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DRSSTC Current Transformers

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UbuntuNinja

Wed May 26 2010, 10:03PM

Registered Member #2677 Joined: Mon Feb 08 2010, 03:06AM
Location: Palo Alto, California, USA
Posts: 64

Okay I have two questions regarding feedback current transformers in a DRSSTC system. First of all can I use primary feedback with a driver designed for secondary feedback? The schematic of the driver I am using is http://www.extremeelectronics.co.uk/coils/sstc2/sstc2.pdf. My second question is what is the relationship between the burden resistance and turns on the secondary of the CT and things like secondary current or voltage?

Dosinski

Wed May 26 2010, 11:34PM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
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Posts: 45

You can use the same schematic, but u'll need more turns. I suggest 150 turns. I don't know the answer to ur 2nd question.

Dalus

Thu May 27 2010, 11:30AM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

It doesn't work as a CT. It's task is to take the current and convert it into a high voltage signal. This signal gets clipped to form a square wave that drives the rest of the electronics. 100 turns should suffice. The site where you got the schematic from has a calculator for current transformers. And a quick google search shows the math.

UbuntuNinja

Wed Jun 02 2010, 02:37AM

Registered Member #2677 Joined: Mon Feb 08 2010, 03:06AM
Location: Palo Alto, California, USA
Posts: 64

So 100 turns on the secondary do I need a burden resistor? If so do I put it in series or parallel with the secondary? I am aware that extreme electronics has a CT calculator but there is a bug in the Javascript or something because it does not calculate the voltage or the other figures when you enter a turns value and a resistance. By "It doesn't work as a CT" do you mean that I cannot use it with primary feedback so I would be forced to use secondary feedback? Thanks

Mrkva

Wed Jun 02 2010, 03:24AM

Registered Member #2655 Joined: Wed Jan 27 2010, 04:09PM
Location: Czech Republic
Posts: 18

Either me or you are missing something. If you are using current transformer as a source of gate drive signal, then you aren't using any burden resistor at all. It's almost same as the secondary current feedback. The only change is the number of turns on your CT and the fact that you're not connecting CT between GND and bottom of your secondary but between primary coil and a side of H-bridge.
CT with burden resistor is used when you need to measure current with the CT like for overcurrent protection.

Goodchild

Wed Jun 02 2010, 02:38PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Mrkva wrote ...

Either me or you are missing something. If you are using current transformer as a source of gate drive signal, then you aren't using any burden resistor at all. It's almost same as the secondary current feedback. The only change is the number of turns on your CT and the fact that you're not connecting CT between GND and bottom of your secondary but between primary coil and a side of H-bridge.
CT with burden resistor is used when you need to measure current with the CT like for overcurrent protection.

Umm no that's not completely right, you do need a burden but, it's to load down the CT as when you use it as feedback.
When you use a CT for feedback you are only looking at timing of the waveform. Then as an extra bonus is you want you
can wind a second CT and use that for an OCD circuit, with an OCD circuit you are looking at the amplitude of the wave
and not the timing and the burden become a bit more important.

UbuntuNinja

Thu Jun 03 2010, 12:38AM

Registered Member #2677 Joined: Mon Feb 08 2010, 03:06AM
Location: Palo Alto, California, USA
Posts: 64

Ok I am going to use 100 turns for the secondary but I am going to wind two secondaries at the same time on the core, one for feedback and the other to connect to an oscilloscope. Should I burden the scope winding? The maximum primary current is 34.1 amps and the max primary voltage is 326 volts. Will 100 turns provide a sufficient feedback signal?

HV Enthusiast

Thu Jun 03 2010, 12:46AM

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

Typically, current transformers (such as Pearsons, Ion Physitcs, are terminated with 50 ohms. Then you would hook up to your oscilloscope and set input to 50 ohms as well. Just note, that the amplitude in this configuration is actually twice what you would read on the scope.

Dr Hankenstein

Fri Jun 04 2010, 01:22AM

Registered Member #1642 Joined: Sun Aug 17 2008, 11:36PM
Location: Black Canyon City
Posts: 96

I must agree that the termination resistance on some intstrument transformers is 50 ohms; but, if one looks a little closer, the the actual "burden" is normally less than 3 ohms or so. Fact is, this low "burden resistance" is "impedance matched" to 50 ohms (which is typical of most inputs to test equipment). The actual question is: what should the burden resistance (not the termination resistance) be? Well, acurate reproduction of the primary current signal requires a very low burden resistance; usually les than two ohms. We are looking for currrent, not voltage. After about 5 ohms, or so, severe distortion of the waveform begins to occur. (try it for your self) If you are winding your own CT's, I would highly recommend a burden of less than 3 ohms. (Refer to Meterman's Handbook, Chapter 11, "Instrument Transformers") as there are too many other factors too list here to take into consideration. I have spent many years lab testing PT's and CT's for accuracy and verifying correction factors to less than 0.01% and phase angle errors of degrees, minutes and seconds. I know that at burdens of more than 2 ohms, just very a short length of wire will destroy the accuracy of a CT. Drop to less than 0.2 ohms, and the accuracy improves several hundred times!

"Keep it low; it's the way to go!"

Regards,
Hank

Steve Conner

Fri Jun 04 2010, 10:37AM

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

So, Hank, what do you think of Steve Ward's practice of not terminating the feedback CT at all?

I always thought he was crazy, but his coils worked, so I can't argue.

UbuntuNinja: If you wind two windings on a CT, you only have to burden one of them. They'll have the same voltage induced in them.

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