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Theoretical question about HID ballast contol

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walk on

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Someone was kind enough to direct me here about a theoretical question I have listed below:

While doing some reading on HID bulbs and ballasts then somehow ending up on youtube (it always happens), I came across an interesting article about different methods of regulating the output power aka intensity. There are several methods practiced by manufacturers of dimmable ballasts, but to me the simplest (but not most efficient) way seems to be simply slightly adjusting the input. Attached is a Pout vs Pin graph from one of the articles. I'm not familiar with the rules regarding outside links, but if it's allowed I will gladly post it.

Assuming efficiency wasn't of great importance and the input voltage was always kept above the minimum required for proper operation, wouldn't a voltage divider before the ballast's input theoretically do the trick? Part of me says it makes sense, part of me says its too simple to work. Perhaps it's so simple because it isn't efficient? (Possibly why this simple dimmer knob isn't offered as a low cost attachment)
I don't have the parts (or courage) to see if it would work, but I was just curious and decided to ask if anybody could share their input.
View attachment 67179
 
Hi,

A resistive voltage divider to control the power to a 400 watt load would consume a HUGE amount of power, making it so very inefficient you wouldnt want to use it.
 
A voltage divider is used to create a voltage that is in proportion to the original voltage, but you cannot draw an appriciable amount of power from it. The voltage divider CAN provide the feedback to a controller that can.

If your actually doing intensity control, then you have to measure the intensity. I have some experience using a 1000 W arc lamp source.
 
I see your point about a such a large load, but I was thinking about it this way (copied from my other post):
"Those super bright lights that end up blinding people on the streets aren't properly marketed. They advertise them as a simple replacement bulb which just end up producing a ridiculous amount of light scattered in all directions. The proper ones are real HID lights, use a ballast, focus the light as not to be too bright for other drivers, and cannot be dual filament (since there is no filament). The attraction to the falsely advertised "HIDs" I guess is that the driver has dual filaments.

With a 12v source, a driver would be able to control the brightness with a simple switch to either engage or bypass a voltage divider that would serve as an intensity control (high beam - low beam). That's what brought me to do some light reading on the matter. The appeal of having the control to have 2 levels of brightness is what makes those super bright falsely advertised "HID's" so appealing. "

I was just sick of thinking that ETs space ship came back to earth every time i saw one (especially in the rain). That's what got me thinking about it.
Perhaps the (approx.) 30 watt bulb that the ballast drives is still too large to make this possible. Im no EE and still am not sure where that magical power comes from and if its safe to regulate it from the supply. Anyway this was just something I thought about and wasn't able to get a answer on by myself. Kind of like a kid who doesn't understand politics asking why the government is so (insert your political belief here).
 
Here is some info on HID lamps: https://en.wikipedia.org/wiki/HID_lamp

An HID lamp is an arc lamp. what this means is that sometimes the arc doesn't strike in the exact same place. It also means that a wide range of intensity isn't possible primarily because it degrades lifetime and performance.

It also means that the current to the lamp is the controlled variable.

On of the issues I saw in the 1000 W precision light source that I used is that over time the spectrum had less blue light in it and the intensity for a given amount of lamp power was reduced. You could see this discoloration on the lamp. We were concerned with both spectrum and intensity.
The lamp was about $800 and you had to "suit up" to change the lamp. Heavy clothes, a full face shield and gloves. The lamps were changed at 1000 hours of operation regardless.
 
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I see. I understand also that dimming near 50 percent would greatly reduce the lifespan of the bulb, but wasn't aware that even minor dimming would have such a major effect on durability. I skimmed something about the changes in color earlier, but got sidetracked and forgot all about it. As for the $800 bulb, wow...
 
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