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Capacitor

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sniper007

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Hi guys!

Which capacitor will sufficient after bridge rectifier to supply 4 white LED diodes (series circuit)

AC voltage is 12V.


Note: I won't using regulator because space is very limited, just AC voltage -> bridge rectifier -> capacitor -> resistor and diodes
 
It depends on what you're doing.

Are you taking about a smoothing capacitor or a capacitive ballast?

For a capacitive ballast, connect the LEDs as two series pairs wired in reverse parallel and use a 150R resistor and a 33µF capacitor to limit the current. Note that the capacitor needs to be bipolar, an standard electrolytic is no good.
 
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homework help needed

a series tuned circuit has a frequency of 1 mhz. It has a .01 uF capacitor and a 300 uH inductor and a 20 ohm resistance. What would the bandwidth be
 
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smoothing capacitor of course

There's no of course about it, here's another way of doing it without a bridge rectifier.
12vac-led-ballast-gif.29048

If you go for the bridge rectifier option, you don't even need a smoothing capacitor. If you want to get rid of the flicker then a 220µF capacitor will well and truly squash the ripple to below 2V.

Note that all resistor calculations should assume the output from the rectifier to be 15V.
 

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On behalf of flicker sensitive people everywhere if you use a bridge rectifier PLEASE use a filter cap =) I made an LED bank once that ran off AC power and the flicker was pretty bad.
 
There's no of course about it, here's another way of doing it without a bridge rectifier.
12vac-led-ballast-gif.29048

it looks perfect. Is it possible to connect parallel another 4 LED that would be 8 LED all together ?
 
You're better off connecting two strings of 4 in reverse parallel for 8 LEDs.

Also, you might as well get rid of the capacitor and use a 47R resistor.
 
Please let me know if i understand you correctly, here is circuit.
 

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Looks fine to me, except if you want to simulate it you'll need to enter 17V as LTSpice works in peak voltage, not RMS.
 
So i tried to simulate it and current seems to be very high through diodes. Does it current (300mA peak) won't damage diode ?
 

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Sounds like you need to use a bigger resistor. Just watch for the power dissipation in the resistor as well. Also make sure the voltage charactoristics of the LED's you're using in the simulator match your real diodes.
 
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It depends on the forward voltage drop across the diodes.

What's the Vf of the diodes in the simulation?

I went on the fact that white LEDs have a forward voltage of about 3.5V, four in series makes 14V. (17-14)/47 = 63.8mA and the LEDs will only be conducting every half cycle for less than half a cycle, giving an average current of much less.
 
Vf for this LED diode is 0.8V (LTspice) and therefore is current so high in simulation. Real Vf for white LED diode is about 3,5V....

Now i'm going to soldering and thanks guys for your helps ;)
 
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