Laser Return Current (HY031-D)

In many CO₂ HVPS the laser tube return is shunted directly to ground; often thru a front panel ammeter so the user can monitor the current. The HVPS neither directly monitors nor regulates the laser current. However, in some HVPS the laser tube return is one of several components that the SMPS incorporates into its regulation algorithm.

The tube return current is shaped by a component network before being passed to the SMPS control circuitry. This network has the effect of expanding the range for currents less than 10mA, which may come in handy for pre-ionization regulation.

 

This network has the effect of expanding the range for currents less than 10mA, which may come in handy for pre-ionization regulation.

Analysis:

• For I_LaserRet <10mA, only R20 and R5 are important (along with minor contribution from the series combination of R1 and R2).
• R5 (2KΩ) represents the 2KΩ potentiometer which is used to calibrate the I_LaserRet current.
• Taken together (390Ω) || (2K) || (10K + 2K) gives 317Ω, which nicely matches the measured value of 314Ω.
• Taken together (390Ω) ∥ (2K) ∥ (10K + 2K) gives 317Ω, which nicely matches the measured value of 314Ω.
• Above 10ma the U1 (TL431) is active and introduces an additional 100Ω (R21) in parallel with the above combination.
• This gives 76Ω (which is in the same ballpark of 88Ω (the slope measured off the graph).
• ZD2 (9.1V Zener diode) provides a hard limit for currents above 80mA and is likely for protection.

This is a plot of actual data collected from the HY031-D HVPS.

 

It nicely matched the simulation.

 

 

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Enrollment in local colleges, 2005

Source: Fictitious data, for illustration purposes only ⊥ ∥

Tube installed Oct 31, 2011

 Note the tube was built Feb 16, 2011. This means that when the tube was installed it was 8 ½ months old.

When it started failing in mid-Feb 2012 it was over 1 year old, even though it only had 80 hours of actual activation since being installed on Oct 31, 2011.

 

Some of the failure mechanisms we’ve heard mentioned include:

• Electrode erosion (due to excessive current)
• Optical failures (mirror or exit window)
• Leakage (degradation of resin seals)
• Out-gassing (contamination of gas in tube mainly by seals)
• Exhaustion (change in gas chemistry, notably CO₂ into CO)

 

DrawingQAT

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