Jeff Liebermann wrote:
On Thu, 6 Jul 2017 07:15:39 -0000 (UTC), frank
wrote:

I'll try to make a test setup with constant current on a GDT and try
to measure the frequency of the oscillation.
Frank

Could I trouble you to disclose the maker and model of the gas
discharge transient suppressor that you're testing, and the model

I don't know, I've pulled them out of a scrap PCB.
They are marked as 201M TM4M (on two separate lines).
201 might be the discharge voltage which is in good agreement on what I see
on the curve tracer.

number of the Heathkit curve tracer? I don't have a curve tracer but
I do have a high voltage variable power supply that I can manually
sweep to see if there are any negative resistance regions.

it's IT-3121, connected to an HP-1801A/180AR as display.


The manufacturers of gas discharge tubes should have something on the
gas discharge tubes and negative resistance.
http://www.littelfuse.com/~/media/electronics/product_catalogs/littelfuse_gdt_catalog.pdf.pdf
Yep, the graph looks much like a neon lamp. The tube triggers at 75
to 600VDC, and rapidly drops to 15VDC.

I have a different guess(tm). When the gas discharge tube conducts,
it effectively shorts the terminals of the device. I don't think your
Heathkit curve tracer will not like a short circuit. It should have
some kind of short circuit or overcurrent protection circuit which I
would guess(tm) is doing the oscillating. Insert a large value
resistor in series with the gas discharge tube and try again.

series resistor was already selected to limit the current peak value, it's
a knob on the curve tracer.
I've tried going from 5K to 50K and oscillation remains, of course peak
current varies. Using 100K as series resistor doesn't make the tube ignite
at all. Max voltage value on this curve tracer is around 240V.
Do you mean I should add an external resistor? I believe it would just go
in series with the internal one.
Indeed these tubes are meant to adsorb transients, so once fired, their
impedance could become very low, that would explain what I'm seeing.
Frank