Author Topic: eevblog #50 constant current power supply voltage  (Read 2333 times)

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Offline tome

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eevblog #50 constant current power supply voltage
« on: August 18, 2011, 06:24:39 pm »
I noticed for this blog project Dave chose a Meanwell constant current power supply (LPC-35) that is rated for 9V-48V, yet the LEDs are only rated up to 3.75V.  Did he add a voltage divider to it or something? I don't see that mentioned anywhere.
Thanks,
Tom
 

Online IanB

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Re: eevblog #50 constant current power supply voltage
« Reply #1 on: August 18, 2011, 07:27:12 pm »
I don't have time to go back and re-watch the whole blog right now, but LEDs are current rated devices. As long as you don't try to pass more current through them than they are rated for (given appropriate thermal design), then they will be fine. In Dave's case I'm pretty sure he had several LEDs in series, so this would have given an appropriate voltage match to the power supply.
I'm not an EE--what am I doing here?
 

Offline tome

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Re: eevblog #50 constant current power supply voltage
« Reply #2 on: August 18, 2011, 07:43:12 pm »
In Dave's case I'm pretty sure he had several LEDs in series, so this would have given an appropriate voltage match to the power supply.

Ah, yes thanks.  Looking closer at his photo I see that you are right, they are wired in series.  At first glance I thought he had them in parallel.
Thanks,
Tom
 

Online IanB

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Re: eevblog #50 constant current power supply voltage
« Reply #3 on: August 18, 2011, 09:05:29 pm »
At first glance I thought he had them in parallel.
Again, I can't remember, but I think Dave may have covered this also. Just FYI, you should not attempt to wire LEDs in parallel. This is an outcome of the previously mentioned point that LEDs are current rated devices. Voltage loads go in parallel, current loads go in series. Connecting LEDs in parallel is something that will usually cause poor performance or premature failure of the LEDs.
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Online ejeffrey

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Re: eevblog #50 constant current power supply voltage
« Reply #4 on: August 18, 2011, 09:41:26 pm »
You can connect LEDs in parallel with small load-sharing resistors.  That will waste a small amount of power, but there is a big advantage that you can power a whole bunch of LEDs without getting into high voltage.

How big your current sharing resistor must be depends on the type of LEDs, the operating conditions, and how unevenly you are willing to let the current split.
 


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