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Low voltage (12V) AC/DC incandescent dimmer

Low voltage (12V) AC/DC incandescent dimmer

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Dimming circuit for halogen lamp secondary side dimmer.
Chop the input voltage by the antiseries (to operate at AC polarities) MOSes to reduce the rms voltage, so the lamp brightness. Work for both AC and DC (for DC the positive pole should be on "INP").
Uses well known 555 circuit, where the duty ratio is controlled by altering the voltage on the internal resistor divider (pin 5). The 555 drive directly gate of two FETs in antiseries conection: One interrupt one polarity, second the other one.
Supply for the 555 is derived from the transformer by rectifying it via one diode (there is low current consumption, so it suffice). If the pot has an end-position switch, it can be connected in series with the diode to switch the power fully OFF. In this case i would recommend to add bleeder resistor ~10kOhm between "OUT" and "GND" pins of the IC to ensure no remaining voltage on the FET gates (so they would stay OFF and do not dissipate).

Ballast12V.GIF SelfOscClamped.GIF HalogenDim.GIF TrafficLightPCB.GIF

Light Information

Light Information

Lamp
Lamp Type:Incandescent
Electrical
Wattage:Any, what used FET's can handle
Voltage:12V AC or DC
Current:Any, what used FET's can handle
Physical/Production
Factory Location:Your home...

File information

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Filename:HalogenDim.GIF
Album name:Medved / Homebrew lighting
Keywords:Gear
File Size:16 KB
Date added:Dec 22, 2010
Dimensions:1003 x 514 pixels
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Dec 23, 2010 at 12:14 AM Author:
Nice!...The one I built has one MOSFET and a voltage regulator for the pin 8 and 4 of the 555.
SeanB~1
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Apr 18, 2011 at 01:10 PM Author: SeanB~1
My version is slightly different, I used a variac on the secondary of the 12V transformer, one that I could otherwise not use, as it was rated for 115VAC 400Hz operation, rated for 50W input power, max current 5A on the slider. Worked very well.
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Apr 18, 2011 at 01:33 PM Author: Medved
I just spotted a bug on this picture: The C3/R3 should not go to pin 7, but to 2+6.
I do apologize for it to anybody not successful to build it, it was quick-drawn as an indication, how to eliminate the diode bridge and large capacitor (both dissipate quite lot of power) in the classical approach (rectifier + filter to make the DC and then followed by the PWM regulator)

No more selfballasted c***

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Apr 19, 2011 at 10:11 AM Author: SeanB~1
Not too much of a problem ,I like your use of the body diodes of The MOSFET's to provide rectification and isolation when in the off state. The first time I saw that in a PSU I was a little puzzled why the MOSFET's were reversed, then on further thought saw that the body diode was only there for startup, being bypassed by the much lower resistance of the on state when required.
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Apr 19, 2011 at 01:22 PM Author: Medved
The body diode is bypassed even here: Both FET's are controlled together, so when the current flow, both are ON. That is the reason for way lower losses then with the diode bridge (it has voltage drop of two diodes, so >1.5V)...

And i guess you mean UPS (forcing zero voltage phase between active pulses on the modified sinewave)?

No more selfballasted c***

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