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# jeeep

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## Posts posted by jeeep

1. ### LED power

Okay, so If I use a 12VDC power supply with a 450 Ohm resistor (using 20mA) in series with one 3V LED everything will be fine then?

(12V-3V)/20mA = 450 ohms (or the next highest standard resistor)

See, for some reason, I have always thought that the 3V was all the LED could handle. But now its clear that current is the main thing to worry with. Voltage is important when considering how many LEDs in series you want to have. For instance if I wanted to max the amount of LEDs I could have in series on a 12V power supply I would take 80% to be the max amount of volts I should spent LED wise.

12V * .8 = 9.6V

With 3V LEDs only three could be in series. Is this correct?

(I used this as a reference: http://www.theledlight.com/LED101.html)

2. ### LED power

Heres what I have:

12VDC Supply Voltage
3VDC Voltage Drop
20ma LED Current

I used to think that the Total LED Voltage Drop should equal the Supply Voltage. But after doing some research I found that the Total Voltage Drop should be less than 80% of the Supply Voltage.

So the correct circuit would look like this:
3 LED's in series with a 150ohm resistor and a 12VDC Supply Voltage.

My questions are this: does the resistor actually lower the voltage? and what would be the problem if 4 leds with the same Voltage Drop were used?

3. ### 12VDC @ 100ft from source

Omni, thank you very much for your post. It contained everything a perfect reply should consist of: an explained solution, an example with insite, and a interesting/relevant footnote.

4. ### 12VDC @ 100ft from source

I need to light a cluster of leds at varying distances; 50ft, 75ft, 100ft. The cluster of leds would be atop of an antenna and at the bottom would be several switches that controls the cluster. Idealistically, I would like to have 24VDC at the bottom of the antenna and then step the power down to 12VDC at the top to power the leds. But I know with these varying distances the higher up the power runs the lower the voltage will be (is this a correct assumption?). So what would be the best way to supply a constant 12VDC at the top of the antenna?

5. ### 12v power supply?

Ian,
What I would do is get several old phone chargers. Lets say you have two phone chargers that puts out 6Vdc each. Take the positive from one and the negative from the other and check the remaining leads, which should equal 12Vdc. The power source doesnt have to be strickly old phone chargers, perhaps an old remote control car batter charger, etc. Oh yeah be sure to get a multimeter to check voltages!

6. ### switching 12Vdc with 3Vdc

Found a relay in my junk pile that should work, only thing is I cant figure out how to hook it up! I've tried searching google but no luck. Here is a diagram of the relay (from the bottom)

This is what I'm guessing:
2 attaches to the +12Vdc
5 attaches to the -12Vdc
1 COM attaches to +3Vdc
4 NC, 3 NO attaches to -3Vdc

Please correct me if I'm wrong

7. ### switching 12Vdc with 3Vdc

Thank you esp1, that makes perfect sense. Is there a particular relay you can recommend that will handle 12Vdc @ 1amp?

8. ### switching 12Vdc with 3Vdc

I have an AVR who's pin outputs 3Vdc. I would like to be able to turn on, or allow to connect, 12Vdc with this 3Vdc output. There will be two power supplies. What sort of electronic part am I looking for? Thank you

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