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1:33:33 CT accuracy?

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LAN_403

Arcstarter

Sat Feb 06 2010, 05:13AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

If you really want to try the whole shunt thing, i can trade or sale ya mine. It is 500 amps with an output of 1mv per 10 amps (50mv at full current). It is in great shape, big brass bolts for connections, and i even have the associated analog meter, measuring up to 500 amps with 100v increments with notches every 10 amps.

It is a nice one, so i cannot let it go too cheap, but... The biggest problem is shipping, which maybe makes it pointless.

Steve Conner

Sat Feb 06 2010, 10:55AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I've never quite trusted Steve's 1:33:33 thing, it always seemed to me that it might suffer from self-resonance or the like.

On my DRSSTC I used a 1:33 CT with a really heavy burden resistor, 0.33 ohms non-inductive, to give 1 volt per 100 amps. But this doesn't work with the classic driver circuits that all require several volts of feedback signal from an almost-unburdened CT to guarantee a start. I see that as a flaw in the driver circuit, though.

An analog current meter will be completely useless for calibrating a CT that only works with high frequency AC. And even if you get a shunt and hook it to an oscilloscope, it won't be accurate at HF because of its inductance.

The Pearsons use a weird scheme called "Distributed termination", and you can't really say what the burden resistance is because it's distributed throughout the winding.

And finally, yes, your DRSSTC could be running 400 amps :O

brtaman

Sat Feb 06 2010, 01:08PM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247

Thanks for the replies everyone!

I am planning on purchasing the commercial CT's EVR pointed out. I think those will do the trick. I will be sure to post my findings on the subject when I finally get them. (Shipping to Europe is usually $ so I am going to make a stock list of parts I need and just put in a large order.

Mr. Conner: I think you will be most surprised, if I tell you that the driver I am using is just your standard SSTC driver, only interrupted and using secondary feedback. It was just a quick lash-up as I said, to test out my bridge and the CT's while waiting for my driver board parts, I was fully expecting to sacrifice my IGBT's in the name of progress.

Basically I am running a fancy VCO with no flip-flop to sync

, how the IGBT's are still alive and kicking is beyond me. One observation I have made is that as pulse width is increased, the phase-adjustment/VCO potentiometer, must be shifted towards what would be a lower frequency in VCO mode, this has the effect of adjusting where in the current pulse the "hump" is, so by increasing pulse width I have to also adjust the hump locating using the pot to achieve optimal performance. Any thoughts on the subject, I still have a lot of analysis and scoping to do, to find out what is actually going on, and how in the world it is working without even the basic flip-flop. Perhaps I am getting some sort of "lock" at higher pulse widths?

HVguy: I only wish I had friends/acquaintances with such equipment in the area, it would make life a lot easier.

Phillip Slawinski

Wed Feb 10 2010, 05:51PM

Registered Member #1732 Joined: Thu Oct 02 2008, 02:34PM
Location: Huntsville, AL
Posts: 112

Just thought I'd throw my two cents in here. I've only ever used the cascaded CTs 1:33:33 for my DRSSTCs and they've worked great. I even have a 1:33:33 CT that I have used for scoping primary current. Aside from a little more noise, it compares favorably with a pearson, and will absolutely be fine for feedback and OCD. What's more is that these CTs are cheaper than their commercial counterparts.

Dr. Drone

Wed Feb 10 2010, 08:41PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

HV Enthusiast

Thu Feb 11 2010, 01:55AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Phillip Slawinski wrote ...

Just thought I'd throw my two cents in here. I've only ever used the cascaded CTs 1:33:33 for my DRSSTCs and they've worked great. I even have a 1:33:33 CT that I have used for scoping primary current. Aside from a little more noise, it compares favorably with a pearson, and will absolutely be fine for feedback and OCD. What's more is that these CTs are cheaper than their commercial counterparts.

Well, first of all, you really can't compare these cheap cascaded CTs with a Pearson current monitor. Sure, a DRSSTC current waveform will look similar to the Pearson, but thats only a sinewave at a few hundred kHz maximum. Bandwidth of a Pearson can be well up to 35-50Mhz depending on model. Try getting that out of those cascaded CTs.

However, for the application regarding DRSSTCs, I agree that the cascaded CTs work great for that. Of course, I always opt for the $6.00 CST transformer, just cause i'm too lazy to wrap 66 turns.

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