Hi Everyone,

I'm looking to make a light setup that uses 6 color LEDs, is powered by 12v and can be controlled by a PCA9685 and PWM. The LEDs I'm looking at are the following.

15W RGBWA+UV 6IN1 Led Beads, Led Chip, Led Lamp Bulb for LED Stage Lights

PCA9685 Datasheet

I've read a lot of different things that suggest a BJT transistor would be best, but I'm not very knowledgeable with transistors myself. Any tips or suggestions on what to use? It would need to be fast switching and be able to handle PWM.

Any help is appreciated.

Thank you!

 

As I read the datasheet the default output for the LED drivers is totem-pole so I would leave it set there and use a NMOS transistor to interface with your LEDs.

Choose a good logic level N-Chanel MOSFET with the required current capability.

Don't forget the resistors.

 

What heat sink are you using. This part can get very hot. To start out you should use 50mA or 100mA not 400mA.
Note that one color needs only 2.3V while the others need 3.3V.
I hope you are using resistors to limit the current or a constant current source.
12V supply seems high for running a 3V LED. With 3V on the LED you will have 9V on the resistor. At 400mA that is 3.6 watts in the resistor.
What supply voltage for the PCA9685. Use 5V if you can because it is hard to find FETs that work well with only 3V of Gate drive.

 

You can place a couple of those LEDs in series to reduce the wasted power on the resistors. (they don't look like common anode or cathode)

Also consider using a gate driver between the PCA and the MOSFETs.

 

What heat sink are you using. This part can get very hot. To start out you should use 50mA or 100mA not 400mA.
Note that one color needs only 2.3V while the others need 3.3V.
I hope you are using resistors to limit the current or a constant current source.
12V supply seems high for running a 3V LED. With 3V on the LED you will have 9V on the resistor. At 400mA that is 3.6 watts in the resistor.
What supply voltage for the PCA9685. Use 5V if you can because it is hard to find FETs that work well with only 3V of Gate drive.

I'm not trying to be rude, I am genuinely curious, but without knowing what my plans are, is there a particularly good reason you're telling me I should use lower mA? The reason I was going for the higher power is because these will be for an outdoor fixture, which should give plenty of cooling once I figure out the heatsink and I want it to be pretty bright.

As for the heatsink, to be determined. In the end, it might end up being an aluminum backed circuit board attached to a metal housing for cooling. I guess it depends on how hot it will get. Could be many versions before I settle on the best idea for however I decide to assemble it.

I do see the different voltages. I haven't decided on resistors or a constant current source yet. I was even considering something like this, https://www.sparkfun.com/products/13705, but I wanted to get opinions and ideas from people more knowledgeable.

As for the 12v, well, it's part of something else it might be attached to. I'm not opposed to regulation before the LED circuit, but I just didn't want multiple power supplies if I didn't have to. I've considered linking 3 LEDs together maybe as well.

The voltage on the PCA9685 isn't determined yet. I currently use them for other things at the 3.3v, but I don't see why I couldn't run it at 5v if I run into problems finding 3.3v FETs.

 

As I read the datasheet the default output for the LED drivers is totem-pole so I would leave it set there and use a NMOS transistor to interface with your LEDs.

Choose a good logic level N-Chanel MOSFET with the required current capability.

Don't forget the resistors.

I will look into that, and definitely will need resistors, I know. I was thinking MOSFET myself, but was reading things about the BJT transistors being better because they are quicker switching, which I'm told is good for the PWM, though it may not make a huge difference for what I'm doing.

 

You can place a couple of those LEDs in series to reduce the wasted power on the resistors. (they don't look like common anode or cathode)

Also consider using a gate driver between the PCA and the MOSFETs.

Yeah, nothing common on them as far as I know, so I could tie whichever side I wanted to together depending on other components. Any recommendations on whether common cathode or anode would be better?

I'm not familiar with a gate driver, but I will look that up. I'm more of a programmer than a circuit designer, but I try to learn as I go.

 

At those switching speeds there is no consideration for which transistor type switches faster.

The reason I mentioned common anode/cathode is because you can't run that type in series, the type you have chosen can be placed in series.

Gate driver - Wikipedia

 

At those switching speeds there is no consideration for which transistor type switches faster.

The reason I mentioned common anode/cathode is because you can't run that type in series, the type you have chosen can be placed in series.

Gate driver - Wikipedia

Ah, yes, I don't know why I wasn't thinking of that. My mind was still on the individual led I guess. Thank you for the link. That is very helpful information. Definitely something I should probably use to make things easier, faster and not potentially burn anything out trying to drive the MOSFET.

 

Looking around, I came across this.

https://www.infineon.com/dgdl/irlml...46d462533600a401535668048e2606&redirId=112281

Now, assuming I were to control leach color in the LED individually, would this work without a gate driver? Or would you still recommend a driver? Seems like it would handle 400ma no problem as far as current draw and heat dissipation. I believe it would also work from 3.3v without needing the 5v, though I could still use 5v

 

My guess is you could do without the gate drivers, but I would choose to go with 5 volts.

They might work just fine @ 3.3 If I'm reading the data sheet correctly, but I'm certainly no expert.

Assuming you are going to drive a single MOSFET only from any given channel of the PCA.

 

My guess is you could do without the gate drivers, but I would choose to go with 5 volts.

They might work just fine @ 3.3 If I'm reading the data sheet correctly, but I'm certainly no expert.

Assuming you are going to drive a single MOSFET only from any given channel of the PCA.

Yes, if I went this route, it would be a single MOSFET for each individual color of each LED. Though, with 6 colors and 16 channels per chip, I was considering 5 colors being controller individually (15 channels for 3 LEDs) and maybe combining either the white or UV into once channel, which would need a larger MOSFET for at least that output.