Please help, Why is the led off

Discussion in 'Homework Help' started by sensor, Sep 8, 2009.

Sep 8, 2009
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2. bertus Administrator

Apr 5, 2008
19,170
3,836
Hello,

There must be a voltage source at the input of the regulator.
This must be at least 2 Volts higher than the output voltage.

Greetings,
Bertus

3. JoeJester AAC Fanatic!

Apr 26, 2005
4,007
1,770
What is the input voltage to your 7805 voltage regulator?

You need to have a voltage source connected to the input of the regulator.

Your resistor is very large ... at 20 mA, it would drop 30 volts. Your LED is probably 1.5 Vf @ 20 mA. With a 5 volt supply, you would need a much smaller resistor.

4. sensor Thread Starter Member

Sep 8, 2009
19
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Hello,
How did you calculate 30 volts?(V=IR)

5. Mike2545 Senior Member

Mar 26, 2009
116
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Your resistor value is too big.

6. beenthere Retired Moderator

Apr 20, 2004
15,808
294
Ohm's law. E = IR , so .02(amps) X 1500(ohms) = 30(volts).

7. sensor Thread Starter Member

Sep 8, 2009
19
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How can i know this(about a led)?
What's 1.5vf?

8. steinar96 Active Member

Apr 18, 2009
239
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Leds are pretty standard. It's easy to google up drops for different colors of diodes. Vf means the forward voltage drop which is the voltage measured accross it when it is forward biased (LED is actually a diode). . 1.5Vf means that when conducting 1.5 Volts are dropped accross it.

9. sensor Thread Starter Member

Sep 8, 2009
19
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But it was only 1K resistor.
1k=1000(ohms) is it correct?

10. steinar96 Active Member

Apr 18, 2009
239
4
sensor. You want 20mA trough your led correct ?

The led will drop 1.5V. If you have a 5V supply it means you want the resistor to drop 3.5V accross it.

Calculating (5V-1.5V) / 0.02A we get 175 ohms.

So the resistor needs to be 175 ohms for this setup to work. With a 1k resistor the diode is deprived of current

11. JoeJester AAC Fanatic!

Apr 26, 2005
4,007
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Your diagram showed a 1k5 .... that's 1.5k or 1500

12. sensor Thread Starter Member

Sep 8, 2009
19
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Thank you for your information.
I did not know that 1k5 means 1.5K(kiloohms).

13. sensor Thread Starter Member

Sep 8, 2009
19
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http://img35.imageshack.us/i/35855855.png/
You are right.
But i can't find any resistor less than 1k in proteus/ISIS 7 pro.
I got 0R1.
What does it mean by 0R1?

NB:This time the led is on.

14. bertus Administrator

Apr 5, 2008
19,170
3,836
Hello,

A resistor od 0R1 is a resistor of 0.1 Ohms.
Can't you edit the properties of the resistors?
I do not use the simulator but some have the option of a rightclick on the resistor,
so a popup box shows up to edit the value.

Here is a list of standard resistor values:
http://www.educypedia.be/electronics/resistancevalues.htm

These values can be muliplied by 0.01, 0.1, 1, 10, 100, 1K, 10K, 100K, 1M.

More information on component coding can be found in the links of this page:
http://www.educypedia.be/electronics/datacomponent.htm

Greetings,
Bertus

Sep 8, 2009
19
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Thank you.

Mar 24, 2008
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