A2 Starting Current Graph???

Does anyone know where I can find a graph showing the incandescent start up current for the A2 vs. an incandescent without a "soft start" circuit?? A superslow motion video would also be great, however, I know that is asking for a bit much.



thanks,

Paul

The regulator in the A2 is a PWM regulator, and thus is switching on and off a great many times a second. A graph of the current going through the filament for the first second would be very jagged and difficult to understand. The important parameters are probably average current and average power, but you have to define what the averaging period is.

As for the A2 specs, very, very few people actually even know what the steady state average filament current (or power) is precisely because it almost requires something like an oscilloscope to measure, although I found another way to do it as well as using an o-scope.

BUT, the main thing about the startup is that the LVR limits the average CURRENT to the filament, holding it back from the high value it would normally have against the very low resistance of a cold filament. After the initial 50 mS (approx.) it switches to maintaining a constant DC-equivalent voltage across the filament via pulse width modulation.

Very interesting, thanks for all of the info on the regulator which I was not fully aware of. It would be great if you could post any o scope data that you have. Perhaps I am trying to oversimplify it, but correct me if I am wrong. For the lamp to continue to burn at a constant brightness P must be constant. If we adhere to the formula P=E*I where P is the power in watts,E is the voltage and I is the current, then as the voltage drops the regulator compensates by increasing the current. Is this correct in your opinion?

:candle:

senna,

Well, not really. No.

The AVERAGE power must be constant for the output to be constant, but the thing about an incandescent filament is that as long as it is hot it will continue to "burn", i.e. radiate light: think of a blacksmith heating a length of metal up in his or her forge--no electrical power applied, yet light is emitted because it is HOT. An electrical current to a filament just serves to make and keep it hot. That's all. So, as long as the power is constant when averaged over a time interval that is fairly short relative to the thermal time constant of the filament, then it will continue to burn at constant brightness even though the instantaneous power is going all over the map.

The FET turns on and off, alternating between directly connecting the battery stack to the filament, and completely disconecting it, and it does so in just such a way that the AVERAGE voltage (and thus power) is constant when averaged over a time interval that is long compared to the thermal time constant of the filament.

If you want, you can check out my SF A2 review for more details on the A2 and how it works.

I was planning on doing a part 2, with o-scope pictures and lots of technical details, but, frankly, I'm so busy with TigerLight consulting and my regular job and the USL and last of the M6-R stuff, that I doubt I will ever get around to doing that. All of it is a royal PITA, and I doubt that SureFire would really want that stuff in the public domain anyway, and I think I will respect that. It really doesn't matter much in any case. I've said enough already about the A2 in part 1 of my thread.

Senna, the filament would only adhere to that if it could cool and heat instantly. Even so, must PWM have a cap in the output stage to smoothe the ripple. The ultimate objective of the circuit is to control the temp of the filament just as a PWM speed control attempts to control a motor. The inertial forces of the rotating shaft is just like the cooling of the filament. Once the next pulse comes, the temp is jacked back up to the set level

The easiest way to estimate the power is with an analog voltmeter. The slow response of the meter acts like an integrating cap in a way

One should also remember that an ordinary household lightbulb is "pulsing" at 60Hz with the AC current in our outlets.

I can't do the exact lamp but they are all pretty much the same just different scales.

Here is a startup curve with voltage soft-start of an osram 62138 lamp ramping logarithmically up to 12.5V:

(2V/div/3.9A/div/50msec/div)



I calculated the power spike to be 160W at 31A at 50msec.. and 194@ at 24A at 100msec.

now contrast that to a 'current limited' startup like mentioned by JS above..


This time the power numbers mathed out to 80W at 50msec and 136W at 100msec.

notice.. that initially (for the first 20 msec or so) the voltage and current rise at exactly the same rate as with the voltage only soft-start.. but that as soon as the current reaches 20A in this case.. the voltage is clamped and is only allowed to rise to the level that generates 20A.

As the filament heats and becomes higher resistance it takes more voltage to achieve 20A.. and at about 100msec the point is reached where the voltage can resume it's normal logarithmic rise to the set voltage.

To achieve a faster startup and put less heat on the FET, i usually set a startup current about 2 to 2 1/2 x the running current of the lamp.. this is tremendously less power on the filament than 'direct connect'.. and achieves a nice balance of a quick startup (typically 1/4 second or less) and a serious drop on the power spike to the poor filament!

In this same example.. the instantaneous power spike applied to a cold filament of a 62138 lamp wit 12.5V applied will be over 400W easily!

I have measured over 75W spike on a 15W lamp for example.. using only soft start and not current limiting will drop that to 1/6th the power spike. I don't usually bother with current limiting on lamps lower than 50W.. i like the bit of power spike to help heat the filament faster, and it's simpler to program and set the regulator... i typically get power spikes no more than about 50% of running power when i use only voltage ramping soft-start.

-awr

Andrew, could you measure the bulbs resistance at room temp please.