LED circuit help

[Mr Musabe]

Hi guys I am new on here and want to clear something up that is confusing me.

I have never done electronics before so I apologise if this question seems stupid. I need to connect 5 LED's in series.
I am planning on using LED's that are 3 volt. Am I right in thinking I will need at least a 15V supply? or will a 12 V be enough? (5 x 3=15)

What is confusing me is, if Each LED drops 3 volts then I will have used 15 volts, so after the first LED I will have 12 volts, then 9 volts etc. why doesn't the first or second LED blow from that voltage as they receive the highest amount of electrical pressure?

I understand that if it was in parallel each LED would get the same voltage and a resistor would be needed

Thanks in advance

 

[Arouse1973]

Hello

You are correct in your assumption. You will need at least 15 Volts. However it's not quite as straight cut as that. The LEDs voltage drop is prodominatly dependant on current. So the LED will drop 3 Volts at a specific current only. If you look in the data sheet it will show you this. What you need to do is decide on what current you will be passing through the LED, this will determine the voltage drop across the LED.

What you should do really is use a current limiting resistor which will drop the excess voltage you don't need across it. Otherwise if your supply fluctuates or some other environmental conditions effect the LEDs forward voltage then you might damage the LED from excessive current draw.

The LEDs in your series chain will only have say 3 Volts for simplicity's sake across them (each). It's the voltage drop that matters and not the voltage on a particular node of the circuit measured referenced to 0 Volts. Do you have the data sheet for the LEDs you are thinking of using?

Thanks
Adam

 

[Mr Musabe]

Hi Adam thanks for your reply, that does make a lot of sense. I should have realised that volt drop is dependant of current as V= IR


I haven't got a data sheet all I know is they are 3 volt white 10mm LED's. they are on ebay and don''t have any other information.
Looking around the general assumption is that they would run at around 15 to 20 milliamps?

If they were in series would the last LED not be dimmer as it gets the last of the voltage?

 

[Arouse1973]

As long as you have enough voltage headroom and the current is within spec you should be fine, if the LEDs are from different batches you can sometimes get a difference in brightness. The best way to drive LEDs in series is with a current source. Then you don't have to worry at all about the LEDs voltage drop. As long as you have enough voltage to supply the desired current. I am guessing the LEDs are either Toyoda Gosei or Nichia.
Adam

 

[Mr Musabe]

I can't see who makes them, it doesn't say. Hopefully they will all be from the same batch. I was thinking of buying a breadboard so i can experiment with it a bit and see what works before soldering it all together.

 

[Martaine2005]

A bread board is always a good idea for any kind of circuit first.
A part from what Adam has told you, you can have a good read of the resourse section here for 'anything led'.
It's a great read and very informative.

Martin

 

[Arouse1973]

I can't see who makes them, it doesn't say. Hopefully they will all be from the same batch. I was thinking of buying a breadboard so i can experiment with it a bit and see what works before soldering it all together.

Good idea Mr M.

 

[Mr Musabe]

Thankyou both for your help. I will read that section on LED's.

 

[Colin Mitchell]

" resource section here for 'anything led'."

There are lots of mistakes in this resource, including this diagram:

(Image 2.4 from http://www.blue-room.org.uk/lofiversion/index.php/t32607.html)

 

[(*steve*)]

There are lots of mistakes in this resource, including this diagram:

Colin, I'd love to use some very un-parliamentary language to describe exactly how helpful you are.

 

[Colin Mitchell]

What is the diagram supposed to represent?
Is is a string of diodes or LEDs???
If is is LEDs, 12v will not be sufficient to produce any illumination.

 

[Ratch]

Hi guys I am new on here and want to clear something up that is confusing me.

I have never done electronics before so I apologise if this question seems stupid. I need to connect 5 LED's in series.
I am planning on using LED's that are 3 volt. Am I right in thinking I will need at least a 15V supply? or will a 12 V be enough? (5 x 3=15)

What is confusing me is, if Each LED drops 3 volts then I will have used 15 volts, so after the first LED I will have 12 volts, then 9 volts etc. why doesn't the first or second LED blow from that voltage as they receive the highest amount of electrical pressure?

I understand that if it was in parallel each LED would get the same voltage and a resistor would be needed

Thanks in advance

Not to worry, Since the LEDs/diodes are in series, one individual diode cannot possibly use more current that any of the other diodes do. The current rises to the same value in each diode simultaneously. Some diodes might light up at slightly different currents when starting up, but that is no problem. If one of more of the diodes should short out, then the rest of the string will take the extra voltage and possibly burn out, too. That is where some current limiting or fusing would be advisable.

Ratch

PS. Electrical circuits use volts (joules/coulomb), not pressure (pascals).