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aN4rK1	#1 24/01/2008 - 02h23 Top
Class : Apprenti Posts : 7 Registered on : 21/01/2008 Off line	I just have a few questions about this power supply I want to use. Firstly, its a 4.5v, 300mA power supply and that means i dont have to use a resistor right? Also, the two wires that come out of the power supply are both black except one has a white stripe. I did a bit of googling and I understand the one with the wite stripe is the ground wire and the other one is the positive lead, correct? Thank you!
tristan68	#2 24/01/2008 - 07h41 Top
Class : Electronicien en chef Posts : 651 Registered on : 10/07/2007 Off line	Hello aN4rK1 : I just have a few questions about this power supply I want to use. Firstly, its a 4.5v, 300mA power supply and that means i dont have to use a resistor right? false A resistor (or another current limiting device) is MANDATORY in a led setup. This is due to the Led steep voltage/current characteristics. You could very easily fry your led with only a little voltage vairation, without a current limiting device. so ALWAYS use a resistor. remember : Leds are not light bulbs but diodes, so the art of doing and thinking with light bulbs doesn't apply to leds. the two wires that come out of the power supply are both black except one has a white stripe. I did a bit of googling and I understand the one with the wite stripe is the ground wire and the other one is the positive lead, correct? Not sure . The best way if to check it using a voltage tester. This way you'll also have another important information : the output voltage of your power adapter. Depending of your power adapter, you could have far more than 4.5v as output voltage , so better check. If you don't have a voltage tester, look for a friend having one , or ask at your local electronics shop. cya
aN4rK1	#3 24/01/2008 - 15h13 Top
Class : Apprenti Posts : 7 Registered on : 21/01/2008 Off line	Hello, Alright thank you. I do have a voltage tester thats good. Also, about the resistor, I used the resistor calcuator on the website main page, and I saw that it said I need a 13 ohm resistor @ 4.5v. The Power supply I have is variable meaning I can change the voltage from 1.5v all the way upto 12v. So would it be better I use as less voltage as possible and a smaller value resistor or higher voltage?
tristan68	#4 25/01/2008 - 13h05 Top
Class : Electronicien en chef Posts : 651 Registered on : 10/07/2007 Off line	hello aN4rK1 Your REAL power supply output voltage has to be a little bit more than the led bloc voltage (about 1 volt more is OK). What kind of leds do you plan to use ? regards
aN4rK1	#5 26/01/2008 - 14h01 Top
Class : Apprenti Posts : 7 Registered on : 21/01/2008 Off line	Hello Again, I just tested the power supply with a my old Multi meter. I put the red lead to the wire with the white stripe and the ground lead to the wire WITHOUT the white stripe, and the meter seemed to move in the positive direction. If i reversed this, the meter moves in the opposite direction. So i guess my initial guess was wrong, the wire with the white stripe seems to be the positive lead, and the just the black wire seems to be negative/gnd lead. Can anyone confirm this? Also, I cant quite read the voltage output because the multimeter I have is analouge and the way the marks are made is very confiusing. Lastly, I plan to use the SFH485P leds that I bought from Digikey. They are sort of purple-ish, have a flat top, and from the datasheet, the annode is the small leg and the cathode is the longer leg.
tristan68	#6 27/01/2008 - 11h33 Top
Class : Electronicien en chef Posts : 651 Registered on : 10/07/2007 Off line	Hello aN4rK1 Yes i confirm. According to what you say , the white marked wire is the positive wire. Can you please take a shot of your multimeter while measuring the voltage, and post it ? I need to see correctly the neddle and range selector. This way , i can confirm the measured value. cya.
aN4rK1	#7 27/01/2008 - 21h41 Top
Class : Apprenti Posts : 7 Registered on : 21/01/2008 Off line	Hello Tristan, Ok that solves my question about the wires. Now I got my self a digital multimeter from a friend and measured the voltage. I think the power supply faulty because It said there was an output of 17 volts! Since the power supply was variable, I tried to changed voltages and no matter where I went It was still 17v. So I got rid of it and got my self another power supply, this one was rated at 4.5v and 500 ma. I measured the output and it is really 6.4 v. Now I went to the led calculator on the home page and found out that I need a 39 ohm resistor. So I think iam all good except now I have 1 last question. The power supply says 500ma. Now is that 500ma MAX or 500ma Constant? Iam asking if it can provide UPTO 500 ma in current or it only supplies a steady stream of 500ma? thanks!
tristan68	#8 28/01/2008 - 08h08 Top
Class : Electronicien en chef Posts : 651 Registered on : 10/07/2007 Off line	hello aN4rK1 it's : up to 500 mA. but better not reach this value , specially on small power adapters.... Anyways, you only need less than 100mA, so its ok. cya
aN4rK1	#9 28/01/2008 - 17h52 Top
Class : Apprenti Posts : 7 Registered on : 21/01/2008 Off line	Hey Tristan, Ok so Iam good to go then? Everything is ok? The ma, voltage, the resistor and so on? thanks again!

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