Dear All


I've recently disassembled an LED lamp because the touch-buttons for it stopped working. I now intend to replace the electronics inside with physical controls, more specifically a potentiometer (with an end-switch if possible). This would allow me to set a dimming level permanently by leaving the potentiometer at a certain level and I could turn the lamp on and off via the switched power-socket (instead of having to press the annoying touch-button every time it gets power).

The lamp is made up of two seperate LED array rings: a larger one pointing to the ceiling and a smaller one intended for reading.

I now require some help designing a circuit to dimm the LED rings.


Here's what I'm working with:

Electronics:
The pre-existing electronics showed a maximum current rating for both lamps. Using this I tested for a maximum voltage until that current is used. I’ve also tested the minimum required voltage to have the LEDs visibly turn on.

Power supply (existing)	24 VDC	1.5 A
LED ring (large)	14 - 19.3 V	0.9 A
LED ring (small)	12 - 16 V	0.25 A

Space requirements:
The housing is an oval cylinder with two 12 mm rods running through it. The front has two 16 mm holes that used to house the touch-buttons and where I now want to mount the potentiometers. Enlarging the holes is possible if necessary.

Additionally, I’d like to keep costs as low as possible, say below 50 bucks.


What I’ve attempted so far:

I’ve tried designing a control-circuit as seen in the picture below, trying to regulate the current flowing through the LED by using a BJT transistor controlled through a potentiometer.

The circuit is then doubbled to controll both LEDs. Given the different voltage requirements, the compontents would need to be different for each LED ring.

I’ve attempted to calculate the needed components for this layout, however I find myself out of depth when it comes to variable circuits and components working with a voltage range.

For example: I’ve found a transistor (TIP47, see data sheet, (for the larger LED ring)) which can handle the current needed and through that calculated the value of the resistor. This component has a 1V voltage drop from Collector to Emmiter. The current is 0.9 A and the current coming from the base is negligable. Meaning the voltage over the resistor is between 3.7 and 9V, leading to a resistance of 4.1 - 10 Ohms.

This is where my knowledge ends however. I don’t know how to decide on what resistor to finally use and how it affects the circuit when the LED is dimmed. Additionally, I don’t know how to figure out the resistance range of the potentiometer and wether I’ve chosen the transistor correctly.

I now ask for your support here.
Is this circuit itself even correct?
Does it do what I expect of it?
Is there potentially a better or easier way of achieving my goal?
What components with what exact values do I need to complete this circuit?

Kind regards and many thanks in advance
yoxi

 

Welcome to AAC!

Is this circuit itself even correct?

Power dissipation will be an issue. Something will have to dissipate about 5W for the large ring when it's at full brightness.

Is there potentially a better or easier way of achieving my goal?

I think it would be better to use a PWM and use a MOSFET that can handle more than 24V on the gate for switching.

Here's something I posted in another thread.

IIRC, the duty cycle range was around 20-80%. It would require different, and more, parts to get 0-100%.

 

IF the TS is able to select the supply voltage to provide the maximum light when the series resistance is close to zero, then the heating will be less.