# How to illuminate an LED regardless of voltage level (12VDC and 6VDC)

#### DAP2

Joined Dec 28, 2023
8
I am trying to solve a scenario where I want an LED to illuminate regardless if its given 6VDC or 12VDC. I cannot control the voltages "given", so the circuit would start with a single DC source that alternates between 6 and 12. It would be more beneficial for the 6v input to be dimmer and the 12v to be brighter. Is it as simple as sourcing dimmable diodes, or would there need to be some type of relay/controller in the circuit?

#### sghioto

Joined Dec 31, 2017
5,388
Would suggest a constant current supply. What is the LED exactly?

#### Ian0

Joined Aug 7, 2020
9,777
I am trying to solve a scenario where I want an LED to illuminate regardless if its given 6VDC or 12VDC. I cannot control the voltages "given", so the circuit would start with a single DC source that alternates between 6 and 12. It would be more beneficial for the 6v input to be dimmer and the 12v to be brighter. Is it as simple as sourcing dimmable diodes, or would there need to be some type of relay/controller in the circuit?
Calculate the resistor value to give the maximum current for the LED from 12V. It will be approximately half as bright when connected to 6V.
Most LEDs have a maximum current of 20mA. Red LEDs have a volt-drop of 1.8V.
(12V-1.8V)/20mA = 510Ω

#### Ya’akov

Joined Jan 27, 2019
9,101
Welcome to AAC.

It first seems that you are talking about an LED in the rôle of an indicator but “dimmable LED” suggests illumination. While that doesn’t really make sense, it would be very helpful if you’d explain why the LED is doing this—that is, describe the problem the LED is solving for you, the application.

LEDs are not subject to the idea of “dimmable”. You may be confused but “dimmable LED bulbs”. This is a distinction because there is a power supply in the bulb for the multiple LEDs it uses, and that power supply must be compatible with the method of dimming you might use.

An LED’s brightness depends on how much current you supply. As @Ian0 suggests, it would be a natural outcome of simply using an LED wired to work for 12V to get a half-bright illumination at 6V. But really, please tell us what this LED will be doing for you, it’s important for us not to root around in the dark if we are going to really help.

#### Ian0

Joined Aug 7, 2020
9,777
it’s important for us not to root around in the dark if we are going to really help.
Such a good pun.

#### DAP2

Joined Dec 28, 2023
8
Welcome to AAC.

It first seems that you are talking about an LED in the rôle of an indicator but “dimmable LED” suggests illumination. While that doesn’t really make sense, it would be very helpful if you’d explain why the LED is doing this—that is, describe the problem the LED is solving for you, the application.

LEDs are not subject to the idea of “dimmable”. You may be confused but “dimmable LED bulbs”. This is a distinction because there is a power supply in the bulb for the multiple LEDs it uses, and that power supply must be compatible with the method of dimming you might use.

An LED’s brightness depends on how much current you supply. As @Ian0 suggests, it would be a natural outcome of simply using an LED wired to work for 12V to get a half-bright illumination at 6V. But really, please tell us what this LED will be doing for you, it’s important for us not to root around in the dark if we are going to really help.
It is for the purpose of illumination (automotive). Maybe it is my old school knowledge, but I thought an LED's cutoff voltage would be above 50% (6V would be no illumination, 12V would be full). Are you saying that an LED should be half as bright at half the voltage, no different than how traditional bulbs work?

#### Ian0

Joined Aug 7, 2020
9,777
It is for the purpose of illumination (automotive). Maybe it is my old school knowledge, but I thought an LED's cutoff voltage would be above 50% (6V would be no illumination, 12V would be full). Are you saying that an LED should be half as bright at half the voltage, no different than how traditional bulbs work?
The combination of LED and resistor will behave that way.
A red LED needs 1.8V to make it light up. Green LEDs about 2V, and Blue LEDs about 3V

#### LesJones

Joined Jan 8, 2017
4,188
LEDs are current driven devices. So the brightness is about proportional to the current through them. If you connect any single LED without some form of currebt limiting to 6 volts it woulf be destroyed. The forward voltage of red LEDs is about 1.8 volts blue & white LEDs it is about 3.3 volts. This is at the rated current.

Les.

#### LowQCab

Joined Nov 6, 2012
4,052
What is the Part-Number of the LED ?
Please supply the PDF Spec-Sheet, or at least a Picture and a description.

Why is "6-Volts" a thing ?
Why would the LED ever be operating on 6-Volts ?
You said it's purpose was "Illumination",
so it is not being used as a "Diagnostic-Tool", therefore, "6-Volts" is completely irrelevant.

Do You want to make the LED "Dimmable" ?
For what purpose, and in what manner ?
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#### ericgibbs

Joined Jan 29, 2010
18,830
Hi DAP,
If you are using 12V OR 6V, this circuit will give an apparently brighter LED at 12V than at 6V.

E

#### BobTPH

Joined Jun 5, 2013
8,928
Note that the solution is simple because the voltages you picked are well above the voltage required by an LED. If you had said 2V and 4V the solution would be more complex.

#### Alec_t

Joined Sep 17, 2013
14,312
Note that the simple circuit will give a current at 6V which is about a third of the current at 12V. If that variation is too great then a more complex ciruit will be needed. We do need to see the spec of the 'LED' you intend to buy. Some include a series resistor and some don't.

#### dl324

Joined Mar 30, 2015
16,898
Maybe it is my old school knowledge, but I thought an LED's cutoff voltage would be above 50% (6V would be no illumination, 12V would be full). Are you saying that an LED should be half as bright at half the voltage, no different than how traditional bulbs work?
LEDs are non-linear devices. Some will start turning on at very low currents.

LEDs aren't always "drop-in" in automotive. Some lighting circuits for incandescent bulbs run a small current through them to checked for burned out bulbs. LEDs don't work well in that scenario.

#### panic mode

Joined Oct 10, 2011
2,733
cannot really think of a car using 6V, but...
since the application is illumination rather than indication, series resistor is not a good idea since current is large and efficiency would be terrible. so why not use led driver? i am thinking of something like LM3519 for example.
this one is for low input voltage but it is just an example ( one that comes to mind), even so one can use DC/DC regulator to step voltage from 6-12V down to something suitable for this product though i am sure there is no need for double conversion as other products may support wide input voltage range.

#### panic mode

Joined Oct 10, 2011
2,733
or... you can boost voltage 6-12V to something higher (like 24V), then use AL5810Q

at any rate, you really need to provide the specs. what is the LED current, forward voltage drop, how many are used to make one "light". first need input, then we can try to think of a solution.

#### MisterBill2

Joined Jan 23, 2018
18,426
None have asked yet, I don't think. There are bare LEDs that do not include any additional circuitry. Then there are also low voltage "BULBS" that are intended for some specific voltage, usually 6, 12, or 24 volts. These usually include a resistor although some may also include an electronic circuit.
Understand that an LED by itself is a diode with a forward voltage drop, like all diodes, although that voltage drop is usually more than a non-LED diode, such as a power diode.
A bare LED will start to illuminate when current starts to flow, which will be at some voltage a bit lower than the rated forward voltage. But the neither the current nor the brightness will vary in a linear manner with the forward voltage.
Many bare LEDs without additional components are rated for 20 milliamps at some low forward voltage.
THEN there are also higher powered ILLUMINATION LEDs that may have current ratings up to several hundred milliamps. THAT is the more common ILLUMINATION LEDs that are rated at higher voltages WHEN IN A PACKAGE with additional components that you may not see.

So we really do need to know just what sort of LED you are asking for advice about.
MY experience with a 12 volt auto-accessory LED type 12 volt bulb is that it would light a it at 6 volts and a lot more at 12 volts just as it was packaged.

#### eetech00

Joined Jun 8, 2013
3,926
I am trying to solve a scenario where I want an LED to illuminate regardless if its given 6VDC or 12VDC. I cannot control the voltages "given", so the circuit would start with a single DC source that alternates between 6 and 12. It would be more beneficial for the 6v input to be dimmer and the 12v to be brighter. Is it as simple as sourcing dimmable diodes, or would there need to be some type of relay/controller in the circuit?
LEDs are "current driven" devices. So you would need a circuit to provide a constant current, regardless of the voltage, to light the LED. There are many ways to do this...simple a BJT or JFET current limiter, IC current regulator, etc., are just a few.

#### ThePanMan

Joined Mar 13, 2020
792
None have asked yet, I don't think.
As @MisterBill2 began to state - I think I'm wondering and wanting to ask the same thing: Exactly WHAT is the LED being lit? This could be a simple 5mm LED or could be an automotive side marker rated for 12V. A marker has its own resistor built in to limit the current to meet the specs of the LED. THEN there's automotive LED's such as turn signal indicators, brake lights, backup lights, parking lights and even interior lights. STILL MORE - there's also LED Headlights. All those animals are different types of LED. So exactly what are we talking about?

#### Jon Chandler

Joined Jun 12, 2008
1,045
So many answers before the type of LED was established. And those answers are geared more towards a single 5mm LED running at 20mA than say an cob LED assembly consisting of many LED elements and designed for 12 volt operation.

Don't be in a rush to answer a question if necessary facts are absent. Or at least preface your answer with appropriate bounds.

#### LowQCab

Joined Nov 6, 2012
4,052
I asked way back in Post-9, but it seldom, if ever, works.
I really don't get it,
it always seems that getting more information is like pulling-teeth in this Forum.
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