Questions about LED strip circuit and current

Question

I have several questions but I will split it up into two (main) questions.

The first 'series' are about double LED strip 'controller' circuit. Below is the the

I'm intending to use a (5V) 74HC595 through a level shifter as I want it to use on a 3.3V ESP32. The level shifter I didn't show (not so important for the questions I think).

The red dashed box is a part that will not be part of the main PCB, as I don't want to add the 24V power supply on the PCB. Instead it will be a small separate manually made PCB.

The LED strips should be max 3.3A per led strip (of 5 m). Are my following calculations/insights correct (not having an electronic background):

I intend to use a GX16-8 connector for connecting the LED strips to the enclosure, and a JST_XH connector to connect to the PCB. For the JST_XH and GX pins 1 to 4, the max current is 3.3 A / 4 = 0.83 A. Both well within specs of 3A per pin.

Questions:

1. Can I use a transistor after the LED strip? In most cases I see them before the LED strip, but afaik it doesn't matter. This way I can place the transistors on the main PCB.
2. For the JST_XH and GX pins 5 to 8, the GND is 3A. Since this is just on the limit, I use all remaining pins resulting also in 0.83 A / pin. Maybe there is a small deviation between the GND pins 5-8 as GND finds the shortest way but I assume it will be 'more or less' spread around the pins?
3. In the picture below I show the power (the GND of the 12 V LED strips assumes to go through this, I assume this is ok, as all GND needs to be connected together?). I guess I also would need 4 pins for that (thus not unlike the picture just 1 JST XH pin, having 3A max rating, or using a different kind of connector able to handle more than just 3A). Is this true?

Answer 1

I'm intending to use a (5V) 74HC595 through a level shifter as I want it to use on a 3.3V ESP32.

If you use a HCT595 then you don't need the level shifter, it will read 3V3 levels correctly when powered by 5V.

Since ESP32 has a very nice PWM peripheral, you could use that and a HCT245 as 8-way level translator/mosfet driver instead of the HCT595.

If you want to get rid of the 5V power supply and only keep the 12V, you can use a canned switching converter.

Can I use a transistor after the LED strip? In most cases I see them before the LED strip, but afaik it doesn't matter. This way I can place the transistors on the main PCB.

No idea what "before" or "after" means... your schematic shows low-side switching, which means the switching transistor is on the ground side. So it has to be a N channel MOSFET and this is the most practical way to do it. No problem.

Since RGB LED strips are all common anode you have to use low side switching anyway.

Note the N MOSFETs are connected the wrong way around on your schematic. Look at the built-in diode, current will always go through and you won't be able to turn the LEDs off. Just swap drain and source.

IRLZ44 has 28 mOhm RdsON at Vgs=5V so it will work fine. Personally I would use SO-8 dual MOSFETs for a more compact pcb.

If you put a P channel MOSFET to switch the positive supply, it's called high-side switching. Not a good option here as you'd have to drive the gate to 12V to turn it off, so you'd need an extra transistor.

For the JST_XH and GX pins 5 to 8, the GND is 3A. Since this is just on the limit, I use all remaining pins resulting also in 0.83 A / pin. Maybe there is a small deviation between the GND pins 5-8 as GND finds the shortest way but I assume it will be 'more or less' spread around the pins?

It's common to use several pins to share current. No problem.

In the picture below I show the power (the GND of the 12 V LED strips assumes to go through this, I assume this is ok, as all GND needs to be connected together?).

Yes, all grounds together.

As I said I'd use a 5V dc-dc to get rid of the 5V supply.

If you keep the 5V supply, better use a USB connector to use a phone charger, for example.

The 5V connector only needs to be able to handle the current on 5V. So if it is just powering an ESP32, you don't need amps, the 2 pin JST will be fine.

Answer 2

For the JST_XH and GX pins 5 to 8, the GND is 3A. Since this is just on the limit, I use all remaining pins resulting also in 0.83 A / pin. Maybe there is a small deviation between the GND pins 5-8 as GND finds the shortest way but I assume it will be 'more or less' spread around the pins?

Bad assumption: current doesn't divide equally because of significant variations in the wire crimp and especially in the contact area between mates.

Although it is commonly done, it's bad practice. What happens is that one contact carries more than its far share of the current and overheats. Instead, choose a different connector, one in which each contact is rated for the total current.