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Topic: Mechanically adjust power for LEDs (Read 494 times) previous topic - next topic

Ben1234

I have a strip of LEDs that I want to illuminate stuff (workbench photography etc.) that are in parallel. If I have a fixed power supply say 6v and I want to be able to dim the lights without using any microcontrollers from fully on to nearly off (I would use a switch to guarantee that they are entirely off) how would I do that? Is it really as simple as a potentiometer and if so how would I determine what kind.

I will have 16 LEDs with a forward current of 20mA and 3.4V. Thanks for your help in advance.

fungus

#1
Mar 28, 2013, 09:16 pm Last Edit: Mar 28, 2013, 09:20 pm by fungus Reason: 1
A potentiometer connected to the gate of a MOSFET.

For best results use a logic-level MOSFET and put a resistor R1 between +5V and the potentiometer, and another resistor R2 between the potentiometer and ground. The middle pin of the potentiometer goes to the MOSFET gate.

R1 should be about half the value of the potentiometer 's resistance and R2 about a quarter of it.
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CrossRoads

Sure - you have a pot that controls a PWM signal into a transistor to adjust the average current.
Can be a pot controlling the PWM from a 555 timer, whose output drives the Gate of an N-channel MOSFET in the - lead of you LED strip.
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Ben1234

What if I had something like this where I had a 10K pot? I don't know though that value I would need for R5. If that would work do you have any suggestions.

fungus


What if I had something like this where I had a 10K pot? I don't know though that value I would need for R5. If that would work do you have any suggestions.


I'd put another resistor on the other side of the pot as well. Basically you're creating a resistor divider chain so that the voltage arriving at the gate is in the MOSFET's linear region. The exact values will depend on the MOSFET - look at the curves on the datasheet.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Ben1234

#5
Mar 28, 2013, 09:33 pm Last Edit: Mar 28, 2013, 09:35 pm by Ben1234 Reason: 1
Quote

I'd put another resistor on the other side of the pot as well. Basically you're creating a resistor divider chain so that the voltage arriving at the gate is in the MOSFET's linear region. The exact values will depend on the MOSFET - look at the curves on the datasheet.




What kind of values should R6 and R5 be? I am not sure how to calculate it. The LEDs will draw 320mA per hour and I was thinking of using a https://www.sparkfun.com/products/522 . I will need to cut the total current to 200mA.

fungus


What kind of values should R6 and R5 be? I am not sure how to calculate it. The LEDs will draw 320mA per hour and I was thinking of using a https://www.sparkfun.com/products/522 . I will need to cut the total current to 200mA.


The MOSFET should go between R4 and ground.
The wiper of the pot should connect directly to the gate of the MOSFET (with no resistor).
Now put a resistor R1 from +5V to one leg of the POT and another resistor R2 from the opposite leg of the pot to ground.

R1 should be about 20k, R2 should be about 5K.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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