# IGBT + T8 Fluorescent + Simple Ballast = Headache

[SOLVED] IGBT + T8 Fluorescent + Simple Ballast = Headache

My first post! Hopefully first of many.

So my story is this, I started this job not too long ago fresh out of school and there are literally no EE's around to actually give me guidance. As a result I'm forced to make just such a post.

So I have a very simple ballast to give a short 1000VDC pulse to turn on a T8 bulb for a short period of time. I'm interested in the propagation of the plasma inside the lamp, so it starting up is pretty crucial. Anyway I devised an IGBT controller circuit and ballast from a variety of sources and all seems well, sort of...

The whole circuit seems to work just fine when nothing is connected to it but when I plug in voltage probes the bulb never turns on. When the bulb turns on and the plasma connects both the cathode and anode the impedance is very low and most all the current sinks through the bulb until the cap is discharged. When I hook up the probes anywhere on the circuit BESIDES across the IGBT the bulb no longer turns on and the cap discharges gradually through the 15k resistor (it's really odd, when the probe is across the IGBT everything works fine, but anywhere else and the T8 no longer turns on). When I put the IGBT between the ground and the rest of the circuit it works fine, however that leaves the cathode at a high potential all the time and makes my experiment useless. I need the IGBT between the cap and the rest of the circuit.

Can anyone please critique my circuit that drives the IGBT? I'm 80% sure my problems lie with the negative of my 12 volt float and how it interacts with the rest of the ballast. How could I properly couple it to ground without grounding out the 1000V from the rest of the circuit? Any help would be most appreciated!

Circuit:
b.imagehost.org/0795/circuit.png

Oh incase it matters I'm using three Tek TDS 2014B scopes and four Tek P5100 Probes.

berkeman
Mentor
Please send me the link via PM and I'll post it. You aren't able to post links until you have a few more posts (it's an anti-spam feature that we use here).

berkeman
Mentor
from Czar's PM:

CzarDestructo said:
http://b.imagehost.org/0795/circuit.png [Broken]

Last edited by a moderator:
berkeman
Mentor
Oh incase it matters I'm using three Tek TDS 2014B scopes and four Tek P5100 Probes.
Are these floating, battery powered oscilloscopes, or grounded 'scopes with 3-prong AC Mains power plugs?

They're three prong AC powered scopes. I would hope though that these $400 a pop probes are engineered well enough to isolate it from the ground of the scope. berkeman Mentor They're three prong AC powered scopes. I would hope though that these$400 a pop probes are engineered well enough to isolate it from the ground of the scope.
In general, no, the 'scope probe grounds will not be isolated from Earth ground. You can beep them with a DVM to check. If you want isolated probe grounds, you need to use a battery powered 'scope, and even then keep in mind that the multiple probe ground leads are all connected inside the 'scope.

Oye I just tested the ground shield of the BNC connectors on the face of the scope (which I know that the ground clamp of the probe is tied to) and tapped it to the backplane of the ballast and they're indeed connected. This is with nothing connected to the scope, just the power and the 1000V supply off. So what does this mean it terms of my ability to monitor this circuit? Really all I want is the voltage across the lamp x 3 (for scope triggering) and the voltage across the 900ohm resistor for current.

Thank you so friggin much for the insight. The coupling between the probes and the scopes were the issue all along. I took apart the probes and removed the clips and I'm just using them as a single point rather than a differential, works beautiful! :D :D :D

berkeman
Mentor
Thank you so friggin much for the insight. The coupling between the probes and the scopes were the issue all along. I took apart the probes and removed the clips and I'm just using them as a single point rather than a differential, works beautiful! :D :D :D
Glad it's working for you. Yeah, I've seen 'scopes blown up because folks didn't realize that the metal ground clips on the probes were hard connected to Earth ground throught the 'scope. Boom!

Be careful with those high voltages