zzz_julian_zzz wrote ...

Hi Everyone,

I'm convinced that in order to make a good GDT for coil, one of the criteria is to have a correct core, but what other than this??

In my test, wire being used has very huge effect on the leakage inductance / ringing on the GATE - EMMITER signal.
with UTP cable (awg# 24) leakage is still a bit high and causes high tops (38volts secondary on a 1:1 GDT with primary swing of 24v[N30 core]) .. Not all of this can be answered by just dampening the signal on a resistor on the gate. I tried using insulated magnet wire also awg 24 thick, and surprisingly the leakage L went down and the signal are much better (the pic with signal with low tops is the magnet wire)

I tried breaking the insulation of this MAGNET wire by supplying it with a fused 400vdc power. I twisted it like in GDT fashion and sourced it with 400vdc for about a minute. No shortage/flashover occur. My question is, is this 400vdc I'm planning to apply on a bridge will be the one i should be concerned about in terms of wire insulation? because the GDT might see more than 400vdc here since it is introduced in a AC inverter circuit that outputs +/-400vAC.

Can anyone enlighten me here? Thanks!

loneoceans wrote ...

fr LONEOCEANS,

That’s correct, it's clear that reducing the thickness of the insulation of the wires will reduce the leakage inductance, and you can reduce it even further by using shielded twisted pair. Most commercial pulse transformers are specifically designed with thin insulation to reduce leakage inductance. However I think in practice, the usual twisted pair wires using Cat5 cable works fine.

In fact your GDT output on the gates looks fine (even with your conventional UTP cable). I would recommend using 30V TVSes to avoid damaging your gate insulation.

As for magnet wire insulation, it depends a lot of the wire insulation material and thickness. See this page here for detailed information on calculating the breakdown voltage (Ds). If you’re running say a 600V bus, most good quality magnet wire should be ok.

Depending on what GDT core you use, some of them might have hard edges (e.g. rectangular cross section toroids) - you'll need to be careful not to scrape the insulation off the magnet wire in those cases. :)

Hydron wrote ...

Kizmo (and I'm assuming others too) has used co-ax to make super low leakage inductance GDTs - using the core as primary and shield as secondary or vice versa.
He's got some pictures floating around of the setup (possibly on the forum), worth looking/asking if you're interested.

Mads Barnkob wrote ...

Are your familiar with the tests thedatastream made long ago?

... wrote ...

I would shy away from magnet wire, the single GDT I wound with magnet wire was fine for many hours but eventually there was enough physical stress placed on the leads (due to moving the coil, etc) that it arced over and blew up a quite pricey transistor. Maybe if you pot the sucker in epoxy like a commercial transformer, otherwise I would recommend kynar insulated wire (ex, surplus wirewrap wire) if you really need the improved performance, or normal UTP for general work.

klugesmith wrote ...

Today in a coilgun thread, Der Albi pointed out a cringeworthy video of magnet wire physical abuse.

Back to GDT's, has anyone tried bonded bifilar wire? has a representative picture:


I have a spool of untwisted bifilar magnet wire somewhere, with two contrasting insulation colors and a 50 ohm characteristic impedance. Traditional impedance value for twisted pairs of "wire wrap wire" is more like 110 ohms. It matter in baluns; I don't know about GDT's.

nzoomed wrote ...

I believe that UTP cable works quite well.

Im in the middle of constructing mine that will be used for steve wards universal driver.

Im unsure if im assembling it correctly though.

do you basically pair up each colour of wire?
I believe you parallel all the white wires together for the primary, is this correct?