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

[Ray von Postel <vonpostel@xxxxxxxxxxx>]

> 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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