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L.E.D.`s - series or parallel?

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Community Veteran
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L.E.D.`s - series or parallel?

I want to wire up 4 or possibly 6 white l.e.d.`s ( 5mm 3.2v - 3.8v)

I have a small "battery pack" that I can insert 3 X AA batteries for the power supply

The completed "chain" will be used as a back light through a dimmed perspex screen to underlight drawings for copying purposes... a "kind of" lightbox affair.

As the l.e.d.`s will be in use for some length of time... possibly up to 1 hour .. continuously,  I would like to know which would be the best way to wire them......

series or parallel.

I could also use a 12v battery power pack, if a higher voltage supply would be needed for the long "on" time envisaged.

 

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Re: L.E.D.`s - series or parallel?

I would be inclined to use parallel to provide a better balance of power and it will also allow the circuit to work should one of the LEDs fail. If you've going to use a 12v supply then you'd need to include a voltage regulator in the circuit to prevent things from getting cooked.

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Hero
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Re: L.E.D.`s - series or parallel?

@shutter 

I believe that white LEDs have a forward voltage drop of around 3 volts so your supply voltage limitation will preclude putting them all in series.

The other thing to consider is that once the diode reaches its knee voltage the current starts to rise very rapidly with voltage, you therefore need a current limiting element, typically a resistor though if you want to go for the bells and whistles solution with a transistor you could make a constant current generator.

With parallel connections you need one resistor per LED and therefore multiple LEDs will be a drain on your power supply.

The low power solution would to have well matched LEDs (illumination v current) connected in series with a single resistor if fed from a regulated voltage or constant current generator if the voltage is variable, but I doubt that you have the voltage available.

This leaves you with the parallel power hungry version. At least if the illumination is uneven you can tweek the series resistors.

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Re: L.E.D.`s - series or parallel?

@Baldrick1  A lot of what you said, is a misty mystery, hence my initial question...but I "kinda" understand what you are saying...

no good with circuit mathematics.... so applying ohms law to find the resistor(s) in a circuit would baffle me !.

I COULD use various 1 amp, (or other amperage) 12v wall power supplies, but I was hoping to use battery power ( as indicated) purely from the portability point of view.

I also have available, a NiCad battery charger for 3, 6,  9, and 12 volts available....( the kind used for small motorcycle batteries/alarm batteries )

I can follow a circuit diagram to obtain parts, and solder them together... ( hopefully in the right order ! !! ) and the thing will work.. ( again hopefully ) ! ! ... if it does not work, then a visual check of the joints with a magnifying glass, and "rudimentary" follow through the connections sequences/locations would be all the trouble shooting I would be able to do ! ..  if that did not find the fault, then I would de solder it all and start again ! ! !...

 

These are the leds I have bought....

https://www.ebay.co.uk/itm/Ultra-Bright-LEDs-3mm-5mm-10mm-Red-Blue-Green-White-Yellow-LED-Clear-Bulb...

 

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Superuser
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Re: L.E.D.`s - series or parallel?

@shutter  a typical LED has a forward operating current of 20mA, although there's no actual specs for the ones you have. Effectively once you get above the operating voltage (typically 3v ish!) then as @Baldrick1  says the led is almost a shortcircuit. Therefore you should size the resistor to give 20mA when the led is operating i.e 3v drop across it.

If Vp is your power supply voltage, then using ohms law , R = (Vp-3)/0.020.

Actually a quick google revealed this http://www.ohmslawcalculator.com/led-resistor-calculator

It would be good to have actual specs for the leds you have though.

Without those , probably best to err on the side of caution and assume a lower current, hence larger resistor. Exceeding the max current (likely around 30mA) will brick the led.

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Re: L.E.D.`s - series or parallel?

@MisterW  Thanks for that... and the linky... looks like I`m on the way to building the next space rocket now ! ! !..

Grin

 

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Hero
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Re: L.E.D.`s - series or parallel?

@shutter 

Sorry I did not mean to confuse you.

From the spec sheet LEDs each have a forward voltage of 3.5 V plus or minus  0.3V. The currnet for the stated illumination is 20mA with a max of 30mA.

The simplest option is to just put your LEDs straight on to a 3V supply and see how bright they are. You could also use a series parallel combination on your 12 V supply.

Noting your reluctance to engage in ohms law this might help.

Power Supply V

Configuration

Series Resistor

Comment

3V

Direct connection

None

May be dim

4.5V

Single diode

47K

 

6V

Two in series

None

May be dim

6V

Single diode

68K or 82K

Ideally 75K

9V

Three in series

None

May be dim

9V

Two in series

39K

 

9V

Single diode

100K or 120K

Ideally 110K

12V

Four in series

None

May be dim

12V

Three in series

68K or 82K

Ideally 75K

12V

Two in series

220K or 270K

Ideally 250K

12V

Single diode

390K or 470K

Ideally 425K

 

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Re: L.E.D.`s - series or parallel?

@Baldrick1 WOW !... thanks very much... you have just secured a place in the front of the rocket ! !  Cheesy

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Superuser
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Re: L.E.D.`s - series or parallel?

@shutter I've just noticed that in one of the pictures on that listing they DO have the specs.

Seems @Baldrick1 noticed it earlier.

 

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Hero
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Re: L.E.D.`s - series or parallel?

@shutter 

Is this rocket the Ark from the Hitchhikers Guide to the Galaxy?

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Re: L.E.D.`s - series or parallel?

O.K... so ... what I was mainly concerned about, was how long the battery "powerpack" would last, if used for about 1 hour continuous with 4 or 6 leds.  .   I do have rechargable batteries, ( so supply of batteries is not a problem)  and the usage would be, probably very limited... say, .. once a month... for an hour or so, at a time.. .. ..  ..

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Re: L.E.D.`s - series or parallel?

@Baldrick1  Can`t be sure...but probably an updated version, as I seem to recall that was way back in the eighties, when steam was just being phased out ! ! ... Smiley

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Hero
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Re: L.E.D.`s - series or parallel?

@shutter 

Hold fire, I've just noticed some errors, I shall returm!

 

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Superuser
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Re: L.E.D.`s - series or parallel?

@shutter  I do have rechargable batteries

AA rechargeables are about 1500mAH IIRC. So at 20mA per led going to get 1500/(20*no of leds)  hrs of use.

@Baldrick1  aren't those resistor values in your table a bit high. By my calcs a single led at 20mA with a 4.5v supply needs around 47 ohms not 47k

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Hero
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Re: L.E.D.`s - series or parallel?

@MisterW 

I have screwed it up and am reworking it. I'm having a bad day! Apart from everything else I'm getting ohms and K ohms wrong. It must be my age! Damn, it's too late to delete the table. It's a bit embarrassing really!

Right, the new truth:

Power Supply V

Configuration

Series Resistor

Comment

3V

Direct connection

None

May be dim

4.5V

Single diode

47

 

6V

Two in series

None

May be dim

6V

Single diode

120

 

9V

Three in series

None

May be dim

9V

Two in series

100

 

9V

Single diode

270

 

12V

Four in series

None

May be dim

12V

Three in series

68 or 82

Ideally 75

12V

Two in series

220 or 270

Ideally 250

12V

Single diode

390 or 470

Ideally 425