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Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Hello, i recently received a minibrute CT from Tribologist (thanks Ulf), and i connected it to my sstc. I put 2-3 turns around it, one side to the secondary base, and one to earth ground. I turned it on with the antenna still providing the feed back, but the CT not powering anything. Well, after a minute or two, the CT got very hot, with no load. Also, from the CT output terminals, i could draw tiny little arcs with an ungrounded screwdriver. Is the core not made for RF? Why does it heat so much? And the arcs that i drew from the CT output pins where the same size and thickness as the sparks drawn straight from the ground (I obviously have crappy ground). Is this capacitive coupling or something like that? I do not want to use it until i know whether or not it is suitable.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Arcstarter wrote ...
Hello, i recently received a minibrute CT from Tribologist (thanks Ulf), and i connected it to my sstc. I put 2-3 turns around it, one side to the secondary base, and one to earth ground. I turned it on with the antenna still providing the feed back, but the CT not powering anything. Well, after a minute or two, the CT got very hot, with no load. Also, from the CT output terminals, i could draw tiny little arcs with an ungrounded screwdriver. Is the core not made for RF? Why does it heat so much? And the arcs that i drew from the CT output pins where the same size and thickness as the sparks drawn straight from the ground (I obviously have crappy ground). Is this capacitive coupling or something like that? I do not want to use it until i know whether or not it is suitable.
Thanks.
A current transformer must be burdened, either by a low value resistance or adequate clamping diodes. It should never be operated open circuit.
you are NEVER supposed to use CT unloaded. When no load is pressent on the secondary side, the voltage on the winding will reach very high levels. It is very possible that the CT is arcing internaly and that is why it is getting hot. There is a big chance that you have damaged it.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Dr. H. wrote ...
Good day sir
you are NEVER supposed to use CT unloaded. When no load is pressent on the secondary side, the voltage on the winding will reach very high levels. It is very possible that the CT is arcing internaly and that is why it is getting hot. There is a big chance that you have damaged it.
Update - Beaten by Marko.
Cheers
The core is heating because of losses caused by very high flux density (limited only by saturation) produced by primary MMF, as current can't flow in the open secondary to oppose this flux. Hence the secondary voltage indeed tends to a very large value (again only limited by saturation of the core) which may as well cause breakdown of insulation.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Well, i took it apart and did it again while looking inside, and the was no arcing over, but it still heated badly. In a regular sstc with CT feedback, they just use a 10kohm resistor in series with the .1uf capacitor, then to the 74hc14 and clamped by the diodes. Would this be enough of a burden? Will the diodes have to clamp almost continuously?
I had a ferrite toroid with about 30 or more turns, so i but 2 turns and tried it on the sstc. It would not break out well, and when i drew arcs they where extremely hot, they melted a box cutter blade i was using as a breakout point. the sparks where only an inch or two long, and i was getting arcs to the primary which was only about 7 inches tall where it struck. The primary was on a 6 inch form, so there was quite a bit of space from the secondary and primary. What is going on here? I reversed phasing, which did not oscillate at all.
UPDATE: Well, i used the CT made for the minibrute, and it works great. The sparks are as long as before, and I can now draws arcs very small, and it will not stop oscillating. So, it is much better. No more corona on antenna's, and no more worrying about arcovers, and the best part is that the is no more antenna's that have to come out of the enclosure i will put it in. Also, when you got too close to the antenna, it would sometimes start losing a tiny bit of spark length.
So many thanks to Ulf (Tribologist) for the CT, and thanks to Avi for giving me the idea of using a CT for feedback.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Same rules apply for saturation and losses for current transformer as well as for voltage transformer. The main limiting factor will be volts per turn the core can support. I try to make it no make no more than 1-2V/turn for small material 77 cores and no more is really needed.
Hence depending on number of secondary turns, the burden resistor for current sensing should be no more than few tens of ohms.
For feedback, antiparallel pairs of zener and fast diodes should be used to clamp the output voltage to approximation of squarewave with amplitude of zener diode voltages. This insures symmetric burdening of the transformer. Then, a dropping resistor, DC blocking capacitor and another rail-to-rail diode clamp should be used to extract an +5-0V squarewave signal.
This is an example where such a configuration is used:
Registered Member #1879
Joined: Mon Dec 22 2008, 04:11AM
Location:
Posts: 40
Glad the CT is coming to use. Look at Wards or McCauleys driver designs, you have all the info there.
I gave you a pair of the 1N5818, not sure I gave you any 4.7 Volt Zeners (1N5337) I put a pair in a envelope today so you will have them early next week. Use a bunch of regular diodes in series in mean time, Aim for 5V.
Don't use it with no load, you will damage it (might not make it useless but it will not be useful for accurate measurements, Worst case you can probably open it and rewind it with magnet wire
Good learning if nothing else. I don't have any more of them but someone else might have a spare to share with you if it died.
We need to get you a scope. I have a HP1740A with only one working input, I'm trying to find a new cheap scope and If I do I will send you the old HP if you promise to be careful with the remaining channel. Maybe someone else have a junky scope that you could get for free. Otherwise your best bet would be to go to a hamfest, You can't imagine what people toss on the last day so they don't have to take it back home...
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Minibrute CT for sstc feedback.
