REFLECTOR: HID lamp cable length

[Brian Michalk]

Here I go talking to myself again.

I went and found some schematics online for HID drivers.  There are two 
parts to the circuit:  A high frequency transformer (~20KHz.), and a 
diode voltage multiplier like here: 
http://en.wikipedia.org/wiki/Voltage_multiplier .

Normally the bulb is a high impedance, read this as an air gap.  It 
takes a bit of a spark to get it started similar to how flash bulbs 
work.  Actually, is very close to a flash bulb except it stays lit.  
Once the several Kvolts jumps across the gap, the resistance drops to 
several ohms.  The arc is maintained from the transformer.

The high voltage starter is a DC source which is blocked by a capacitor, 
and forced through the bulb.  Once the delta voltage across the bulb 
drops, the high voltage source really doesn't deliver more than a few 
milliamps.  However voltage multipliers are noisy!  You don't want an 
antenna radiating this noise.

So, what Andy referred to as the module on the bottom of the bulb is 
most likely the high voltage multiplier that keeps the noise down.

 From what I can tell researching patents; the digital ballasts are more 
like modern power supplies that sense the output current and regulate it 
with digital electronics rather than old analog feedback.  The digital 
ones would seem to be much more efficient, but I'm not seeing a lot of 
those claims in the literature I've found.  It would also seem that the 
digital ballasts manage the ignition in a quieter way because they 
theoretically would not need the multiplier.  Seems like a large 
capacitor dumpped through the transformer would ignite as well as the 
diode multiplier.

 From all this I surmised several things:
1) Yes, you can lengthen the cable from the ballast to the bulb.  Use 
shielded twisted pair grounded at each end.  Yes, each end.  It makes 
the assembly appear as a single enclosure.  This is different than 
signaling and ground loop noise.
2) Digital ballasts should be quieter and more efficient.
3) Take Andy's advice on the bulb/igniter configuration.

Brian Michalk wrote:
> When I was at TxDOT, we designed a system for automated crack counting 
> on pavement.  The lighting system was very simple.  A row of LED's, two 
> mirrors and a cylindrical lens.
>
> Scott Baker just replied that the canard leading edge is 1/4".  That 
> just doesn't seem right.  A .5 inch diameter rod fits the profile of our 
> canard?  I would have thought about 1 to 1.5 inch diameter.
>
> I'll see if I can cobble up a drawing of my idea.
>
> Andy Millin wrote:
>   
>> Brian,
>>
>> You are working beyond the limits of my research.  Sorry man.
>>
>> I agree that the D series shouldn't have the cable length limitation.
>>
>> With the igniter and reflector the unit will be fairly deep.  If you have
>> the old style canard (foam only outboard), and I think you do, it might
>> work.  If the spar ran the full length, I don't think it would work.
>>
>> Where will you find the reflector to fit the profile?
>>
>> Andy
>>
>> -----Original Message-----
>> From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
>> Behalf Of Brian Michalk
>> Sent: Sunday, July 20, 2008 11:08 AM
>> To: Velocity Aircraft Owners and Builders list
>> Subject: REFLECTOR: HID lamp cable length
>>
>>
>> Andy,
>>
>> In your browsing of the net, did you find any limitations on cable length?
>>
>> If I were to figure out how to make this work as a canard taxi light, it 
>> would be nice to mount the ballast in the fuselage.
>>
>> For the more expensive D series, where the igniter is mounted to the 
>> lamp, what else is it good for?  Is that where the current regulation 
>> happens?  It seems that RF noise from current regulation would not be a 
>> significant problem.  I can see that ignition phase would be very noisy.
>>
>> I need to find a site of someone who has dissected a lamp and discusses 
>> theory of operation.
>>
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>
>
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