Pulsing halogen Headlight Shorten Life?

Thinking of building a headlight pulser. I was wondering: if I let it pulse all the way on-off, will it shorten the life of the bulb? The required pulse rate is about 3Hz, so the filament would be heating and cooling a fair amount between cycles.

Another thing: I believe most of them don't pulse fully ON-OFF, they go from about 30% brightness to 100% on when they pulse. It would be a lot simpler to just toggle fully OFF to ON, I suspect that would shorten the bulb life even more.

Any specs on this as a function of light bulb life?

Actually I believe that they pulse from Hi to Low. But either way no it would not shorten the life in fact it would lengthen it. The life of a filament is determined by total on time.

otakar wrote:

But either way no it would not shorten the life in fact it would lengthen it. The life of a filament is determined by total on time.

Actually, filaments are severely stressed by heating from cold to hot, hence most light bulbs blow when you flip them on. The cold filament has significantly less resistance than hot, so it gets hit with a huge surge current at turn on.

In TV sets, they used to leave the picture tube filament on at half voltage (most were a 6V filament) to keep them warm. That gave "instant on" but also multiplied tube life. I have one that ran about 25 years and the tube was still good. That design practice had to be discontinued for "green technology" because it was too wasteful to run the filament at half throttle all the time it was off.

I was worried that pulsing from full off to full on would be hard on the filament of the headlight.

otakar wrote:

Actually I believe that they pulse from Hi to Low.

Leave the low beam on continuously and pulse the high beam on?

Or, pulse back and forth between them?

Pulse back and forth between them.

otakar wrote:

Pulse back and forth between them.

according to the Feds, that's not legal. Their spec says it's either one or the other:

S7.9.4.1 A headlamp on a motorcycle may be wired to modulate either the upper beam or the lower beam from its maximum intensity to a lesser intensity

www.gadgetjq.com/headflash_dot_regs.htm


It also says you can NOT pulse from full on to off on each cycle, the "lowest" power of the cycle has to be at least 17% of max:

The lowest intensity at any test point shall be not less than 17 percent of the maximum intensity

Here is an article that knows a lot more than I do. I run an LED for daytime city riding and a HID for night time.
users.snip.net/~WCLAMB/MODULATOR.HTM

This will tell you the DOT regulations for modulation.
It looks like you will have to copy and paste the address the link wont work right.

I think the link I posted is to the DOT specs. The link you posted sends me to a page asking me to sign up for something.

Here's the actual modulator specs:

TITLE 49--TRANSPORTATION

CHAPTER V--NATIONAL HIGHWAY TRAFFIC
SAFETY ADMINISTRATION, DEPARTMENT
OF TRANSPORTATION

PART 571--FEDERAL MOTOR VEHICLE SAFETY STANDARDS--Table of Contents

Subpart B--Federal Motor Vehicle Safety Standards

Sec. 571.108 Standard No. 108; Lamps, reflective devices, and associated equipment.

S7.9.4 Motorcycle headlamp modulation system.
S7.9.4.1 A headlamp on a motorcycle may be wired to modulate either the upper beam or the lower beam from its maximum intensity to a lesser intensity, provided that:
(a) The rate of modulation shall be 240 <plus-minus> 40 cycles per minute.
(b) The headlamp shall be operated at maximum power for 50 to 70 percent of each cycle.
(c) The lowest intensity at any test point shall be not less than 17 percent of the maximum intensity measured at the same point.
(d) The modulator switch shall be wired in the power lead of the beam filament being modulated and not in the ground side of the circuit.
(e) Means shall be provided so that both the lower beam and upper beam remain operable in the event of a modulator failure.
(f) The system shall include a sensor mounted with the axis of its sensing element perpendicular to a horizontal plane. Headlamp modulation shall cease whenever the level of light emitted by a tungsten filament light operating at 3000 deg. Kelvin is either less than 270 lux (25 foot-candles) of direct light for upward pointing sensors or less than 60 lux (5.6 foot-candles) of reflected light for downward pointing sensors. The light is measured by a silicon cell type light meter that is located at the sensor and pointing in the same direction as the sensor. A Kodak Gray Card (Kodak R-27) is placed at ground level to simulate the road surface in testing downward pointing sensors.
(g) When tested in accordance with the test profile shown in Figure 9, the voltage drop across the modulator when the lamp is on at all test conditions for 12 volt systems and 6 volt systems shall not be greater than .45 volt. The modulator shall meet all the provisions of the standard after completion of the test profile shown in Figure 9.
(h) Means shall be provided so that both the lower and upper beam function at design voltage when the headlamp control switch is in either the lower or upper beam position when the modulator is off.
S7.9.4.2(a) Each motorcycle headlamp modulator not intended as original equipment, or its container, shall be labeled with the maximum wattage, and the minimum wattage appropriate for its use. Additionally, each such modulator shall comply with S7.9.4.1 (a) through (g) when connected to a headlamp of the maximum rated power and a headlamp of the minimum rated power, and shall provide means so that the modulated beam functions at design voltage when the modulator is off.
(b) Instructions, with a diagram, shall be provided for mounting the light sensor including location on the motorcycle, distance above the road surface, and orientation with respect to the light.

One of the circuits I designed is very simple and would pulse from 0 - 100% (total off to on) each cycle which is NOT legal by the spec, but I was wondering if it also shortened bulb life. I suspect it would.

users.snip.net/~WCLAMB/MODULATOR.HTM

This is the part I had a question about:

2. Won't my headlight bulb burn out from all that flickering?

NO again. Since the average power applied to the bulb filament is lower, the use of a modulator actually extends the usable lifetime of the modulated headlamp's bulb.

MAYBE


Average power applied is NOT the critical consideration, it's actually the range of cooling and heating which stresses the filament. Just sitting there at one temp will give a very long lifetime, turning it on and off repeatedly (and letting it cool each time) will kill it quickly. I used to do exactly that to electronic devices to stress test them and get worst case life data.

I believe the fact that it goes between 20% power and 100% power never lets it cool MUCH, so maybe it doesn't impact service life significantly. However, if it is switched between 0 and 100% I believe it would shorten the life.

Just do one filament and a pulse-width modulation. Very simple. No heat dissipation losses in the controller since it will be doing full on-off cycling, but the frequency will be high enough that the filament doesn't fully cool in each cycle.

Half the time the filament will be on 100%, then the other half of the time the filament will be turning on and off at a 30% duty cycle but at a high frequency. A single 556, and a few resistors and capacitors should handle the timing and a simple 15A Darlington BJT can do the driving (or any FET of your choice, I know you like your FET's ). Put the first half of the 556 somewhere in the 5 to 10 hz range, then put the other half in the 200 hz range at a 30% duty cycle. The first half will enable/disable the second half.

There will still be some stress on the filament. The duty cycle-method doesn't prevent that, but does eliminate the need for big heat sinks and high wattage transistors. (Still needs high-current transistors of some sort, though.)

Anytime the filament goes through changes in heat dissipation levels (regardless of how it is achieved), it contracts and expands. That's enough to shorten the life, but nowhere near as much as a full bright to full dark cycling. (As you already said.)