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?
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?
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.
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.
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.
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.
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.
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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