RE: [TCML] mmc cap protection by spark-gaps-any ideas?

["Lau, Gary" <Gary.Lau@xxxxxx>]

The resistors you discuss in DC power supplies are used for an entirely different reason than why they're used in MMC's.  In MMC's, the power is AC, and the voltage division is determined solely by the ratio if the capacitances.  The bleeder resistor values are far higher than the AC capacitive reactance, so the resistors have no bearing on voltage division ratios.

The wire wound resistors used in the radio PS need to be high power (which makes them more failure-prone) because the leakage current of electrolytics is extremely variable over time and temperature, so the resistor must be chosen to be significantly lower in resistance to guarantee that IT determines the division ratio and not the variable leakage resistance.  That's just not the case in MMC bleeder resistors.

If it might diminish your horror, consider that the likelihood of _ALL_ of the many bleeder resistors in an MMC failing open is very remote.  The consequence of just one failing isn't that great.  And even if they did, the discharge path formed by the NST or other transformer would ALSO have to fail for there to be a genuine hazard.

It's not clear what you are proposing as an alternative.  If you are suggesting that folks not use bleeder resistors and instead use a manual shorting stick every time they turn off their coil, I would counter that:
1) Shorting just the ends of the MMC won't guarantee that the individual caps are all at zero charge.
2) I would bet any money that users are more likely to neglect to use the shorting stick, than there would be massive failures of all bleeder resistors.

I believe that it's far safer to have a 2nd tier safety mechanism (bleeders) that is always present (the 1st tier is the presence of the transformer winding), than to have a 2nd tier mechanism that requires considerable diligence and is likely to be ignored.

Regards, Gary Lau
MA, USA

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Ray von Postel
> Sent: Tuesday, August 19, 2008 4:41 AM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] mmc cap protection by spark-gaps-any ideas?
>
>
> > Hi!
> I have been following this post with horror expecting to read that
> some one has relied on the resistors used in an MMC constitute a
> SAFETY precaution.
>
> THEY ARE THERE TO PROTECT THE CAPACITORS AND NOT THE
> PERSONNEL WHO
> WORK ON THE EQUIPMENT!!!!!!
>
> The reason for the series parallel connection of capacitors as is done
> in an MMC is
> 1.  A cheap way of getting a capacitor with a high enough working
> voltage.
> 2.  To achieve a capacitor with the correct capacitance.
>
> If you have the money you buy one that meets your needs. You don't use
> an MMC.
>
> The practice of connecting capacitors in series is an old one stemming
> from vacuum tube radio days.  Tubes run on d.c. That requires a power
> supply unless you run on batteries. The power supply consists of an
> a.c. voltage source, a rectifier, and a filter.  The  filter consists
> of choke coils, resistors, and capacitors (condensers).  The filter
> capacitors must have a voltage rating commensurate with the voltage.
> If you need a capacitor with a working voltage of 450 volts and all
> you have are some rated at 200 volts, it is the dead of night in the
> midst of a hurricane, you resort to series connection of three 200
> volt condensers and if that doesn't give you enough capacitance you
> parallel additional strings of three until you do.  Despite anything
> you can do, no two condensers are exactly the same. They each have a
> different internal resistance which means that when you connect a
> string of them across a voltage source, they will not have the same
> voltage across each condenser in the series string.  It was found in
> practice, especially with electrolytic capacitors that the voltage
> across a capacitor in a string could exceed the working voltage of the
> individual capacitor.  The solution is to put a nominally high
> resistance across each capacitor in the string to equalize the voltage
> across each capacitor in the string.  That is the reason each resistor
> in the series string of capacitors in an MMC has the same resistance
> value.  IT IS THERE TO PROTECT THE CAPACITOR AND NOT THE
> TECHNICIAN.
> The fact that resistor across the capacitor will discharge it is just
> a bonus but the fact that resistors fail is a fact of life.  This is
> particularly true in high voltage circuits.  Should a resistor open up
> you have lost any so called protection you thought you could count on.
> Don't!!.  The resistors used in an MMC are a two bit items seldom
> rated for the power supply voltage.  They do what they are designed to
> do which is to help protect the series capacitors from over voltage of
> either a steady state or pulse form.  In a perfect world they are
> superfluous.  This is not a perfect world or haven't you noticed?
>
> I have previously made postings along the foregoing lines.  If you
> have ever been present when a friend reached into a turned off radio
> transmitter to change coils and got fried you wouldn't wonder why.
> Burning human flesh has a distinctive odor.
> The high voltage was only 6000 volts d.c.  The bleeder resistor  that
> had opened up was one of a series string of  200 watt wire wound 5000
> ohms made with nichrome wire on a ceramic form and covered with
> vitreous enamel.  The capacitance was 10 mfd. rated at 10 kv. There
> was a permanently installed grounding hook which was not used. The
> court ruled that my friend met his death by accident due to his own
> carelessness. The funeral was on a Friday over 60 years ago.  I did
> not enjoy accompanying the widow and his small children. Do I need to
> get more graphic before you will stop talking about bleeder resistors
> protecting you?  Such statements only indicate a lack of knowledge and
> experience.  You now have the knowledge.
>
> Ray

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