How to Add a Dimmer Facility to a LED Bulb

In this article I have explained how to make an LED dimmer circuit for enabling a dimming facility to any mains operated LEDs bulb.

Update: [Using Capacitors, it Works]

If you think PWM dimming control might not work for commercial LED bulbs, then the best method is to use series capacitors, which might just do the trick.

Using selectable high voltage capacitor series capacitors can be used for dimming control of all ready made commercial LED bulbs without any problems, as shown below.

Understanding The Basic Idea

Let us first understand that this circuit is working only because we are putting different capacitors in series with the LED bulb supply line.

So we can say that we are using the capacitors as the main current controller. Now you select one capacitor at a time through the rotary switch, and that capacitor decides how much current will flow to the LED bulb.

How The Capacitors Control The Current

We know that a capacitor does not behave like a resistor. Since a resistor drops power by heat, a capacitor does not do that.

A capacitor creates a reactive impedance that is called Xc. So this Xc will limit the AC current. The formula for this is written as:

Xc = 1 / (2 × π × f × C)

Now we can see that when C becomes smaller, then Xc becomes bigger and current becomes less.

When C becomes larger, then Xc becomes smaller and current becomes more. So you can say that small capacitor gives dim light and large capacitor gives bright light.

How The Rotary Switch Makes The Steps

The rotary switch is simply selecting only one capacitor at a time. Let us say you rotate to the 0.047uF position, then you get very low current and very dim LED.

When you rotate to 0.1uF then brightness increases slightly. When you rotate to 0.5uF then LED becomes medium bright.

When you rotate to 1uF then LED becomes almost fully bright. So the rotary switch is like a step-by-step brightness selector.

How The LED Bulb Reacts Internally

Inside the LED bulb there is a small driver circuit. This driver wants to maintain some constant power or constant current inside the LEDs.

So when you limit the input current with the capacitor, the driver automatically reduces its output to the LED string.

That is why brightness reduces smoothly. Since the internal driver is stabilizing everything, then you normally do not get flickering also.

Why There Is No Heat Produced

Let us note that there is almost no heat in the capacitors because they are reactive parts. So they do not burn any power like resistors. That is why this dimmer becomes very efficient for LED bulbs.

Why Only High Voltage Capacitors Are Used

We must use 400V rated polyester or polypropylene capacitors. We must do this because the capacitors have to handle the full AC voltage which has a peak of around 310V.

So these capacitors must tolerate high stress. You should not use low voltage parts here, since then they will fail very soon.

Brightness Steps You Usually Get

With 0.047uF you get very dim night lamp kind of light.

With 0.1uF you get dim light.

With 0.5uF you get medium brightness.

With 1uF you get almost full brightness.

The exact brightness can change from one LED bulb model to another, but the general idea stays same.

Important Things To Know

Now let us discuss some important points slowly.

When the LED bulb has isolated SMPS, active PFC, or high level constant-current driver, then sometimes the bulb may not dim smoothly at very low current.

The bulb can even shut off. So this method works best for simple non-isolated driver bulbs or basic capacitor-dropper type bulbs.

Since the entire circuit is connected directly with main AC, then there is no isolation. So every part of the circuit remains at high voltage potential, and you must handle with care.

Since capacitive dimming cannot reduce the voltage to zero, then you cannot get completely OFF brightness unless you add one OFF position in the rotary switch.

Summary

So in this method we simply put different capacitors in series with LED supply. That capacitor limits the AC current according to Xc.

The LED driver receives less current and then it reduces its output power and you get dimming. The rotary switch selects the amount of current and that gives you different brightness levels. This becomes a very simple and efficient dimmer because it uses no heat and no TRIAC.

Using PWM Dimming Control is the Best?

We know that our ceiling fans and incandescent bulbs can be easily controlled using triac dimmer switches, and we are quite used to with dimmer switches in our homes installed for controlling such devices.
However with the advent of LED bulbs and tubes, incandescent bulbs are slowly making an exit, and our home bulb holders are getting replaced with LED bulbs.

LED bulbs come with a built in SMPS driver within their holder cabinet, and an SMPS circuit makes it difficult to operate or control through a triac dimmer switches, until and unless its suitably modified for the application.

Because, the SMPS driver inside LED bulbs and tubes strictly employ inductor or capacitive based circuits which are never recommended to be used through triac dimmers, since triac dimmers utilize phase chopping technology for the dimming purpose which unfortunately does not suit inductive/capacitive load control.

If used then the LED bulbs do not dim correctly rather show erratic dimming or brightening behavior, due to an incompatible reaction.

The best method and probably the technically correct approach is the PWM technology which can be effectively used for controlling or dimming LED bulbs or tubes. The figure shows the design may be implemented.

You may also like: Simple DC Lamp Dimmer Circuit

Problem in the above Method

A deeper inspection of the above concept shows that the concept might not work due to the presence of the internal filter capacitor in every LED bulb circuit, right after the bridge rectifier.

This filter capacitor will hold the charge and keep the LED bulb ON even during the OFF times of the PWMs, preventing the dimming effect.

This means that dimming an LED bulb through an external means can be impossible.

However, the dimming effect can be perhaps implemented by connecting the series LED section of the LED bulb with the IC 555 circuit, as indicated in the following diagram:

We know that an LED bulb circuit is nothing but a small AC to DC SMPS circuit, which employs a small ferrite transformer for stepping down the mains voltage to a lower LED DC voltage. The secondary side of the transformer produces the stepped down voltage which is rectified by a single diode and a large filter capacitor.

The rectified DC is then transferred to a series LED assembly for lighting it up.

We have to modify this LED section and connect it with the IC 555 PWM stage as shown above.

This can be implemented wiht the following steps:

• Open the LED bulb container.
• Cut the wire of the LED assembly which goes to the negative line of the DC supply.
• Connect this negative LED wire to the transistor collector of the 555 pwm circuit.
• Finally connect the 555 pwm circuit's positive/negative wires with the LED DC supply, coming from the ferrite transformer secondary.
• This also means that the 555 IC circuit does not need an external DC, and it can be derived from DC supply from the smps, meant for driving the LEDs.
• Finally, connect the LED smps input to AC mains and check the dimming effect by varying the IC 555 pwm pot.
• Remember the smps circuit primary side is not isolated from mains and therefore extremely dangerous to touch in switched ON condition.