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5v pwm to 24v dimming

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ginojo

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Hello,

I'm trying to dim a led-strip that is running on 24v. When I hook it up to a power-supply it's nicely dimming from 24v to 18v. Turns off at +/- 18v.

Now I' trying to regulate the brightness with an esp32 (arduino like). I have created the following circuit:

Screen Shot 2018-02-09 at 20.38.29.png

But the problem is that when the PWM is 0, the led-strip doesn't turn off completely. It's 'stuck' at 19v.

I'm probably using the wrong FET. Could any of you guide me? I'm kinda hopeless at the moment.

Thanks
 

I assume this is just the initial stage of your experiments, due the output expander IC in the current configuration is perhaps unnecessary (a single bipolar transistor could do the job); anyway, adding between the gate of the IRFB4610 and the 74HCT245 output, a Diode-series/Resistor (pull-down) network, could ensure a sufficient voltage drop for the MOSFET do not enter on its ohmic region. Keep in mind that the logic level 0 in the TTL (HCT on this case) standard is not always zero volt.
 

Hi,

In opposite to the true TTL ICs ... the HCTs switch pretty good to GND.
Typ 0.001V @ 20uA.

You use the HCT245 to level shift from 3.3V to 5V. You maybe could avoid this with a Mosfet with even lower V_gs_th.
Or you use a single gate IC, like 74HCT1G00.

The circuit should work. The PWM duty cycle determines brightness.
50% duty cycle is about the same as 50% average current and 50% brightness.
But the human eye is not linear...

Klaus
 

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