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My rationale is this: neg rail is floating in respect to earth so that chip inputs are not ground referenced. Thus being in the E field of the TC does not impose a high voltage on the components when no shielding is used. As the voltage imposed on a point in the E field is a result of the radius between the origin of the field and the location of that point, two points only see the difference in radius. Given that the distances between sensitive nodes inside the driver are small, there will be negligible field gradient change and thus negligible voltages between all sensitive points. If you tie negative rail to earth you lose this.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Sigurthr wrote ...
FWIW I always leave my LV supply floating.
My rationale is this: neg rail is floating in respect to earth so that chip inputs are not ground referenced. Thus being in the E field of the TC does not impose a high voltage on the components when no shielding is used. As the voltage imposed on a point in the E field is a result of the radius between the origin of the field and the location of that point, two points only see the difference in radius. Given that the distances between sensitive nodes inside the driver are small, there will be negligible field gradient change and thus negligible voltages between all sensitive points. If you tie negative rail to earth you lose this.
Thanks, leaving it floating does make the most sense.
I think I'm going to simply install the interrupter into the main driver's grounded enclosure in the base of the coil, since I won't be dodging any huge sparks. I'd need a multi-core shielded cable, or two RCA AV cables (or other coax connectors), to connect the handheld interrupter to the coil: for 12V power, interrupter output, and protective earth for the metal case. Don't really feel confident about relying on RCA cables for protective earthing in a mains-voltage environment. It makes more sense to put the money and effort towards making the coil's enclosure look a bit nicer.
Quick question, has anyone seen affordable digital panel meters for DC voltage and current, something around 1000VDC and 50A absolute maximum. Power measurement would be nice, but it's not important. I found these for $20 USD, but having both voltage and current displayed at the same time would be perfect:
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
TwirlyWhirly555 wrote ...
Why not use fibreoptic between the interrupter and SSTC driver ? , Just power the interrupter with a battery .
I've been considering it. At first it seemed a bit extravagant for such a small coil, but it's likely I'll end up building something larger too. I've got a big spool of green enameled 23 AWG wire and another 150mm x 38mm aluminium toroid here already. Lengths of PVC up to 250mm in diameter aren't very expensive here, may as well start winding! Having a slightly fancier interrupter with battery power, more features and a fiber optic link is becoming more appealing now that I'm making plans for bigger coils in the future. More research is certainly needed, I've yet to use fiber optic gear in any projects, but after some quick googling it seems simple enough.
I've almost finished off the half-bridge and DC supply, and will now tackle the driver and feedback CT assemblies. Out of curiosity, does anyone have an estimate to how much power these FCA47N60F MOSFETs and their small heatsinks will handle at 420-460 kHz? They have rather long total switching delays, so I'm not expecting much.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
GrantX wrote ...
Quick question, has anyone seen affordable digital panel meters for DC voltage and current, something around 1000VDC and 50A absolute maximum. Power measurement would be nice, but it's not important. I found these for $20 USD, but having both voltage and current displayed at the same time would be perfect:
Is it normal to use digital meters in such a 'noisy' environment? Moving coil meters may be more reliable, and can be biased to read any range you want with the appropriate resistors.
Moving coil meters are fairly immune to noise, due to their inherent inductance.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Ash Small wrote ...
Is it normal to use digital meters in such a 'noisy' environment? Moving coil meters may be more reliable, and can be biased to read any range you want with the appropriate resistors.
Moving coil meters are fairly immune to noise, due to their inherent inductance.
I was a bit concerned about noise, but for DC voltage measurement only I think a digital meter should be work. It would be measuring directly across the smoothing capacitor, with MKP decoupling caps. The meter's supply would be isolated and decoupled directly at the input to the meter itself. But DC current and power measurements might be more susceptible to interference. Now it's starting to sound cheaper and less risky to go with reliable analog panel meters. If I could find some fancy illuminated meters that aren't crazily expensive I'd be very happy.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
GrantX wrote ...
Now it's starting to sound cheaper and less risky to go with reliable analog panel meters. If I could find some fancy illuminated meters that aren't crazily expensive I'd be very happy.
You'll probably have to make your own scales for them, so make them translucent and put some LED's behind, or wherever. Paint the needles with luminous paint, or whatever. Make them as fancy as you like. Shine UV on them if you want, and get the scale and needle fluoresce if you want.
It won't cost much and won't take much time. The bit that will take the time is the scales, but you can print those out on a printer, after using a CAD program, or something (or getting someone else to supply a DXF file, or whatever).
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