Investigating a dimming LCD, an engineer discovers the best diagnostic tool is the one that does the job
An engineer proves once again that the best diagnostic tool is the one
that finds the source of the problem, not the newest or prettiest one
in the bag
About a hundred engineering years ago, I worked for a company that made
sunlight readable (high bright) LCDs. They used thin CCFLs (cold
cathode fluorescent lights) and optical films to do this; the CCFLs
used high-voltage inverters and worked very well. I had released a new
design to production and things went along fine for a while.
One day I was called out to the production floor – brightness was
way down. The usual swaps fixed nothing. The voltage across the tubes
was way down, but disconnected (open-circuit) it was normal.
We had just gone through a 6 Sigma training and certification
session and the engineering manager was anxious to show that 6 Sigma
was the universal cure-all. I had the normal engineer’s love of
paperwork and ceremonial procedure – I wanted to find the cause, fix
it, document it and go back to shipping product. I understand 6 Sigma’s
power in high volume production, but this was a 20-unit-per-month
problem, not millions-of-ballpoint-pens-per-day problem. While the
wizards pondered, I grabbed a failing unit and headed back to the lab.
I had been bit by this high voltage too many times before during
design and debug and wanted to think more with my brains than my
fingers. What looks (electrically) like a digit without the bloodshed
and puncture burns? I got a Q-tip, dipped one end in water, clipped a
lead to it, grounded the other end of the wire to circuit ground and
held onto the dry end. I then traced out the HV wire paths from
inverter through the chassis up to the backlights. We used flexible
silicone wire between the inverter and the lamps. As I ran my "finger"
across a tie wrap fastening HV wires to the chassis, the lights went
from dim to OFF. I shut off power and looked at the tie wrap. It was a
soft metal pair of ears embedded in a nylon tab backed with PSA. The
metal tabs were anchored in the nylon with a wide metal tab – think of
a stamp with two flexible arms.
These metal tabs wrapped around the HV wires created a capacitor to
ground and, since the blocking cap from the HV secondary of the
transformer was on the order of a few pF, the tabs created a voltage
dividing cap and my probe was shorting out this cap. The tie-down was a
good manufacturing part, easy to place anywhere, cheap and adjustable.
It just wasn’t good for tying down HV wires. Manufacturing changed to a
nonmetallic part and my problem was solved without another puncture
Management hailed this as another case of "smarter by the numbers."
I smiled and nodded and went back to the next design problem.
About author John Linstrom: "I'm a degreed EE with 30 years experience
in analog/low level/low power instrumentation in
medical/communications/industrial environments. I currently work for a
contract manufacturer in NW Montana and for myself at SUNSPOT."