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Topics - sam

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Hi all,
I'm hoping to tap into the collective brain for a problem I'm working on.  I've been using my lab's Chronos for recording turbulence patterns in my soapfilm tunnel.  Here are some test videos I took:

The pressure distribution on the flowing soap film causes light and dark interference fringes when illuminated with a monochromatic light source.  Currently, I use a 55 watt Philips Low Pressure Sodium lamp (  To drive the lamp, I'm using a Fulham workhorse 5 (, as I saw many people had had success using this ballast with LPS lamps.  When I bought it, I thought I would be free from 60Hz flicker because it is advertised as an "electronic" ballast.  Unfortunately (as you can see in the videos), the 60 Hz flicker is alive and well...

I scoped the output to get a sense of what was going on, and I saw the nice ~100kHz waveform I had expected (TEK00110.PNG attached -- don't worry about the voltage scale; it's measured through a capacitive pickup, so the absolute scale doesn't mean anything).  When I zoom out in time I see the amplitude of this high frequency signal is modulated at 60Hz (TEK00111.PNG attached), explaining the flicker.  So, my ballast is indeed an electronic type, but it is passing through the 60Hz frequency content from the mains. 

Initially, I had assumed the circuit in the ballast was something like the block diagram here:
I thought the flicker could be coming from not having enough capacitance after the rectification to sufficiently smooth the signal.  So, I opened up the ballast and de-potted the pcb from the disgusting black tar... and found what looked like the diagram above (image attached).  My thought was just to replace the blue electrolytic capacitors with larger ones.  But, when I (carefully) probed the circuit, I didn't see what I had expected after the rectification stage.  Instead of a near-DC signal, I saw the sine wave clipped into positive and negative pulses before getting sent into the chopper.  So, I didn't know how to proceed...

So, I'm looking for some advice.

1. Does anyone in this forum has experience with LPS lights and their ballasts?

2. Anybody know other electronic ballasts that work with LPS that might have less flicker?  I tried this one (, but I don't think the striking voltage was high enough, as the arc never formed (just a halo around the electrodes).

3. Is there an external power filter module I can put in series with the lamp to filter the 60Hz (and avoid any depotting...)?

4. DIY no-flicker LPS driver? 

5. any other ideas?

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