24v light bulb

[TimB]

Sorry really OT and a bit long winded

I have been building my new extension and now its about done I'm trying to source 24v light bulbs E27 for a couple of pendant lights.

The issue is that I want to control the brightness using an RF controller. A WT1 by Tayu. It's a pwm dimming system.

It works great on my 24v Led strips but cannot find a E27 light bulb that is compatible. The ones I have are 12-24v so they regulated internally down to around 9v. So for most the the PWM range nothing happens then the bulb starts buzzing and flickering.

I took a bulb apart and see it uses a custom led driver https://www.ledlightbulb.net/Projectengineering/MT7201-specification.pdf

My question is what can I do? I was hoping to hack the circuit but the circuit but I cannot see how.

The other option was to make my own PCB but think I cannot safely have 40w of leds on the pcb using a load of Resistors to get the voltage right. The dia of the circuit in the bulb is 46mm.

Any ideas?

Thanks
Tim

[TimB]

Ok found a board that will work on Aliexpress https://www.aliexpress.com/item/1005007474859920.html?spm=a2g0o.order_list.order_list_main.5.1c991802zDKpAa

[Fanie]

Hello Tim !
The big thing with driving LED's is to be able to limit current...
Here is a basic circuit to do that.



The bottom resistor determines your current.  As soon as the voltage drop over this resistor goes to around 0,8V the transistor pulls the gate of the FET down and the current through the LED's are lessened.
The FET operates in linear mode so you have to draw PCB surface for heat dissipation or add an external heatsink.
Hope this helps.


I want to add - the gate resistor on the left doesn't have to be as high.  It can even be a 1k, but choose a value the driver can drive comfortably and the small transistor can sink without problems.

[Fanie]

Your choice of a FET
You know this, but for the less experienced...
Some FET's require higher voltage to switch the gate properly.

This FET can be switched directly with 3V since the threshold (to keep it on) is low enough (2V).



This one needs at least 5V to switch the FET.  Driving it with 3V for instance will have it perform poorly since the threshold is 4V and will hence always be in linear mode.



The maximum gate voltage is also in the data sheet, this of course should not be exceeded.
If you do not have a choice of FET's and the threshold voltage is too high for the driver voltage, then you can use a pull up resistor (to say 12V) and a small FET to sink the gate voltage, which will invert the PWM, which can be inverted again by repeating the small FET circuit.

[Fanie]

Measure the voltage drop over a LED (it is also in the data sheet).
This will determine the LED count you can have in series from your supply.

If the LED's forward drop is say 2V1, then the 8 LED's in series will drop about 17V.
If the supply is 24V as in Tim's case, all 8 LED's can be in series.  If all the LED's are in series, the current drawed by the LED's are minimum.
If the supply is 12V, then 4 in series x 2 will have to be used and the LED's will drop around 8V4, and the current will be roughly double.

The LED's I use in my LED lights have a voltage drop of 10,5V for each 3 in series so I drive them off the solar's 12V.
12V is also a commercial (automotive) standard, where 24V is an industrial standard.
You get everything in 12V too, even microwave ovens.

I want to add as a note - the Chinese LED strips does not have current limiting (and they do not believe in polarity protection diodes either).
As long as your power supply ie 12V as specified is maintained, the LED strip will last.  If you have a charger for batteries, and the voltage goes to 14V, then the strips will not last very long.