PWM LEDs on high side in RV--need help!

Hello all,
I need some help switching and dimming some 12-volt LED "bulbs" that are in the ceiling of my RV:
I have an LED dimmer working on my breadboard using an n-channel mosfet, Wemos D1 mini, and standard Blynk sketch–I just added a button and a slider in the Blynk app and it works great and dims the LED no problems.
I am trying to control 10 LEDs that are in the ceiling of my RV, I can access the ground wires to the light, only at the fixtures so I cannot use the PWM of the n-channel mosfet, unless I have one at each light or run a new ground wire to the lights–not reasonable.
I do have access to all of the 12v power that is powering the lights from a single-pole switch. So, I transitioned to this setup (picture schematic included below) using a p-channel mosfet. It works, BUT, the LED is blinking/pulsing so badly that it is not usable as it is currently configured. I also tried a different LED "bulb", no luck. Again, both LED "bulbs" worked fine using the n-channel mosfet.
Question, why is it blinking/pulsing so much? Can I change the pwm frequency in the Blynk app or sketch? Can I change the value of the resistors to make it more linear? Do I need to change the LEDs? I am very new to electronics, what am I doing wrong? Thank you! :slight_smile:

Can You make it clear... You start talking about LEDs and then switch to talk about bulbs?
LEDs powered by a DC supply is quite different to bulbs powered by mains, 50 or 60 Hz.

Your circuit looks possible to me.
Sorry but I don't know Blynk.

Hello Railroader, thank you for the reply; sorry for the confusion.
I am trying to PWM 12 volt led “bulbs” via the high side because an isolated LED ground is inaccessible to use low side PWM.
I currently have it wired up according to the attached schematic. However, the 12volt LED-bulbs flash/blink like a slow strobe when applying a PWM signal; they work just fine at regular on/off. The question is why? How do I PWM the high side of 12v led lights at a frequency that is usable?
Thank you!

Either the technology is LED or it is bulb.
You can't use an N channel FET. N channel MOSFET can only switch the low side. Get hold of a P channel MOSFET. Sorry I didn't notice that the first time.

Thank you for the pointer, I will try to use LED and not bulb going forward.

Sorry I am confused, I am using a P-channel MOSFET: IRF4905

Is the LED you used on the bench the same as the LED bulbs in the van?

Is it possible the LEDs in the van have additional internal circuitry that, perhaps, doesn't like being PWMd?

For example, do the van LEDs have internal constant-current regulators?

stansrs:
Thank you for the pointer, I will try to use LED and not bulb going forward.

Sorry I am confused, I am using a P-channel MOSFET: IRF4905

My mistake, thought I was reading N channel somewhere. The diagram says P channel ok.

LED lamps can be dimmable but some are not. Check for that.

Blackfin:
Is the LED you used on the bench the same as the LED bulbs in the van?

Is it possible the LEDs in the van have additional internal circuitry that, perhaps, doesn't like being PWMd?

For example, do the van LEDs have internal constant-current regulators?

Thank you for the reply :slight_smile: Yes, it is the same LED on the bench that is in the ceiling. And that same LED will PWM using an N-channel without issues, it is only when I tried using the P-channel config that I begin to have problems. Do some LEDS only PWM on N-channel? I ordered some new LEDs that say they will dim using a highside dimmer--we'll see.

Railroader:
My mistake, thought I was reading N channel somewhere. The diagram says P channel ok.

LED lamps can be dimmable but some are not. Check for that.

Understood, thank you. I have ordered some new LEDs that claim to be high-side dimmable; they will be here Thursday!

I don't think high or low side switching matters on any device only having 2 connection points. Never mind. Let's see how it turns out.

The 10k pullup on the gate is to high for PWM switching speed, drop to 1k, delete the 1k collector resistor on BC547 and drop the base resistor to 1k.

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