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LED Drive Using Mosfet

gauravkothari23

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Hi all.
I am trying to drive the LED strip using Dual Channel Mosfet SP8M24FRATB and 8051 Controller. (Circuit Diagram attached)
The Gate pin of Dual Channel Mosfet is Connected to PIN P05 (High Side) and (LOW Side) P02. I have 2 LED strips so when High Side is switched ON and Low side is OFF, the Lower LED Glows and When High side is switched OFF and low side is Switched ON, the Upper LED Glows.
The problem what i am facing is the drain Pin of the mosfet always shows 12V even when the high side mosfet is switched OFF and LOW side is switched ON. i have also measured the voltage at high side gate which shows me 12V, but then too the mosfet drain pin shows me 12V. due to which upper LED does not glow. i have even tried changing the mosfet, but still the result is the same.
As i am also sure that i am not switching both the mosfet together ON to avoid short circuit. I initially switch the Mosfet OFF and then wait for approx 500 ms and then switch the other mosfet ON.
 

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KlausST

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Hi,

why HIGH side and LOW side switching.
I thing switching both channels LOW side is more simple.

With your circuit you risk short circuit and immediate damage of MOSFETs when both are ON (accidentally)
****

Klaus
 

gauravkothari23

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Hi,

why HIGH side and LOW side switching.
I thing switching both channels LOW side is more simple.

With your circuit you risk short circuit and immediate damage of MOSFETs when both are ON (accidentally)
****

Klaus
Yes, I agree, switching both low side would be quite simple. Which I would definitely do it. But for now I need to know why in existing circuit I am not able to switch LOW side. Even after switching the low side, I am getting 12V at the output.
 

d123

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Put a 1k to 10k from PMOS drain to ground, might be 'off' leakage.
--- Updated ---

Also, I thought blue LEDs were ~3V. 12V supply. Are there meant to be six in series or three in series when either Q1 or Q2 are on?
 

danadakk

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You have scope capture pics of collectors of the NPN's and the gate
drive V ?

1657134821713.png


The leakage of the MOSFETs gate either absorbed in the bipolars when on or their collector pullups when off.

But when you power up, and 8051 outputs are tristated, you need pull ups/pulldowns on
appropriate pin to make sure MOSFETS remain off. Eg. no floating bases on the bipolars
or they could wind up turning both MOSFETS on, on powerup.

Regards, Dana
 
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KlausST

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Hi,

Please show photos where we can see all your wiring.

If you have a scope, then check signal flow step by step:
Microcontroller pin --> base resistor --> bjt_base --> bjt_collector --> mosfet_gate --> mosfet_drain

Klaus
 

Easy peasy

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If you trigger both the mosfets ON in that package - even if only very briefly - they will short the 12V supply and - > BANG .... POOF ...
--- Updated ---

this is why common low side drive is used by professional engineers and others to avoid timing and other issues...
 

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