How to choose an appropriate LED Driver for PWM dimming

Hello,

I would like to add dimming capabilities to a LED system I recently purchased. Since the LEDs (:Look at LED.jpg) on my system consume a lot of power (120-240AC, 1.2A converted to 30~46VDC, 0.65A: Look at Driver_Transformer.jpg) I figured that using a MOSFET alone would not be sufficient. Thus I found this chip: http://www.ti.com/lit/ds/symlink/lm3404.pdf but am a little concerned that it is a “Buck Regulator”… if I’m inputting 46VDC,0.65A into the chip does this mean that the resulting Vout and Iout will decrease and increase respectively? or does the Vout and Iout reman constant as 46VDC,0.65A? The last thing I want is to burn up my LEDs, so I would appreciate it if someone could shed some light on this matter.

Have you read the guidelines about how to use this forum? If so you did not read the bit about picture sizing. Mind you they are a bit useless anyway.

30~46VDC, 0.65A ..... I figured that using a MOSFET alone would not be sufficient.

Why not power FETs can easily handle this small current at 46V.

What you are missing is information about your LEDs. Do they have a driver in them already? Most systems do, in which case it is hard to tell how they would react to having their input voltage PWMed.

Thank you for the response! and apologies for the picture sizing. I have been a member of the forum for a while now, but was unaware of the sizing limit until now.

Why not power FETs

I extrapolated from this post: http://forum.arduino.cc/index.php/topic,68200.0.html, that high current consuming LEDs should be PWMed with a driver.

Do they have a driver in them already?

It is my belief that the transformer in my system also contains a LED driver but since the system is modular the connection between the transformer and LEDs can be easily disconnected (it is at this location where I would like to add a LED PWM-Capable Driver).

I extrapolated from this post: http://forum.arduino.cc/index.php/topic,68200.0.html, that high current consuming LEDs should be PWMed with a driver.

No that said that high current LEDs need a constant current supply and not just a resistor limiting the current.

It is my belief that the transformer in my system also contains a LED driver but since the system is modular the connection between the transformer and LEDs can be easily disconnected (it is at this location where I would like to add a LED PWM-Capable Driver).

If that is true then you will need another power supply and a constant current driver that you can apply PWM to.

I am currently awaiting a response from the manufacturer regarding the transformer. Hypothetically speaking, if the transformer did not have a driver and was simply a power supply would it be feasible to implement the driver recommended earlier? (http://www.ti.com/lit/ds/symlink/lm3404.pdf )

Thank you for your time and consideration

Hypothetically speaking, if the transformer did not have a driver and was simply a power supply would it be feasible to implement the driver recommended earlier? (http://www.ti.com/lit/ds/symlink/lm3404.pdf )

ONLY if your LEDs take 1A in current and not less.

Thx Grumpy_Mike. I have confirmed that the transformer is actually just a power supply (converts AC->DC) and does not contain any driver, so If I select an LED Driver that outputs constant current less than or equal to 650mA (the max current my LEDs take) I should be fine right?

Also regarding the previously mentioned chip (http://www.ti.com/lit/ds/symlink/lm3404.pdf), if the input current (from the power supply) is only 650mA, shouldn't the output current (from the LED driver) also be 650mA? or does the chip regulate the input power insomuch that a constant output current of 1A is always emitted ? (hence the name buck regulator)

Yes less than or equal to the LED current.

does the chip regulate the input power insomuch that a constant output current of 1A is always emitted ? (hence the name buck regulator)

Yes it reduces the voltage so that only the 1A current flows. A circuit that increases the voltage is known as a boost circuit.

1cecream: Thx Grumpy_Mike. I have confirmed that the transformer is actually just a power supply (converts AC->DC) and does not contain any driver, so If I select an LED Driver that outputs constant current less than or equal to 650mA (the max current my LEDs take) I should be fine right?

That is a LED driver; it states it quite explicitly on the label. It is not a "transformer" and I'll guarantee that if you try to use it as a DC power supply that it won't work.

With regards to controlling it with PWM, either by switching the AC input or the DC output, that won't work with this type of driver either. Try a simple test: turn it on and see how long it takes for the LEDs to light up. It's just a fraction of a second but what you're seeing is the driver attempting to reach a stability in its current output causing the delay.

The driver states a "30 - 46VDC" output voltage but what that means is it's only stable when the Vf of your LED string is between 30 and 46Vf. If you have any 650ma LEDs you can try a simple test by just connecting a few of them (Vf outside of 30 - 46Vf) and what you'll see is the LEDs flickering. Again, the driver is not stable outside of 30 - 46Vf so you can imagine what would happen if you tried to PWM that output. It does not work like a typical DC transformer.