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1  Using Arduino / General Electronics / Re: power leds driver on: March 10, 2014, 12:07:48 am

Yes, some wasted energy comes with these solutions, but I don't think that's the primary issue that most people care about.
Try to somewhat match V source with the vf times the amount of LEDs to use, and the waste is reduced.
Still better than incandescent.

Having a cheap circuit with some control over current seems a good solution as constant  current drivers are a bit pricey.
2  Using Arduino / General Electronics / Re: power leds driver on: March 08, 2014, 03:49:32 pm
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Replace the fet with an npn ...

What's changing to an NPN do for us?  
3  Using Arduino / General Electronics / power leds driver on: March 06, 2014, 10:51:28 pm

For power LEDs, is there a downside of using a resistor, compared to a constant current driver?
(DC wall wart type supply.)
4  Using Arduino / General Electronics / Re: subjective LED dimming on: March 02, 2014, 01:36:25 pm

I'm interested in dimming very bright LEDs to various levels, and just wondered how effective PWM was at these very short flashes. Normally it would be much longer on-times.
5  Using Arduino / General Electronics / Re: subjective LED dimming on: March 01, 2014, 07:53:53 pm
I'm really thinking of 64th notes at 120bpm. Duration is 32ms.

With 15 pulses in 30ms , it seems like PWM would work to dim.
6  Using Arduino / General Electronics / Re: subjective LED dimming on: March 01, 2014, 06:35:28 pm

The math seems good, but subjectively, does that allow wide dimming?
It seems like it would to me.
7  Using Arduino / General Electronics / subjective LED dimming on: March 01, 2014, 06:23:42 pm

If you have an LED that just illuminates for 30ms or so, would PWMing it still make a perceptible difference in how bright it looks?

I want to have an LED flash just for 30ms, but be dimmable as well.
8  Using Arduino / General Electronics / Re: Ac vs DC on: January 21, 2014, 09:34:22 pm
Slightly less current and power, since some of the voltage is dropped out by the diodes, so less voltage across the resistor means less current.

I know the DC drop is 1.4sh
Is the AC drop different then?
9  Using Arduino / General Electronics / Re: Ac vs DC on: January 20, 2014, 10:09:38 pm
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Does putting a bridge into this change the equation?

Without a capacitor, and with a high enough voltage that the diode forward drop is negligible, no.

With a bridge rectifier, there are two diode drops so about 1.4V is dropped out of the "bottom" of the sine wave. So the power would be less. You could get reasonably close if you take into account the power lost in the diodes.

If you add a smoothing capacitor of sufficient size so that the ripple voltage is tiny compared to the peak voltage, then you are looking at a DC voltage just under Vpk - 1.4V.

So just to clarify my original question...  With either AC or DC input, this complete circuit (with a resistor, an LED and a bridge, and no cap) will use basically about the same overall current, yes?
10  Using Arduino / General Electronics / Re: Ac vs DC on: January 20, 2014, 09:13:16 pm
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Note that for a sinusoidal AC current the power in the resistor varies as a sinuisoid
of twice the frequency, varying between zero and twice the average power.  Average
this over a whole cycle or a long enough time and the power is the same as for DC.

Does putting a bridge into this change the equation?
11  Using Arduino / General Electronics / Re: Ac vs DC on: January 16, 2014, 10:06:20 pm
Is there a difference in voltage of the rectified AC right after the bridge, as compared to the same point with DC into that same bridge?

After the bridge, when using an AC input it's pulsed DC.
After the bridge, when using an DC input it's just DC.

Is that a different effective DC level?
Doesn't that in turn affect the overall current consumption?
12  Using Arduino / General Electronics / Re: Ac vs DC on: January 16, 2014, 09:48:42 pm
It is important to realize that we need to be specific when talking about AC.

5VRMS AC will cause the same amount of power dissipation in a given linear resistance as 5V DC.

For a square wave, 10V peak to peak AC has an RMS value of 5VRMS. For a sine wave, 14.14V peak to peak has an RMS value of 5VRMS. I'm talking now about symmetrical waveforms with no DC offset.


Why do you say 14v p to p is 5v rms?
My original question is based on a transformer only for AC, so is a sine wave.
13  Using Arduino / General Electronics / Re: Ac vs DC on: January 16, 2014, 09:39:13 pm
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No, and for 3 separate reasons:
An LED+resistor is a non-linear system, so will not respond proportional
to applied voltage.
A bridge loses 2 diode drops, its again a non-linear system with losses.

But the same for 12vac or 12vdc input, as the components are on the other side of a bridge?
Why is the AC current not constant? 
I'm really only interested in current draw being equal as a theoretical question, not perceived brightness of an LED.
14  Using Arduino / General Electronics / Re: Ac vs DC on: January 16, 2014, 09:31:11 pm
If you would have left the LED out, the power dissipated in the resistor would be the same for 5 V DC and 5 V rms across the resistor.

Why is it different with the LED?  There's a bridge before the LED...   
There's no cap.
15  Using Arduino / General Electronics / Ac vs DC on: January 16, 2014, 12:47:51 am
Lets say we had a circuit with a bridge, a resistor and an LED.
If we power it from 5V DC, we calculate X current draw, based on the resistor.

If we power the same circuit from 5V *AC* instead, is it exactly the same current?

Since the 5V AC is RMS, is it the exact same ohms law calculations as for DC?  Same for wattage?

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