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SCR as a switch control for automatic light dimmer

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New Member
Hi guys, can u guys help me on this?? I wanna design and construct an automatic light dimmer circuit that controls the energy delivered to a 12V or 24V DC light bulb corresponding to the ambient light intensity.
The control of energy will be established through switching action to improve efficiency and i will use SCR as means for switching.
I have no idea where should i start. any ideas guys? thank u very much and hope to hear from u guys soon =)


Why use an SCR? For low voltages like that a Mosfet will likely work better. Turning off an SCR once it's conducting DC can be tricky.


Well-Known Member
Most Helpful Member

I agree with Sceadwian in that a MOSFET would be a good choice.

Turning off an SCR that is conducting a dc current means shorting it out
with a low impedance device (like a MOSFET) anyway, for a time period
that allows the SCR to fully turn off, and then the rate of rise of voltage
has to be limited too once the low impedance device turns off. Too
much of a hassle if you ask me.
Also, you can get MOSFETs that will have lower voltage drop than an
SCR would have so that would mean higher efficiency too (provided
a good driver circuit is also used).
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You should be able to find a plethora of 555 based timer circuits either here in the forum or via Google for basic 555 dimmers. Just use those to feed the gate of a FET to control the actual load. Should be a very simple project.


Most Helpful Member
Buy something else. SCR's are good for phase control of AC but a hassle to work with on DC systems.
All of the added circuitry to make them work can be eliminated by using a Mosfet, transistor, or IGBT.
They are not practical devices for strait DC applications.


New Member
SCR are generally used for high voltage applications.
and if you want to use SCR as switch for light dimmer use should use light dependent resistor (LDR) or any component sensitive to light.
What i can remember that LDR have +ive temp coefficient.(not sure)
You can use this ldr to control the gate current and also Input voltage.


New Member
What Lizkay may not know is that thyristors (SCR's, alternistors or TRIACs) STAY ON when they are fired. So they are perfect for AC (two zero crossings per cycle that allow the device to turn-off) but hardly useable on DC except for crowbars OVP devices.

Lizkay, please consider converting yourself to power MOSFETS! They are virtually the perfect switching devices for your application and as the guys say, do some search on 555 dimmers or if you prefers to work with microcontrollers, they are plenty of PWM capable devices available (I use Microchip"s PICS and many of them have PWM outputs).
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