Powering LEDs without resistors

[Raven Morpheus]

Hey all

I'm currently planning a project and I'm wondering, purely for ease of soldering, if I can get away with not using resistors for my LEDs.

Most of the LEDs will be the 0805 SMD type. And besides making my life easier when it comes to soldering I also cannot work out what resistor I need to use.

The power source is at least 1 CR2032 battery (so 3v minimum), possibly 2 (6v), possibly more if needed (I have considered using 3 to make 9v but cannot find, yet, a suitable battery holder).

I read about using a single LED driver instead of resistors some time ago, but cannot find anything about it in reference to lighting models at this time and all of the LED drivers I've looked at so far are at least 6" by 2" so not suitable for stuffing inside a model where space is at a premium.

Would anyone here have any knowledge of using LED drivers in place of resistors and know of a micro one that will fit into a tight space?

Thanks in advance.

[Ant]

I'm currently planning a project and I'm wondering, purely for ease of soldering, if I can get away with not using resistors for my LEDs.

Short answer. No

Long answer. LEDs are current devices. The current must be limited to specified values or the device will fail. Without any limits, the more voltage you apply to the device, the more current will flow, so for simple LEDs a resistor is used to limit the current for a specified voltage. A driver is a constant current source - this is a bit of circuitry that will ensure the same current always flows, whatever the voltage. These are needed for high power (100's of mA up) LEDs because these generate heat and the operating characteristics will change as they heat up. Thus additional circuitry is required to balance out those changes. But it is a waste of money to use a driver just for a low power LED, a resistor will be fine.

What often catches people out when they say resistors are not needed is just down to the ability(or inability) of the chosen power source to actually provide enough current to do any damage. Coin cells in particular have very little capacity, a few mA tops. Your circuit may appear to be running fine with no resistors, but if you change to a higher capacity battery type (keeping the voltage the same).... poof!

[Raven Morpheus]

OK thanks. I guess that explains why they're all fairly large chunky devices.

Ideally I'd like to wire them all my LEDs/SMDs to 1 resistor for the whole lot close to the power source end of the circuit (my LED/SMD circuit will be a parallel one), so that I don't have a bulky resistor in the way when fitting the SMD's in place.

So given that how do I work out the resistor value I need for my SMD's (I can find online resistor calculators for normal LEDs but not SMDs)?

[EVApodman]

All LEDS have a maximum of 20-25 ma forward current. Typically a single 1.5 cell may allow you to run LEDs without a current limiting resistor. Using batteries with higher voltages or hooking batteries in series will increase the source voltage and require a line current a limiting resistor.

Hooking batteries in parallel will keep the same line voltage but increase your current capacity. Also your final circuit assembly may have more current drain than you figured and this will cause a drop in line voltage and dimmer LEDs than they were when tested individually. If this happens then you need a larger power supply.

[brt]

Or buy pre-wired leds and smds that have a resistor already soldered in at specific voltage values.

[TITAN14]

Ideally I'd like to wire them all my LEDs/SMDs to 1 resistor for the whole lot close to the power source end of the circuit (my LED/SMD circuit will be a parallel one), so that I don't have a bulky resistor in the way when fitting the SMD's in place.

So given that how do I work out the resistor value I need for my SMD's (I can find online resistor calculators for normal LEDs but not SMDs)?

All you need is the basic electrical formula and an understanding of current dividers and voltage dividers.

V=MA
M=resistance in ohms
A = current in Amps. 1 mA = .001A
V = voltage

Resistors in series divide voltage in proportion to the total series resistance.

Resistors in parallel split current in proportion to the total parallel resistance.

You only need one current limiting resistor for a series of LEDs. The problem being you need more voltage than one 2032 is going to put out to drive the series.
If you go parallel then your batteries run out in a few hours (or minutes) because each branch of the parallel network is drawing current. In addition the parallel LEDs can all have different light levels due to natural variances of resistance in the LED and its resistor. You could end up with one power hog that outshines all the others.

I have no idea what you are building but based on my limited understanding of your project you need to run 1 white LED and run fiber out to your illumination points. Then paint the ends of your fibers to match your requirements. This solves all of your power problems efficiently and gets you ease of installation which you stated as a concern with bulky resistors.

You could run 1 20 mA LED on a stack of 3 coins for at least 33 hours. (Provided you can get enough voltage to drive the LED)
You can hold the stack of coins (including wires) together with tape or just build a rig with some sheet styrene. This is for batteries in series.

The reason you can't find a rig to run coins in parallel is that they are other power cells that provide those voltages with much better current capacities. A coin battery is only ~220 mAh. When you run 9v on 3 coin batteries you need to knock the voltage back down to drive the LEDs.
So we use a 360 Ohm resistor to drag the voltage down to 2.2V @ 20mA. But this adds a load of ~7.5mA thats getting burned off as heat in the resistor to run 1 LED. So maybe 8 hours for 3 coins to run 1 led? If you go parallel network the voltage divider resistors dont need as much power but the current load adds up to maybe 3 hours of battery life?