From: Harold Hallikainen <harold@mai.hallikainen.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Sun, 14 Mar 2021 18:45:19 -0700
Subject: {Spam?} Re: {Spam?} Re: {Spam?} Re: [PIC] 1 Controller and 18
	Peripheral PICs
Thread-Topic: {Spam?} Re: {Spam?} Re: {Spam?} Re: [PIC] 1 Controller and 18
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>
>>> Another approach that would be interesting to try is to have + supply
>>> go to an inductor, then to the LED, then to a FET to ground. Place
>>> another LED across the inductor. When the FET is on, the first LED
>>> lights as the inductor current ramps up. When the FET opens, the
>>> second LED (the one across the inductor) lights as the inductor
>>> current ramps down. The inductor saturation current has to be kept
>>> high enough and the pulse duration slow enough to ensure the inductor
>>> does not saturate. But, this would be efficient (no resistive losses)
>>> and pretty minimal on the parts count. You could use a small PIC to
>>> drive the FET (maybe even an 8 pin part).
>>
>> Some careful design would be required. The LED across the inductor
>> would need to be wired so that while the FET was conducting the voltage
>> across the LED was in reverse direction, so that when the FET stops
>> conducting the LED is lit by the 'flyback' current. However it would
>> also require some thought about the inductor value to ensure the
>> voltage across the inductor is not greater than the max reverse voltage
>> of the LED - which would limit the supply voltage to less than the max
>> reverse LED voltage.

Thinking about this further, we could just have a diode from the bottom of
the LED (FET drain) to + supply. When the FET is on, current through the
inductor ramps up through the LED and the FET. When off, the current ramps
down through the LED and the diode to + supply.

Harold

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