Conundrum using resistors? (Raw rookie)

[Mike Perkins]

Hi to all, I'm Mike, subject line describes my incompetence
First off, apologies if this has been queried before, please point me to the FAQ or relevant posting.

I've been trying to reduce 5v DC to 3v DC using a resistor in a test circuit to power 4 x 3v LEDs.
I used an online calculator to find I needed a 100 Ohm resistor to get 3v. However, when I check the output
with a voltmeter there is no difference, still reads 5v

TIA for any assistance, Mike.

 

[Bluejets]

Where is your 5v coming from?
Show a circuit.
Am I correct in assuming you are reading the output without leds connected?

 

[kellys_eye]

100Ω in each LED 'leg' is correct (assuming a 20mA draw on each LED) but as above, if you measure it 'open-circuit' you will see 5V.

Connect the LEDs and measure again - it will be 3V.

 

[shrtrnd]

Yes, we don't know your circuit or where you're reading your 5v from.
The total will be 5v across your circuit. If you're just reading across your resistor with no LED's in the circuit, you'll just read the 5v. If your LED's are in the circuit backwards you'll read the 5v. Most simple LED circuits I've seen have 1K ohms for the current limiting to the LEDs. It'll help if we see what you've actually got.

 

[shrtrnd]

Kellys_eye is pretty good on his advice. I wouldn't doubt his 100 ohm (and your calculation) for the LEDs.
If you can load a picture/schematic of your circuit. Try to describe how you wired it, and exactly where you're taking your voltage readings at.

 

[Bluejets]

Op said" a resistor and 4 leds" so the value of "a resistor" would be incorrect for 20ma requirement.

 

[(*steve*)]

With this voltage, "a resistor and 4 LEDs" will not give you a robust solution.

I would recommend 1 resistor per LED.

If you look in our resource section you'll find one on LEDs that will tell you a lot about using LEDs and may be worth reading.

 

[Audioguru]

Most simple LED circuits I've seen have 1K ohms for the current limiting to the LEDs.

But the power supply is 5V and each LED is 3V. So with 2V across the 1k ohms resistor then the current is very low at only 2mA.

He says only one 100 ohm resistor and 4 LEDs. He doesn't say or show if the LEDs are in series or in parallel. Of course if the LEDs are in series then no current will flow and the LEDs get 5V.

 

[Mike Perkins]

Thanks for all the info' and taking the time to respond gents.
This is the type of LED I'm using https://hobbyking.com/en_us/super-bright-3watt-white-led-lamp-with-aluminium-heatsink.html I'm using 2 white & 2 red.There's 5v dc supplied from the model aircraft radio receiver but I'm using a breadboard 5v supply for test purposes.I had a peep on Instructables and a number of web sites to check my fumblings. I'm still unsure as to use parallel or series. I did see that I can use 2 resistors, a 10Ω & a 15Ω and where they are soldered together I can draw 3v from that point between the 2 resistors. Does that sound correct to you?

 

[BobK]

No, there are all kinds of problems.

First, the specs for that LED shows 2.5V to 3.0V. I have never seen a white LED that operates below 3.0V, so that is almost certainly incorrect. And for a 3W LED is is more likely something like 3.5V. And there is not current specification! You need to know the operating current. If it is really 3W at 3V that would be 1A of current, not the 20mA you previously used to calculate the resistors.

You need to get LEDs that have actual correct specifications. The specification you need is the operating current and the forward voltage at that current.

And the part about the 10Ω and 15Ω resistor is totally incorrect. What is described is a voltage divider. With 5V in it would measure at 3V, but if you connected to the LED to that point, it would drop significantly. The LED you specified, if operating at 3V and 3W would require 1A of current. 1A through a 10Ω resistor would drop 10V, so you cannot pull 1A from a 5V supply through a 10Ω resistor. In fact, to run 1A at 3V through the LED, you would need a 2Ω resistor.

As far as parallel or series goes, you cannot operate two of those in series from 5V unless it really does operate at 2.5V, which I doubt. In that case you use no resistor.

Bob

 

[Audioguru]

Hobby King sells cheap Chinese junk that does not last long.
Their cheap Lo-Po batteries for RC airplanes are so bad that it seems that they are sold without being tested.