Need electonics help (farads/caps and LEDs)
Thread Starter
Former Vendor
Joined: Nov 2005
Posts: 8,995
Likes: 2
From: Spring Texas
St. Jude Donor '08
Need electonics help (farads/caps and LEDs)
Okay here's my application: I'm using a 12v LED strip and i want the leds to fade off completely about a half second from when the power is turned off. What capacitor would make this work?
LED strip has a 160 mA draw at 12-14v
LED strip has a 160 mA draw at 12-14v
Race Director
Joined: Aug 2002
Posts: 16,723
Likes: 1
From: Redacted
You'll need about 5000 - 10,000 µF worth of capacitor.
Figure your load is worth about 85 ohms from ohm's law.
Capacitive time constant formula:
t = RC
Will give us the time to 33% of full charge.
We want a half second of fade time, so t = 0.5.
Algebraic substitution, solving for C,
C = t/R
C = .5 / 85
= .00588 Farads or 5880 µF.
So you'll have to experiment with the value to get the rate of fade you want, but this is a starting point. The visual effect is subjective.
If the circuit is powered by the auto's ACC line, you may need something to isolate it from the rest of the buss, such as a blocking diode to keep the other loads on the buss from discharging the capacitor instead of just the LED.
Figure your load is worth about 85 ohms from ohm's law.
Capacitive time constant formula:
t = RC
Will give us the time to 33% of full charge.
We want a half second of fade time, so t = 0.5.
Algebraic substitution, solving for C,
C = t/R
C = .5 / 85
= .00588 Farads or 5880 µF.
So you'll have to experiment with the value to get the rate of fade you want, but this is a starting point. The visual effect is subjective.
If the circuit is powered by the auto's ACC line, you may need something to isolate it from the rest of the buss, such as a blocking diode to keep the other loads on the buss from discharging the capacitor instead of just the LED.
Thread Starter
Former Vendor
Joined: Nov 2005
Posts: 8,995
Likes: 2
From: Spring Texas
St. Jude Donor '08
You'll need about 5000 - 10,000 µF worth of capacitor.
Figure your load is worth about 85 ohms from ohm's law.
Capacitive time constant formula:
t = RC
Will give us the time to 33% of full charge.
We want a half second of fade time, so t = 0.5.
Algebraic substitution, solving for C,
C = t/R
C = .5 / 85
= .00588 Farads or 5880 µF.
So you'll have to experiment with the value to get the rate of fade you want, but this is a starting point. The visual effect is subjective.
If the circuit is powered by the auto's ACC line, you may need something to isolate it from the rest of the buss, such as a blocking diode to keep the other loads on the buss from discharging the capacitor instead of just the LED.
Figure your load is worth about 85 ohms from ohm's law.
Capacitive time constant formula:
t = RC
Will give us the time to 33% of full charge.
We want a half second of fade time, so t = 0.5.
Algebraic substitution, solving for C,
C = t/R
C = .5 / 85
= .00588 Farads or 5880 µF.
So you'll have to experiment with the value to get the rate of fade you want, but this is a starting point. The visual effect is subjective.
If the circuit is powered by the auto's ACC line, you may need something to isolate it from the rest of the buss, such as a blocking diode to keep the other loads on the buss from discharging the capacitor instead of just the LED.
