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capacitor knowledge

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Can somebody help me please.

So I want to do a simply circuit. Using a LED or lamp that dims on power up and power down. I assume I use a capacitor for this just not sure what size and where at.
 

Mr RB

Well-Known Member
Just attach a huge cap across the led, like about 3300uF. Then a resistor to power. Or use 2 resistors from power to led and the cap from the middle junction of the resistors to ground. It will only take a couple of minutes to connect it up and test the results.
 
Just attach a huge cap across the led, like about 3300uF. Then a resistor to power. Or use 2 resistors from power to led and the cap from the middle junction of the resistors to ground. It will only take a couple of minutes to connect it up and test the results.

Thanks for the info. BTW this system is for a 12V power supply
 

Sceadwian

Banned
You're going to need some series resistance between the 12V supply and the capacitor as well, or the charge current of the cap will be limited only be the series resistance of the power supply IE very large. The resistance you need will depend on the actual power supply and LED you're using. A diode in series with the +12V lead will allow instant on and a dimming effect when it turns off.
 
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You're going to need some series resistance between the 12V supply and the capacitor as well, or the charge current of the cap will be limited only be the series resistance of the power supply IE very large. The resistance you need will depend on the actual power supply and LED you're using. A diode in series with the +12V lead will allow instant on and a dimming effect when it turns off.
I would like dimming on and dimming off.. so like this. or do i need 2 resistors?
 

Sceadwian

Banned
You still need a resistor between the cap and the diode.
 

Sceadwian

Banned
600ohms looks way too high, can you post a link to the LED that you're using?
 
I could use 500ohm.
specs for the LED I will be using:
Specifications:

Size (mm) : 5mm
Lens Color : Water Clear
Reverse Current (uA) : <=30
Life Rating : 100,000 Hours
Viewing Angle : 180 Degrees
Absolute Maximum Ratings (Ta=25°C)
Max Power Dissipation : 80mw
Max Continuous Forward Current : 30mA
Max Peak Forward Current : 75mA
Reverse Voltage : 5~6V
Lead Soldering Temperature : 240°C (<5Sec)
Operating Temperature Range : -25°C ~ +85°C
Preservative Temperature Range : -30°C ~ +100°C

I use this conversion to find out the correct ohm:
Okay so how do I determine the resistor to use then ?

This is where it may get a bit tricky for you but here goes.
Determine what we will have in the circuit.

- Our 3V 20mA LED package
- 12V supply from the PSU.

Okay so our supply is 12V and our LED is 3V ( working or forward voltage )


calculate : supply voltage (12.0V) minus forward voltage (3.0v) equals 9.0v

calculate : new circuit voltage (9.0v) divided by required current through the diode (0.015A) equals 600 Ohms


So we require a 600 Ohm resistor to be fitted to the ANODE (positive) lead of the LED package when placed into the 12V circuit which will limt the current through it to 15mA.

Note: It is important not to mix your units here you want to convert the mA into A hence the 15mA = 0.015A

If you wish to run the LED at it's maximum rated current just substitute 0.020A in where the 0.015A is.
How to: Use resistors and LED's in my rig ? (a guide for the first timer) - Overclock.net - Overclocking.net
 
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Mr RB

Well-Known Member
No, put the 2 resistors before the led.

Then the cap from the middle point of the resistors, to ground.

---R1--*--R2------LED-----

(cap goes from * to gnd)

Charging ramp is basically set by R1, discharge ramp by R2.
 

Sceadwian

Banned
Hey had it right, the last schematic he posted is functionally the same.
 

Sceadwian

Banned
Docile if you look at it you'll realize that it's the same circuit =)
 

Sceadwian

Banned
Woops, apparently I'm blind =P Sorry that was stupid.
Looks like the lower one is better. The upper one has 1200ohms in series with the power supply, the lower one has 600 from the power supply and 600 from the cap.
 
Woops, apparently I'm blind =P Sorry that was stupid.
Looks like the lower one is better. The upper one has 1200ohms in series with the power supply, the lower one has 600 from the power supply and 600 from the cap.
yep thanks. that is what is was thinking. I'll be trying the one first.
 

Sceadwian

Banned
Wow... I missed that too =O Yeah, that'd be bad =)
 
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