Adding a Transistor to Fade LED

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  • #2
berkeman
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Would it be possible to add a transistor in between the LED and the driver, as shown in the application photos, to PWM and fade the LED with 0-5V logic (Assume the LED has a forward voltage of 3V)?

http://www.superbrightleds.com/more...icropuck-dc-buck-boost-constant-current/1053/

I would think probably not, at least not the PWM part. That DC-DC module's feedback control loop would likely be confused by any pulsing of the load impedance.

A better solution would be to make your own boost-buck (SEPIC) DC-DC circuit, and include the dimming capability in the circuit itself.
 
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  • #3
meBigGuy
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It's just an 8V buck/boost switching supply. You could probably PWM it just like you could for any switcher supplying LED's. If the switcher design is really crappy you may need to add a large output capacitor. I'd say give it a go.

I don't see where, in the app photo's, there is a transistor so I can't comment on a PWM circuit. There are PWM constant current drivers available like CAT4101.
 
  • #4
NascentOxygen
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Would it be possible to add a transistor in between the LED and the driver, as shown in the application photos, to PWM and fade the LED with 0-5V logic (Assume the LED has a forward voltage of 3V)?

http://www.superbrightleds.com/more...icropuck-dc-buck-boost-constant-current/1053/
The application sheet carries a note alongside the boost driver configuration (and also the buck/boost configuration): "LED must always be connected whenever power is applied!" I would be cautious about interrupting the LED current as it seems to be flying in the face of that warning. (But then again, nothing ventured, nothing gained. :smile:)

Perhaps you could, instead, switch a lower-voltage diode string in parallel with the LED to rob it of drive? If the puk's output is floating, a BJT switch may be tricky, though. [Broken]

The drive is loosely pitched at a few hundred mA, while varying according to the supply voltage, is that how it works? It sounds like a limited niche module.
 
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  • #5
meBigGuy
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I didn't open theapplications notes. Based on the applications note I'd say this is not a basic switcher. It is, as NO stated, a "limited niche driver". I was wrong. I don't think you should try without details of the internal design.
 

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