What voltage to expect when probing CT?

[thedoc298]

I built my 1000 to 1 current and phase ct for the drsstc im working on.   I just got around to test them.

I have a small 2 inch spark gap coil for testing, I hooked up a nst 7500/30 and it works fine, so I ran the primary wire through the ct and connected the other end to my testing scope.

My question is what voltage should I be expecting on the scope. I watched several people do this but I missed that point. Thanks

edit: please make the title of the topic clear

[davekni]

My question is what voltage should I be expecting on the scope. I watched several people do this but I missed that point. Thanks

You need a burden resistor across the CT secondary for testing.  UD2.7 has a 51ohm load on the feedback CT.  Other values in the 10 to 100 ohms are fine for testing.
If you want to verify CT ratio, you will also need to know the spark gap firing voltage and primary impedance of your test coil, sqrt(L/C).  Current is spark gap voltage divided by primary impedance.  CT output should be that current divided by CT turns ratio.

[thedoc298]

My question is what voltage should I be expecting on the scope. I watched several people do this but I missed that point. Thanks

You need a burden resistor across the CT secondary for testing.  UD2.7 has a 51ohm load on the feedback CT.  Other values in the 10 to 100 ohms are fine for testing.
If you want to verify CT ratio, you will also need to know the spark gap firing voltage and primary impedance of your test coil, sqrt(L/C).  Current is spark gap voltage divided by primary impedance.  CT output should be that current divided by CT turns ratio.

Thanks again, I was getting high values, and it suddenly hit me, forgot the resistor and that makes a big difference over open end. Now can't wait till tomorrow, but I did see they worked good. Thanks for the extra info. Im getting close on my drsstc

[Hydron]

Safest to start with a low value - with 1 ohm a 1000:1 CT will give 1mV/1A, 10 ohms 10mV/1A etc. I have no idea what sort of peak current you'll get with a spark gap coil sorry!

[thedoc298]

Safest to start with a low value - with 1 ohm a 1000:1 CT will give 1mV/1A, 10 ohms 10mV/1A etc. I have no idea what sort of peak current you'll get with a spark gap coil sorry!

Thanks for the tips, going to play in a little bit. These are for for my drsstc, just wanted to see em work and watch on scope. Thanks again

[Hydron]

For measuring a normal-sized DRSSTC I'd go straight to 10mV/A (i.e. 10R with 1000:1 CT) - keeps the voltage output in a sane region, and you have a nice simple ratio to work with. I only mentioned starting with a smaller value resistor as it sounded like you were gonna test on a spark gap coil first.

BTW, make sure you put the CT on one of the bridge outputs, _NOT_ between the tank capacitor and the primary coil. This avoids needing to worry about the high voltage present on the latter node.

[thedoc298]

For measuring a normal-sized DRSSTC I'd go straight to 10mV/A (i.e. 10R with 1000:1 CT) - keeps the voltage output in a sane region, and you have a nice simple ratio to work with. I only mentioned starting with a smaller value resistor as it sounded like you were gonna test on a spark gap coil first.

BTW, make sure you put the CT on one of the bridge outputs, _NOT_ between the tank capacitor and the primary coil. This avoids needing to worry about the high voltage present on the latter node.

I don't understand, I have 3 1000:1, and a ud2.7c. One ct for ocd, one for phase and one for just a monitor. I thought that all three ct's would be between the tank and primary. Are saying put one in-between the tank cap and the primary and the other on one of the bridge out's. That is just where I read to put the ct, statements like, (maker sure to put ct's between tank cap and primary).

Anyway I have just started this area of the drsstc and don't know it all. Thanks for the help, it always make sense.   

If i put one ct of 1000:1 between cap and primary, I will measure it all, If I put one on one of the igbt's bricks that will be only half and won't that mess up the ratio of the ct.
Hope I did not confuse you too bad, thanks

[Hydron]

Refer to this diagram I found on wikipedia:
- Uin is a representation of the bridge (it can be approximated as a simple square wave voltage source)
- Cprim and Lprim are the tank cap and primary coil



The CTs (all 3) should go between Uin (the IGBT bridge outputs) and Cprim/Lprim. It does not matter which side of the bridge, so this can be between Uin and Cprim or Uin and Lprim. Everything is in series so the current will be equal wherever it is measured, but the voltage (compared to the grounded control electronics) will change, so for safety it is best to put the CTs at the lowest voltage point, which will be next to the bridge in the series circuit.

Hopefully this is clear enough - I didn't intend any confusion either!

[thedoc298]

Refer to this diagram I found on wikipedia:
- Uin is a representation of the bridge (it can be approximated as a simple square wave voltage source)
- Cprim and Lprim are the tank cap and primary coil



The CTs (all 3) should go between Uin (the IGBT bridge outputs) and Cprim/Lprim. It does not matter which side of the bridge, so this can be between Uin and Cprim or Uin and Lprim. Everything is in series so the current will be equal wherever it is measured, but the voltage (compared to the grounded control electronics) will change, so for safety it is best to put the CTs at the lowest voltage point, which will be next to the bridge in the series circuit.

Hopefully this is clear enough - I didn't intend any confusion either!

Thanks, that made perfect sense. I had in my mind something stupid as far as hook up and that was the problem, I needed the pix

[thedoc298]

Safest to start with a low value - with 1 ohm a 1000:1 CT will give 1mV/1A, 10 ohms 10mV/1A etc. I have no idea what sort of peak current you'll get with a spark gap coil sorry!

So are you saying to remove the 51 ohm burden on ud2.7c and replace with what work the best, 1,10,20....ohms, thanks

[Rafft]

For measuring a normal-sized DRSSTC I'd go straight to 10mV/A (i.e. 10R with 1000:1 CT) - keeps the voltage output in a sane region, and you have a nice simple ratio to work with. I only mentioned starting with a smaller value resistor as it sounded like you were gonna test on a spark gap coil first.

BTW, make sure you put the CT on one of the bridge outputs, _NOT_ between the tank capacitor and the primary coil. This avoids needing to worry about the high voltage present on the latter node.

Hi Hydron

Ive been meaning to ask these. how is this ratio calculation/math done with cascaded CT? would a 1:33Turn + 1:33Turn make  for a 1:1000?  this is usually what I see. how would it be if I needed something more sensitive like 0-300Amp only max?

with a 10R burden resistor for OCD. thank you.

[Hydron]

So are you saying to remove the 51 ohm burden on ud2.7c and replace with what work the best, 1,10,20....ohms, thanks

No, I was only referring to the measurement CT which you were proposing to test on a spark gap coil.

The UD2.7c burden resistors should be good for a normal sized DRSSTC without modification, and you're probably OK with 10 ohms on the measurement CT (for 10mV/A).

As for the CT ratio, you multiply both numbers together - personally I used 1:31 and 1:32 (equals 1:992) to get closer to 1:1000. To get the sensitivity in V/A you divide the burden resistor value by the ratio, i.e. with 10 ohms you get 10/992 = 0.01008V/A (close enough to 10mV/A).

If you find that the CT is not sensitive enough, you can either increase the burden resistor value or reduce the CT ratio. Nothing is particularly critical, but the default values should be pretty reasonable for a mid sized coil as mentioned and are unlikely to need changing.

[Rafft]

copy that. thanks

on my smallish DR, im using UD2.1b circuit with 1:33:33 for OCD(10R burden. set the trimmer to 1v = 100Amp) and 1:8:8 for the primary feedback. its happily DRing with 100Vdc bus (36vac input ->> doubler)

[thedoc298]

So are you saying to remove the 51 ohm burden on ud2.7c and replace with what work the best, 1,10,20....ohms, thanks

No, I was only referring to the measurement CT which you were proposing to test on a spark gap coil.

The UD2.7c burden resistors should be good for a normal sized DRSSTC without modification, and you're probably OK with 10 ohms on the measurement CT (for 10mV/A).

As for the CT ratio, you multiply both numbers together - personally I used 1:31 and 1:32 (equals 1:992) to get closer to 1:1000. To get the sensitivity in V/A you divide the burden resistor value by the ratio, i.e. with 10 ohms you get 10/992 = 0.01008V/A (close enough to 10mV/A).

If you find that the CT is not sensitive enough, you can either increase the burden resistor value or reduce the CT ratio. Nothing is particularly critical, but the default values should be pretty reasonable for a mid sized coil as mentioned and are unlikely to need changing.

Thanks for reply, I thought that was what you meant. This stuff is fascinating, too bad I don't have too many good years left, thanks for taking the time to answer.

Mod edit: moved reply outside of quote for clarity