Halogen soft start circuit

Thread Starter

AlbertHall

Joined Jun 4, 2014
11,504
Is a soft start worth the loss of filament life due to DC versus AC?
Everything I've seen says that it makes no difference whether they are run on AC or DC.
In this case it is being fed full wave rectified AC so the voltage still goes up and down just the same - it just doesn't reverse direction.
How do they know the difference?
 

wayneh

Joined Sep 9, 2010
17,153
Everything I've seen says that it makes no difference whether they are run on AC or DC.
In this case it is being fed full wave rectified AC so the voltage still goes up and down just the same - it just doesn't reverse direction.
How do they know the difference?
Any incandescent bulb vaporizes the tungsten filament. A halogen bulb is designed to keep the tungsten from depositing inside the bulb, and instead to redeposit on the filament thereby lengthening its life. That much is not disputed.

I've read that AC allows the vaporized tungsten to redeposit more uniformly than does DC. I cannot find that reference again, and barely any recognition that it matters its all. Maybe it's an urban legend. Seems like such a simple question - the answer should be easier to find! Here's one mention of it:
https://electronics.stackexchange.c...cts-of-ac-vs-dc-for-an-incandescent-bulb-life
 

wayneh

Joined Sep 9, 2010
17,153
Last edited:

Thread Starter

AlbertHall

Joined Jun 4, 2014
11,504
Here is the latest product of my twisted mind. R4 represents the bulb. For U1 and M1 I would use LM393 and IRFP460 but In the circuit shown I have used parts available in LTspice.
Initially C3 is discharged so the MOSFET is switched off but the intrinsic diode supplies half wave rectified power to the lamp. As C3 charges the MOSFET is switched on for lengthening periods of time until after about 0.9s it is on all the time. Initially the lamp is being fed with DC but only for a short period of time and when the filament is relatively cool. After 0.9s the lamp is fed with the full AC waveform.

Pull this one to piees please.
1615286167821.png
 

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