DC regulators can't drive car headlight DC bulb... why?

dr pepper said:

Picbits with ref to post #20, I think theres something amiss with your supply.
I also have a weir, 0-30v 2a or 0-60v 1a it is 2 identical isolated supplies which are either paralled or series'd.
Mine has a max o/p of 2a and it lights a 12v 21w vehicle indicator lamp immediatley no stress.
If yours is a 5a and it struggles something's wrong, when I got mine a couple of years back some of the cans had to be replaced to get a clean o/p, maybe yours has dried caps too.

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I need to strip them down and replace the caps - they are getting on a bit now and I suspect the output cap has dried out over the years. It is perfectly fine for driving a constant 4-5 amps into a resistive load but doesn't like car headlight bulbs.

Qaisar Azeemi said:

thanks to all. i want to know if a high amount of current for a fraction of second due to cold resistance can turn the LDO regulator LM338 off then why this cold resistance is not effecting the switching regulator LM2576??? it lit the bulb just if it is directly connected to a 12V battery. what is the reason??

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Hi again,

As we've said, it's a function of the actual regulator being used. The curve of the regulator during current limit has a lot to do with it and the level of current at each point on the curve. In other words, there are some regulators that will never work right for this and there are other regulators that will always work ok for this.
It depends mostly on the current level itself and also the voltage difference across the regulator (difference between input and output voltages). For some regulators it's only a matter of current but for others it's more than that.