Re: calcs: Parallel R to equiv series R affect on MMC perf
> Hi Nick,
> Thanks for the reply.
Thanks for continuing this thread
> I've been so busy with work and robotics afterward (FIRST national
Nice work if you can get it
> I agree with what you say for the higher freq AC case.
> The equalizing resistors do become effective as the charging source
> approaches DC. The finite Rvalue moderates any variance in the dielectric
> resistance (T ohms +- G ohm ?)
> It should be easy to calculate at what frequency reactance is no longer
> effective in equalizing individual Cap voltages for the case where
> is max'ing out and they are not due to discharge for a TBD time.
I can see your logic, but I think the zero voltage flux (note: not dc - zero
voltage flux - the differance is important) case will (or should) never
exist within a tc tank circuit, if you're using it efficiently.
> Some assumptions need to be made
> or measurements of the variance of the dielectric of a lot of Caps would
> I believe those using DC (especially at very low BPS)
> must be more concerned about equalizing series string Caps.
I don't see why they should be more concerned - theirs are the systems in
which the 'equalising' resistors actually do what they say they do.
> A quick charge rate will initially provide equal charge as a function
> of capacitance (reactance from ramp up time) but after charging the
It will provide charging as a function of capacitance - the whole idea of
'equalising' resistors was to stop the caps charging as a function of C, as
the caps are probably only accurate to + - 10% in their C rating, and
therefore charging as a function of C leads to voltage imbalance.
If you wanted eq resistors which actually do what they are supposed to you
have to measure the C of each individual cap and then put an R in parallel
with it sized so that RC for every cap-resistor pair is the same. So if
anyone's got a few millenia and a lot of high prescision Rs ..............
> DC relationship dominates and 10M becomes significant relative to
> the much higher dielectric resistance.
Only under zero voltage flux conditions.
> Regards, Dale
> -----Original Message-----
> From: Tesla List [mailto:tesla-at-pupman-dot-com]
> Sent: Saturday, January 29, 2000 7:08 AM
> To: tesla-at-pupman-dot-com
> Subject: Re: calcs: Parallel R to equiv series R affect on MMC perf
> Original Poster: "Megavolt Nick" <tesla-at-fieldfamily.prontoserve.co.uk>
> Hi All,
> for the 'equalising' resistors you want to analyze it as a
> capacitative and resistive divider parallel connected. My point was that
> enough charge cannot flow through the 10meg resisitors to substantially
> change the capacitor voltages, therefore the equalising effect is almost
> nil. The resistance is so large that it doesn't figure very much in the Q
> calcs - it will have a small inverse sine damping effect, but not much
> the total in a string being 100meg.
> Nick Field
> > Original Poster: Dale Hall <Dale.Hall-at-trw-dot-com>
> > Hi Nick & ALL,
> > The series equivalent R of resistance paralleled with a Cap (or L)
> > Rseq = X^2 / R
> > for .056uF (Xc=14.2ohms) paralled by 10 meg at 200kHz:
> > (1/(6.28*200e-3*.056e-6))^2 / 10e6 = .00002 (.02 milli) ohm each
> > 10 in series = .0002 (.2 milliohm) (assume 1 string)
> > increasing series equivalent resistance lowers Q
> > Q = X/R 142/.0002 = 710,000 (yeah right, ! so dielectric is likely
> > so influence of paralleled R is not significant for these values.
> > Rarc (and to lesser degree Rdc) of Lpri will be greater system factor:
> > assume Rarc ~= 2ohms
> > Resonant Q = (XL=Xc)/Rseries_eq = 142 / ~2ohms ~= 71
> > note: a single R across series Cap string will not discharge the caps.
> > (no DC path, just redistributes the charge a little)
> > Nicks I believe refers to one bleeder R for each stage (~10)
> > in multi parallel strings (~3).
> > Regards, Dale
> > -----Original Message-----
> > From: Tesla List [mailto:tesla-at-pupman-dot-com]
> > Sent: Thursday, January 27, 2000 6:51 AM
> > To: tesla-at-pupman-dot-com
> > Subject: Re: Yet Another MMC question
> > Original Poster: "Megavolt Nick" <tesla-at-fieldfamily.prontoserve.co.uk>
> > Hi Jim,
> > It depends.....
> > The mmc bleed resistors do not change the performance of the cap in tc
> > service, but they do help to prevent operator injury through a charge
> > remaining on the caps after the coil has been switched off. If you're
> > likely to touch the caps between runs, then don't bother. You could also
> > just put a single set of resisitors across the whole lot rather than
> > each cap.
> > Regards Nick Field
> > > I about to assemble a small MMC (10 x 2).
> > > I noticed that most builders put a bleeder resistor across each cap.
> > > << Jim