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Registered Member #1571
Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44
Hi All,
I am in the middle of making a resonant CW multiplier. Its quite large and I have an air core transformer that will produce 50kV to be fed into the Multiplier. I am using a PLL controller (thankyou Steve Ward) and have the controller working on the test bench. It requires feed back for the PLL to lock and track. I was considering running a tertiary winding around the transformer however I have been looking at this formum and other very useful sites and for feed back they use a current transformer. Given the freq will be ~20kHz and the voltage output is around 50kVolt (transformer to be housed in oil) how would I approach the design of CT?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
si2030 wrote ... Given the freq will be ~20kHz and the voltage output is around 50kVolt (transformer to be housed in oil) how would I approach the design of CT?
You could use an off-the-shelf part (sometimes called a "current sense inductor") such as , on the grounded end of a transformer winding. Can often scavenge them from switchmode power supplies. Or homebrew the same thing, with any ferrite core big enough to fit around the winding. Number of turns depends on your expected primary & secondary currents. Excessive secondary voltage would allow core saturation, so you need a low enough load resistance. For example, the 100:1 CT in link should not exceed +-14V at 20 kHz.
Why would you need to put a current transformer around a wire with 50 kV at 20 kHz? Then you would have to deal with capacitive coupling as well as HV insulation.
Registered Member #1571
Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44
Hi Klugesmith,
Thanks for the reply.
Well to be frank I have a PLL that needs a signal from the secondary. I was going to wrap a tertiary winding around the coil but it seems I can also use a CT.
Incidentally, the current is ~17mA at 50kVolts and 20kHz would probably be the low end of its spectrum. How do you calculate the correct windings to achieve a desired voltage swing?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
si2030 wrote ... Incidentally, the current is ~17mA at 50kVolts and 20kHz would probably be the low end of its spectrum. How do you calculate the correct windings to achieve a desired voltage swing?
Sorry, Simon, didn't want to leave you hanging there. But I'm not familiar with your resonant transformer and PLL type driver circuit, and why you would want a CT on the secondary winding of main transformer.
Generally, you would start with the required voltage swing at the output of your CT. Choose a CT secondary current and a burden resistor value to deliver that voltage. Then set CT turns ratio equal to its current ratio.
Most CT's have primary "windings" with one turn and many amperes. 17 mA in a single turn won't even overcome the hysteresis unless the core is very small and/or made of exceptionally "soft" material. So one more time: are you sure you want a CT on the secondary?
And do you understand why the 50 kV number is not a factor in CT design, except for insulation thickness, if for some unknown reason you put the CT at the "hot" end of main transformer winding?
Rich [edit]On closer inspection, you didn't specify where the CT would be going. I think it would make sense (and conform to usual TC practice) to get feedback from a CT on the primary winding of your air-core transformer. Then the 17 mA and the 50 kV would both be irrelevant, so you can see why I was confused. [/edit]
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