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Capacitor

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sniper007

New Member
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
 

Hero999

Banned
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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jimjewel

New Member
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
 

sniper007

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Hero999

Banned
smoothing capacitor of course
There's no of course about it, here's another way of doing it without a bridge rectifier.

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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Sceadwian

Banned
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.
 

sniper007

New Member
There's no of course about it, here's another way of doing it without a bridge rectifier.
it looks perfect. Is it possible to connect parallel another 4 LED that would be 8 LED all together ?
 

Hero999

Banned
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.
 

sniper007

New Member
Please let me know if i understand you correctly, here is circuit.
 

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Hero999

Banned
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.
 

sniper007

New Member
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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Sceadwian

Banned
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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Hero999

Banned
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.
 

sniper007

New Member
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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