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How to Add a Dimmer Facility to a LED Bulb and How its Work?
Posted By Teena Garg
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In this article, we learn how to make an LED dimmer circuit for enabling a dimming installation to any mains-operated LED bulb.
How LED Bulbs Work
We know that our ceiling fans and incandescent bulbs can be easily controlled using Triac dimmer switches, and we're quite used to dimmer switches in our homes installed for controlling similar devices.
Still, with the arrival 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 press, and an SMPS circuit makes it difficult to operate or control through a Triac dimmer switch, until and unless it’s suitably modified for the operation.
Because, the SMPS driver inside LED bulbs and tubes strictly employs inductor or capacitive-based circuits which are nothing recommended to be used through Triac dimmers, since Triac dimmers use phase chopping technology for the dimming purpose which unfortunately doesn't suit inductive/ capacitive load control.
Still, due to an incompatible response, if used also the LED bulbs don't dim rightly rather show erratic dimming or brightening behavior.
The best system 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 that may be implemented.
How it Works
The idea is actually really simple, thanks to the MOC series OPTO couplers which make Triac control through PWM extremely easy and compatible.
The right side of the figure comprises a standard MOC3063 IC-based Triac controller circuit which is operated through an IC 555-based PWM circuit shown on the left side of the figure.
The IC 555 is configured as a standard adjustable PWM generator which feeds the asked PWM to the input pin#1/2 of the MOC IC.
The adjustable PWMs are appropriately reused by the IC through its built-in zero crossing sensor circuit and print Triac which is eventually used for controlling an external Triac BT136 via its output pin#4/6.
The connected LED bulb now responds to the PWM content applied by the 555 circuits and proportionately adjusts its brightness as per the user's preference.
The PWM control is executed through the associated 100K pot, which must be suitably insulated, as the whole circuit isn't isolated from the main current.
The circuit isn't isolated from mains despite the OPTO coupler due to the fact that the IC 555 requires a DC supply for operating which is supplied from anon- isolated transformerless power supply, this is done in order to keep the design compact and avoid the use of costly SMPS module which could have been else an overkill.
Still, you can express them through your comments, if you have any concerned questions regarding the above-explained dimmer circuit for LED bulbs.
UPDATE
This filter capacitor will hold the charge and keep the LED bulb ON indeed 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.
Still, the dimming effect can be maybe 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 also transferred to a series LED assembly for lighting it up.
We've to modify this LED section and connect it with the IC 555 PWM stage as shown over.
This can be implemented in the following way
- 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.
- Eventually 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 doesn't need an external DC, and it can be derived from a DC supply from the SMPS, meant for driving the LEDs.
- Eventually, connect the LED SMPS input to the AC mains and check the dimming effect by varying the IC 555 PWM pot.
- Remember the SMPS circuit primary side isn't isolated from mains and thus extremely dangerous to touch in switched ON condition.
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