright but I am having a problem building the regulator switch. The LED's draw about 1A and the voltage is set btween 3 and 4. What I want to do is be able to dim the LED's as and when I need to. I've built a circuit using a power fransistor on the recommendation of a friend. But can't seem to get it working. Some one else has mentioned a ' power Mosfet' as a voltage controlled resistor. Would this work? And if so which mosfet? I'm realy quite in the dark here, forgive the pun, and would appreciate any imfo and technical help you have. Thanks.

Paul

You will save energy by using a series-parallel arrangment of LEDs, and you will need a higher voltage to operate the mosfet anyway. I dashed this circuit off to give you an idea of how to do it.

Russlk said:

You will save energy by using a series-parallel arrangment of LEDs, and you will need a higher voltage to operate the mosfet anyway. I dashed this circuit off to give you an idea of how to do it.

Click to expand...

I agree with Russlk about running the LEDs in parallel. this is not good practice since you cannot accurately predict the current through each LED due to the spread of LED characteristics.

The circuit he posted applies a square wave signal to the LEDs and their brightness is controlled by the pot which controls the mark/space ratio of the waveform. This is the most energy efficient way to do it and it minimises the FET dissipation since it is either on or off.

Len