Simple LED power question

[Macwarrior]

Hey --

I'm trying to make a small LED lamp that runs on a 9-volt battery. It would use 4 white LEDs, 3.5v each. I can't run them in series (3.5 * 4 = 14), but if I try to run them in parallel (at 20mA nominal current), my calculations tell me that I'll need at least a 1W resistor to dissipate all that extra power. I don't really want a giant hot resistor eating up my battery, so here's my question:

Can I hook the LEDs up in a parallel-series circuit, with 2 sets of LEDs in series, connected in parallel? It simplifies to a pair of (7-volts-drop at 20mA) circuits in parallel, which manages to decrease the resistor requirement to something like 0.08W/56ohm. In addition to less heat and wasted power, it's nice that I can use cheap 1/4Ws.

This seems okay, but seeing as I have very little experience with electronics, I thought I'd check it out here - am I defying any laws of physics or electronics by doing this?

Thanks very much

Macwarrior

(oh, and if the power is backwards in the diagram, ignore it - I always forget which way a diode diagram is polarized)

 

[Nigel Goodwin]

Hey --

I'm trying to make a small LED lamp that runs on a 9-volt battery. It would use 4 white LEDs, 3.5v each. I can't run them in series (3.5 * 4 = 14), but if I try to run them in parallel (at 20mA nominal current), my calculations tell me that I'll need at least a 1W resistor to dissipate all that extra power. I don't really want a giant hot resistor eating up my battery, so here's my question:

Can I hook the LEDs up in a parallel-series circuit, with 2 sets of LEDs in series, connected in parallel? It simplifies to a pair of (7-volts-drop at 20mA) circuits in parallel, which manages to decrease the resistor requirement to something like 0.08W/56ohm. In addition to less heat and wasted power, it's nice that I can use cheap 1/4Ws.

This seems okay, but seeing as I have very little experience with electronics, I thought I'd check it out here - am I defying any laws of physics or electronics by doing this?

It's very bad practice to parallel LED's like that, you should have a seperate resistor feeding each series pair - so instead of one 50 ohm, use two 100 ohm instead. Essentially giving two completely seperate circuits.

 

[Macwarrior]

Thanks for the suggestion; I'm sure it will save me some problems in the future, not the least of which being that I can find 100s more easily than 50s. What's the reason for it being so bad to drive them on a single resistor?

 

[Nigel Goodwin]

Thanks for the suggestion; I'm sure it will save me some problems in the future, not the least of which being that I can find 100s more easily than 50s. What's the reason for it being so bad to drive them on a single resistor?

Because one pair of LED's will always take more of the current than the others, unless they are 100.00% exactly the same - which isn't going to happen!.

 

[monkeytree]

leave out the resistor because electronics can work fine without maths

 

[bonxer]

leave out the resistor because electronics can work fine without maths

:shock:

You must put in the resistors, otherwise you're going to be saying goodbye to those the LEDs pretty quick. The resistor is there for two reasons, to limit current, and provide a voltage drop. If they're not there, then the entire 9 volts will be across an LED pair that's trying its hardest to drop that down to 7 volts. The 20mA limit is removed and immediately exceeded. It draws whatever it can (and heats up until it dies).

 

[Someone Electro]

you can conect an LED directly if the batery voltage is the same as the LEDs forvord breakdown voltage

you can caculate the resistor in this:
(Vbat - Vled) / I = R

Vbat :batery voltage
Vled :LEDs forvard voltage (if the LEDs are in seris this is aded up from all the leds in seris)
I erierd curent
R :Resistor neded

Its that simple!

 

[Nigel Goodwin]

you can conect an LED directly if the batery voltage is the same as the LEDs forvord breakdown voltage

No you can't!.

For one thing the battery voltage will change as it ages, and under load.

Also ohms law applies, with the current being equal to the voltage divided by the resistance - with no resistor in circuit the resistance is approaching zero (actually the internal resistance of the battery) - and the LED's will be destroyed.

 

[Someone Electro]

I can run my UV led directly on three NI-MH button baterys and it can be used a samll flashlight.The LED dosent get warm

and small green leds a ablbe to run directly on 2 Ni-MH button baterys

Seen an simple circuit on some site that can run a white LED of one almost dead button cell.It has chep 4-5 components (transistor,coil,cuple of resistors) It is able to regulate the curent runing thru the LED

 

[Nigel Goodwin]

I can run my UV led directly on three NI-MH button baterys and it can be used a samll flashlight.The LED dosent get warm

and small green leds a ablbe to run directly on 2 Ni-MH button baterys

It's APALLINGLY bad practice :roll:

Seen an simple circuit on some site that can run a white LED of one almost dead button cell.It has chep 4-5 components (transistor,coil,cuple of resistors) It is able to regulate the curent runing thru the LED

 

[monkeytree]

Someone Electro wrote:
I can run my UV led directly on three NI-MH button baterys and it can be used a samll flashlight.The LED dosent get warm

and small green leds a ablbe to run directly on 2 Ni-MH button baterys




It's APALLINGLY bad practice

it may be bad practice but it does work perfectly well unless the current limit is exceeded by more 7 times and the voltage limit can be exceeded by usually half as much again.

 

[jrz126]

It may work just fine, but for how long? When you abuse the led like that it may appear to be working just fine, but chances are you are shortening its life. Why not throw a $0.003 resistor on there?

I only had a couple current limiting resistors in the first batch of leds that I put in my headliner. https://www.cardomain.com/memberpage/435572/2
After about 2 weeks of running them, (probably about 8 hours total) about 10 or 15 of them quit on me.

I've been running the second batch for about 4 months now (50+hours) and havnt had a problem with a single one.

 

[audioguru]

Button cell batteries are made for a very low current drain (watches?) and therefore have a high internal resistance. Keychain "torch" manufacturers take advantage of the resistance in the button cells and use it as the current-limiting resistor. The resistor is there, but you can't see it.
Try it with a new 9V alkaline battery that is capable of supplying up to 1/2A and kiss the LEDs goodbye. If the LEDs would not burn so fast, you would actually see them dimming as the battery voltage runs down:

Three 3.0V LEDs in series without a current-limiting resistor.
9V battery measures 9.5V when new--LEDs are blinding drawing 300mA.
9V battery at exactly 9.0V--------------LEDs are normal drawing 20mA.
9V battery at 8.5V-----------------------LEDs are very dim drawing 1mA.
9V battery at 8.0V, almost like new----LEDs are extinguished drawing no current. Etcetera!

 

[Macwarrior]

I'm pretty sure that 300mA would melt these LEDs' semiconductors...I know (from some idiotic mistakes) that it's enough to heat the leads to a painful temperature. You _can_ overdrive the LEDs to about 50% more (30mA), increasing the brightness, but that results in a much shorter life.

Thanks again, Niigel.

 

[panic mode]

you can conect an LED directly if the batery voltage is the same as the LEDs forvord breakdown voltage

No you can't!.

well sure you CAN connect it but it's really not good idea. did you know that all electronics works on smoke and once you let the smoke out it's dead... :lol: