Transformer configuration possible?

[waveformer]

I'm currently failing on my transformer making experiments and I want to check that what I'm trying to accomplish is even reasonable.
I have a 4kV pulser that I'm trying to step up to 40kV using a DIY 1:10 transformer. So far it seems to always be shorting on the secondary and I can only get around 20kV out. Is this unreasonable and would potting the transformer help?
FYI I'm using this series of cores/bobbin/yokes, suggestions on better models welcome (torroidal?).
Thanks for any help!

 

[Reloadron]

What are you using for wire and at what frequency are you working with? How much current are you looking to get into what load? Normally something along the lines of a high voltage multiplier would be used, as an example the older CRT television sets. Winding a 40 KV secondary leaves plenty of room for breakdown of the wire used. There are other issues also.

Ron

 

[waveformer]

24 AWG Remington wire, it's a fairly quick pulse (around 100 ns), so i guess 10MHz? Not very high current, basically charging caps to 4kV, then dumping the voltage with a MOSFET. Would a high voltage multiplier be fast enough?

 

[Reloadron]

What I was getting at is Remington 24 AWG Magnet Wire uses an enamel insulation coating. The enamel should have a defined breakdown voltage in a data sheet You need to know what the breakdown voltage is. If your secondary is shorting it could be as a result of the wire enamel insulation breaking down. I can't find a good data sheet for Remington wire. Quality wire will spec a DiElectric Breakdown Voltage and looking for 40 KV is way up there. Another thing is often cheap wire sold on Ebay or Amazon is frequently bulk respooled wire which can have nicks in the enamel insulation coating.

I am not at all a transformer type guru but with a PRT of 100 nS that would be a pretty high frequency transformer where saturation of the core comes into play. Another member much more versed than I may have something to offer on that note.

Ron

 

[ronsimpson]

That wire will not hold 40kv. It might hold 1kv for a while. Not long term.
My transformers are wound so the start wire and end wire are not close.

I wind one layer of wire, then a layer of transformer tape, then a layer of wire.
-----------------
oooooooooooo-----------wire
-----------------
oooooooooooo
-----------------
oooooooooooo
|
|
wire
This way the wire insulation only has to hold off 10 volts. The tape holds off 1000s of volts.
Watch out that the wire from one layer does not fall off the edge of the tape and drop down to a lower down layer.

Adding the tape will lower the capacitor in the transformer and allow it to work faster.

 

[waveformer]

Thanks all, ronsimpson that is how I have wound it, 3 layers of 10 turns with kapton tape in between.
I've managed to clear all visible breaks and am getting around 32kV out, but its still shorting somewhere internally.
The data sheet for the wire I'm using states a breakdown voltage of 8700 V/mil for 44AWG and 5800 V/mil for 30 AWG (i'm using 24 AWG, so its presumably lower). Why is the breakdown voltage higher for thinner gauge wire?
Reloadron I'll have to look into high frequency core saturation. My scope measures the frequency as just under 3MHz.
Anyone have any leads on high voltage magnet wire?

 

[ronsimpson]

3 layers of 10 turns

30 turns for 40kv? 1000 volts per turn?

Don't let the wire touch the core.

 

[waveformer]

yes, that's correct. I have found a company (MSS) that does quadruple insulation magnet wire, but I seem to have gotten this txr to function properly as it is. My trigger circuit is outputting around 3kV and the txr outputs 30-32kV.
Now to figure out why the trigger isn't putting out 4kV...

 

[gary350]

If your primary winding is 200 turns at 120 volts transformer has to step up 120 to 40,000 ratio = 334 to 1. Secondary has to be 200 x 334 = 66800 turns. That = 1.67 volts per turn on the secondary. 40Kv will ARC about 1/4" to 3/8" depending on air temperature and humidity. You need to space your secondary winding layers with good thick insulation other wise the winding will ARC around the sides not through the center. You need a safety factory about 1/2" minimum from the first turn of secondary coil to the last turn on the secondary coil. If you put the transformer in oil that will stop ARCs around each end of the secondary but will not stop arcs through the center of the coil. Give yourself a good safety factor your go to a lot of trouble building this your don't want it to self district easy. You also need to consider ARCs from the secondary to the EI core keep all wire away from metal core that will conduct electricity 1/4" or more. If you wind the secondary over the primary then you need an insulation gap between the primary and secondary but if your have side by side windings the gap between primary and secondary needs to be half your safety factory or half of 1/2" if you use 1/2" as your safety factor = 1/4".

 

[JimB]

40Kv will ARC about 1/4" to 3/8" depending on air temperature and humidity.

I think that you are being a bit optimistic there.

It is generally accepted that dry air at sea level pressure has a dielectric strength of about 1000v per millimetre, or 25kV per inch.

JimB