Ideas for inexpensive True-RMS Metering?
in [Design]
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From: whit3rd on 5 Jul 2010 18:29 On Jul 5, 2:22 pm, Grant <o...(a)grrr.id.au> wrote: > I have some small CTs to measure AC current up to 10 Amps, would > putting a bidirectional 16V TVS (1.5KE16CA) across the secondary > be enough to protect the CT from overload or disconnection? > > Would they affect normal measurements much? It's troublesome to put high fault currents, possibly with fast risetimes, through some fine wire in a secondary; you might want to put your bidirectional protector onto a purpose-built protective (tertiary?) winding. Maybe just a few windings of stiff wire into an antiparallel diode pair? Small wires and fast current risetimes is the 'exploding-wire' scenario. One can imagine a washer-like chunk of MOV semiconductor, with the core going through its midhole, that only conducts when a big EMF (beyond the calibration range of the circuit) comes along. That's be a very cheap and reliable kind of protection.
From: Tim Williams on 5 Jul 2010 18:53 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:5bg436lfrq2a05j86fdn3s3t0opi82fr3p(a)4ax.com... > A CT driving a shunt isn't bad. You can use series resistance after > the shunt to protect an opamp. At extreme currents, the CT will > saturate, limiting the power dumped into the shunt. No, limited flux, unlimited power/energy. The amps delivered during the pulse are proportional to the fault current. Only the voltseconds are limited. Blown fuse ~1kA? Maybe not a big deal. Short circuit ~10kA? A challenge. Lightning strike ~100kA+? Ball up into the fetal position.. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: whit3rd on 6 Jul 2010 15:29 On Jul 5, 3:53 pm, "Tim Williams" <tmoran...(a)charter.net> wrote: > "John Larkin" <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in messagenews:5bg436lfrq2a05j86fdn3s3t0opi82fr3p(a)4ax.com... > > A CT driving a shunt isn't bad. You can use series resistance after > > the shunt to protect an opamp. At extreme currents, the CT will > > saturate, limiting the power dumped into the shunt. > > No, limited flux, unlimited power/energy. The amps delivered during the pulse are proportional to the fault current. Only the voltseconds are limited. The saturation of the core makes the coupling of primary and secondary go away; only the primary side gets the fault current, the secondary current is no longer proportional (unless you choose a really odd winding scheme that stays highly flux-coupled when the core is removed).
From: Grant on 6 Jul 2010 18:04 On Mon, 5 Jul 2010 15:29:02 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> wrote: >On Jul 5, 2:22 pm, Grant <o...(a)grrr.id.au> wrote: > >> I have some small CTs to measure AC current up to 10 Amps, would >> putting a bidirectional 16V TVS (1.5KE16CA) across the secondary >> be enough to protect the CT from overload or disconnection? >> >> Would they affect normal measurements much? > >It's troublesome to put high fault currents, possibly with fast >risetimes, >through some fine wire in a secondary; you might want to put >your bidirectional protector onto a purpose-built protective >(tertiary?) >winding. Maybe just a few windings of stiff wire into an >antiparallel diode pair? Sounds better, thanks! Things I don't think of... > >Small wires and fast current risetimes is the 'exploding-wire' >scenario. I've seen too many exploded MOVs, exploding wiring, closest I've seen is three phase cables strapped together in a 1500A converter to stop them jumping about while waiting for the fault interrupters to open. > >One can imagine a washer-like chunk of MOV semiconductor, with >the core going through its midhole, that only conducts when a big >EMF (beyond the calibration range of the circuit) comes along. >That's be a very cheap and reliable kind of protection. Except it goes boom and nothing there for next time? Grant.
From: whit3rd on 6 Jul 2010 19:13
On Jul 6, 3:04 pm, Grant <o...(a)grrr.id.au> wrote: > On Mon, 5 Jul 2010 15:29:02 -0700 (PDT), whit3rd <whit...(a)gmail.com> wrote: > >One can imagine a washer-like chunk of MOV semiconductor, with > >the core going through its midhole, that only conducts when a big > >EMF (beyond the calibration range of the circuit) comes along. > Except it goes boom and nothing there for next time? The energy transfer is limited by the saturation of the core, still; taking 95% of the high-slew energy available before the core saturates is unlikely to cause a 'boom' event. MOVs have wire joints that seem to fail before the material does (don't have full data on dead MOV properties, though, so that could be wishful thinking). I'm wondering if some of the nonlinear interference-suppression ferrites have some current percolation internally... grains of conducting ceramics (B4C) and low-breakdown semiconductors (InSb) could be quite easy to add to a ferrite bead mix. |