# What does a HID ballast do?



## AilSnail (Oct 4, 2003)

I am thinking specifically of the Solarc, but any info might be helpful.

Thank you.


----------



## James S (Oct 4, 2003)

Initially it has to provide a voltage spike to strike the arc, once the arc is established it's basically a short circuit and so the ballast must limit the current to something reasonable. Of course also has to boost the input voltage to something high enough to maintain the arc, but nowhere near the voltage necessary to strike it in the first place.

It also has to be smart about restriking a hot bulb i think.

For bulbs that try to start brighter sooner it will provide more current initially and as the halides (or mercury or sodium or whatever) evaporate the arc will get brighter and it can reduce the current in the arc to keep the same brightness. This reduces the life of the bulb but makes them brighter faster. Some car HID lights do something like this, I don't think the solarc lamps do.


----------



## JSWrightOC (Oct 4, 2003)

This is kind of a broad explanation of ballasts in general, as the term originally did not apply to HID lights. /ubbthreads/images/graemlins/smile.gif

Originally, the term "ballast" was used to describe large inductors used to limit AC current for fluorescent lighting and other gas-discharge lights (especially sodium vapor and mercury vapor streetlights). The application of the term "ballast" has now been broadaned to include a host of other devices which more closely resemble power supplies. 

The modern electronic ballast used for HID lights (which are essentially metal halide gas-discharge lamps) is a DC to AC inverter with a current-limited or sometimes regulated output. There is also a circuit called a "striker" which is usually integrated into the ballast module. The striker generates a train of very high voltage (2-5kV) pulses which cause the gas inside the lamp to break down and ionize. Once the gas ionizes, it will conduct electrical current at a much lower voltage, and then the high frequency AC output of the inverter continues to keep it ionized and in operation. In this mode, the voltage present across the lamp's terminals may be anywhere between 70-300V, depending on the lamp and application.

A similar "ballast" is used for CCFL lighting found in LCD panel backlights (laptops, camcorders, digital cameras, and what have you) and some newer CCFL flashlights. These use a similar to gas-discharge lamps, but without the striker. Instead, the AC output of the inverter by iteslf is high enough to cause the fluorescent tube to ionize and begin conduction. A capacitor in series with the AC output limits the current.

The purpose of such DC-AC inverter-type ballasts is to take a low voltage DC source (such as your car's electrical system, a few 123's, or any other source for that matter) and convert it into the voltage required to start and operate the lamp.

Magnetic ballasts for gas-discharge lamps still need the use of a striker circuit to get the tube started, however the lamps they power are designed operate at a voltage lower than the supply voltage (120-240VAC). The impedance of the inductors limits the current to the lamp (at a given frequency) to a safe value.

Magnetic ballasts for standard hot-catholde fluorescent lamps are operated in a similar fashion, however they do not require the use of a striker. Older designs rely on the operation of a starter module, which is usually a miniature mercury vapor lamp which, when operating, conducts current through the filament electrodes of the primary lamp, allowing them to emit electrons at a lower voltage. Once the emission voltage is low enough, the primary lamp ionizes and the starter ceases to operate, now that the voltage across its terminals is below its operating voltage. Newer so-called "rapid-start" ballasts often use a solid-state device as part of a circuit that takes the place of the starter module.


----------



## AilSnail (Oct 4, 2003)

Well I got a glimpse of what you said, anyway.
This one gives watt in a linear relation to input voltage. I am a bit concerned that the initial input voltage will be too much for the bulb since I am counting on the load to pull the four 123's down to 10-11 volts. The bulb gets 10w at 10.2 v and the ballast is 85% efficient. Duracell's datasheet and Paul_W's magnificently simple empirical SF123 formula (V=3-(0.3*I) suggests that the steady state voltage will be somewhat around 10v. I believe 11v should not be exceeded, but will be so during the startup phase? Any thoughts? 

Edit: And Iforgot to say: THANK YOU for the elaborate answer, both of you!


----------



## AilSnail (Oct 5, 2003)

I will maybe install the voltage limitation cirquit depicted here: 
http://www.walamp.com/Product_Pdfs/Front%20End%20Circuits.zip

There is also a voltage limiter with softstart, and i wonder what that is for?


----------



## Zelandeth (Oct 6, 2003)

Check out the HID section here, if I remember correctly, there's a pretty in depth explanation there. (http://members.misty.com/don/index.html)


----------



## Kiessling (Oct 6, 2003)

does this mean a HID ballast is actually regulated?
and what happens if the batteries start getting weak? will it shut down the buld entirely or will it fade away?
bernhard


----------



## yclo (Oct 6, 2003)

Will it be possible to install a voltage regulator (i.e. Willie Hunt's LVR) along with the ballast? What voltage should it be set at, ballasts sometimes have a xx ~ xx range.

-YC


----------



## PsycoBob[Q2] (Oct 9, 2003)

You can likely do it, but I think you'll need to add filter capacitors to smooth the PWM output. I don't know how the LVR would handle having voltage applied across the outputs when it's not supplying it. Shouldn't be a problem, but I haven't looked at the schematics. 

[/edit] One possible problem is the advanced features- 'Dimming' and 'Low-battery flash' would get real unhealthy for a HID. The LVR3G has very high amperage and input max voltage capacity. If you ordered one with a reprogrammed controller to disable those features, it could work. [edit]

According to Willie's FAQ:

"Can I run LED's, strobe lights or other electronics off the LVR?

No, the LVR does not put out DC. It puts out Pulse Width Modulation of the battery voltage. It is only suitable for filament light bulbs. LED's can be hooked up with a series resistor, but this isn't an efficient way to do this.

LED's can be run on a constant voltage DC source but really they need constant current. There are many DC to DC current mode convertors that can be used for LED's. The problem with white LED's: are far less efficient (when driven hard IE > 50mA) at producing light than a good halogen incandescent light bulb at the lumen levels that most climbers, cavers, and cyclist need. If you only need 1 to 5 lumens LED's are nice, but if you want 40 to 100 lumens they are very expensive way to produce less light and light of poor color temperature with more power consumption than a halogen light bulb. However, for low light needs they can work great and if you build large array and keep the per LED current low (< 20mA) you can achieve higher efficiency than light bulb. 

Strobe lights have a DC to DC convertor in them and so need DC as an input. Many other electronics like radios, tape players etc. also need a DC input. "


----------



## yclo (Oct 9, 2003)

I emailed Willie and he said that his LVR's are not suitable for running with HID ballasts. However there are people out there who build regulated ballasts.

-YC


----------

