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Old 20th March 2009, 12:08 PM   #1
raghs is offline raghs  India
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Default Pilot LED for my new car Amplifier

Hello Gurus,

I have just finished building my own Car audio amp around LA4440 chip (19 Watt RMS). I need to connect a normal 3V LED as pilot lamp just to indicate that the amplifier is switched on.

Can I connect the LED, in series with a 100K 1/2W resistor, directly to the on/off switch? Or could you suggest some better way or better values for the resistor?
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Old 20th March 2009, 12:38 PM   #2
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LEDs are not generally specified as having an operating 'voltage'. They are categorised by current.
Typical values are 15ma to 25mA.
Under normal operating, a fixed and very specific voltage drop will appear across the diode, typically around 1,7V, but higher for blue or white ones.

The computation goes like this:
Power supply = 13.5V or close to that.
Voltage across LED = 1,7V
Desired current = 20mA

The voltage drop that the resistor must handle is thus 13.5 - 1.7 = 12V (close enuff, the actual values are not at all critical)
12V at 20mA requires a 12/0.02=600 Ohm resistor.

630 ohm is the closest E12 value, and will work just great, but something as high as 1k ohm will work perfectly well.

Power of resistor = 0.02 X 12 = 0.24 W
1/4 W is just enough, but 1/2 w is better - will run cooler.

And just connect it between ground, and the switched power supply line - found at the 'out' side of the on/off switch.
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Old 20th March 2009, 12:53 PM   #3
acid_k2 is offline acid_k2  Italy
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I like to use normal LEDs with less current - about 10mA - for on/off indicator. They lasts longer.

for 12V supply:
- 1 Kohm resistor is ok for red/green/yellow LEDs (normal voltage drop on the led: 1.7V).
- 820 ohm for white/blue LEDs (normal voltage drop on the led: 3.2V).
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Old 20th March 2009, 12:59 PM   #4
raghs is offline raghs  India
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You guys are suggesting 1Kohm resistor is enough..but I am connecting 100K in SERIES with the LED..still the LED glows very bright, and it even fused once!

Do I need to connect the LED & resistor in a specific way - like voltage divider?
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Old 20th March 2009, 01:45 PM   #5
acid_k2 is offline acid_k2  Italy
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If you are speaking about a car audio amp, your power supply is probably 12Vdc (battery). If not, correct me.

At 12V, with a simple 100k (100000 ohms) series resistor, you have about 0.1mA flowing trought the LED (very low current). It's bright enough? probably not.

so:

are you sure you are using a 100k resistor?
are you sure your power supply is 12V?
for burned led: leds has polarity, wrong connection may burn them.
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Old 20th March 2009, 03:07 PM   #6
djQUAN is offline djQUAN  Philippines
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maybe you have 100R = 100 ohm.
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Old 20th March 2009, 03:31 PM   #7
raghs is offline raghs  India
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Quote:
Originally posted by acid_k2
If you are speaking about a car audio amp, your power supply is probably 12Vdc (battery). If not, correct me.

At 12V, with a simple 100k (100000 ohms) series resistor, you have about 0.1mA flowing trought the LED (very low current). It's bright enough? probably not.

so:

are you sure you are using a 100k resistor?
are you sure your power supply is 12V?
for burned led: leds has polarity, wrong connection may burn them.
Acid_k2 - Yes I am sure I am using 100K Resistor..the color code is Brown, Orange & Orange
I am using a transformer of 12V & with the wrong polarity the LED wont glow neither would it burn bcos its a diode
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Old 20th March 2009, 03:34 PM   #8
raghs is offline raghs  India
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Sorry, its Brown - Red - Orange & Silver
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Old 20th March 2009, 03:58 PM   #9
djQUAN is offline djQUAN  Philippines
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Quote:
Originally posted by raghs
Sorry, its Brown - Red - Orange & Silver
brown-red-orange is 12k. have you verified with a multimeter that the supply is 12V and that the resistor is within spec?
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Old 20th March 2009, 05:53 PM   #10
acid_k2 is offline acid_k2  Italy
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raghs,

your "trasformer" is a regolated dc power supply? If yes, you have a 12Vdc stable output. If not (very common), you have a voltage ouput depending on the load applied. At idle, you probably have ah higher voltage (sometime exceding 20V).

You said:
with the wrong polarity the LED wont glow neither would it burn bcos its a diode

This is not true. Leds are diodes, but they have tipically a low breakdown voltage (as low as 5V). If reverse connected, with voltage higher than 10V, the risk of a burned led is very high. White/blue/violet/UV leds are a lot more sensitive to reversed voltage than "normal" old red/yellow/green ones.
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