Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

LED lowers Voltage

Status
Not open for further replies.

kaspar389

New Member
Im making a portable USB charger. I have my circuit

**broken link removed**

So basiclly, I got a 5v+ regulator, a battery snap, a red LED and a female USB socket.

I have everything hooked up, and I connected my 5v reg. to my battery, and from the regulator I connected the female USB socket. I got a steady 4.97 volts from that(almost perfect) but then when I add the LED, the volts drop to around 2.80. How can I fix this? I dont get it...

Any help would be great, I need this by christmas.

Sorry if its confusing, I just started this stuff and Im only 14.
 
I believe you were measuring the voltage across the LED. Try measuring it form the USB port.
 
Last edited:
Krumlink said:
I believe you were measuring the voltage across the LED. Try measuring it form the USB port.
I do, I keep the two prongs on the USB port, get a 4.85(charges my ipod fine :D ) then I connect the LED, and put check the voltage again, and I get the 2.80
 
Hmmm

Try a capacitor (do you know what that is) across the 9V battery Terminals and the 5V output. A 10uf should work I would think?
 
How would a capacitor help? The voltage from a battery is already smooth, and an led is going to draw a nice steady current. A capacitor won't do anthing.

What is this circuit supposed to be used for? I can't imagine charging much of anything with a 9v battery. Do you have a resistor on the LED to limit current through it? What type of 5 volt regulator are you using? LM7805?
 
Last edited:
Krumlink said:
Hmmm

Try a capacitor (do you know what that is) across the 9V battery Terminals and the 5V output. A 10uf should work I would think?
All I have is a 0.001uf and a .10uf

:-(
 
Andy1845c said:
How would a capacitor help? The voltage from a battery is already smooth, and an led is going to draw a nice steady current. A capacitor won't do anthing.

What is this circuit supposed to be used for? I can't imagine charging much of anything with a 9v battery. Do you have a resistor on the LED to limit current through it? What type of 5 volt regulator are you using? LM7805?
Well see, since USB is 5 volts, I thought Id take a 9v battery, and get it to be 5v. My ciruit is exaclly the same as in the drawing, no resistors on the LED, Im using a 5v POS. Volt Regulator 1amp LM340T-5
 
Yeah thats a good point. you would not want to charge anything with a 9V battery, the Ampre is too low (around 300ma). I would suggest using AA batteries in series to get to 9V. Radioshack sells AA 9V battery packs with a Terminal Strip on it, so go try and get one of them.
 
Well, for starters, try putting a resistor inline with the LED. Try somthing around 150:eek:hm: .

I still don't see how you are going to charge a battery with another small battery.
 
That will give you 6 volts when all the AA's are new. Is that okay for the device you are charging? Your regulator won't work with 6 volts, as its too close to 5 volts, and regulators like that need a few volts more the the output voltage to work properly.
 
Then what could I do to make USB charger, and fit it into a altoids tin. Or, lets just say I want to use this as a "battery pack" how could I add a LED to it without killing the voltage by 2.
 
The voltage shoudn't drop that far. Somthing is wrong.

You should add a resistor to the LED. Not sure if it will fix the problem, but you should always limit current though LEDs with a resistor.

An Ipod may be okay having 6v fed to it. I can't help you there. I don't have an Ipod or know anything about them. If it would be, you could just use the 4 AAs and get rid of the regulator since it just wastes power anyway.
 
Yeah, more than 5 volts just..frys it. My and my friend fried his ipod mini that way :-(


What type of resistor should I get for it? I only have 10-Ohms(1/2 watt, 5% tolerance) and I believe .10 ohms.

Are you sure this is the only thing that could be wrong? Becasue, tommorwo will prob. be the last time my mom will take me to my local electronics store, so I need to check today what to buy.
 
A 150-200ohm resistor will limit the current through the LED to around 15-25mA. 1/4 watt 5% will be fine. I don't know what the specs of the red led you have are, so i'm just guessing using typical specs for a red led.

I'm not sure whats wrong, to be honest. I have a curcit that can run several orange LEDs and a mico controller with a 9v and a 5v regulator. So I know it can be done.
 
Last edited:
The LED shorts the output of the regulator. the LED needs to have a current-limiting resistor in series with it. The resistor is calculated by subtracting the LED's voltage from the regulator's output voltage then dividing by the amount of current you want in the LED.

If the LED is a 2.8V green one then (5v - 2.8V)/25mA= 88 ohms which is not a standard resistor value so use a 100 ohm resistor.

The LED would burn out if it didn't have a current-limiting resistor and your 9V battery is brand new.
 
For a red LED, as you say you have in your first post, replace the 2.8 in Audios math with 1.7
 
I dont know the voltage of my LEDS.

This is what it says on my LED box

XC209YJP
Yellow Diffused LED(15mcd,585nm, 50P) 20pk

LED .125 Dia
 
When the LED shorted the output of the regulator without a current-limiting resistor, its voltage measured 2.8V. Maybe then its current was a whopping 100mA.

At a reasonable 25mA then the voltage of a yellow LED might be 2.2V.
Then the resistor should be (5V - 2.2V)/25mA= 112 ohms so use a standard 120 ohm resistor for 23.3mA.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top