[TCML] mmc cap protection by spark-gaps-any ideas?
youngs at relia.net
Tue Aug 19 11:08:52 MDT 2008
Yes, for DC applications, like power supply filter caps in series, the
resistors certainly do help equalize the voltages across each cap. Thus
they do add some protection to the caps.
But in TC usage, the voltage across the series strings of caps is RF,
changing polarity on the order of 100,000 times per second. In this
situation, the RC time constant for a bleeder resistor across an MMC cap is
so large (seconds)in comparison to the roughly ten microsecond cycle time
that the resistors will do practically nothing to equalize the voltage
across the caps.
So in Tesla Coil usage, the bleeder resistors are there as just ONE of the
protections against dangerous shocks, NOT to protect the caps. But as you
say, that should not be relied as the ONLY means of preventing shocks and
electrocution. The cap bank should ALWAYS be discharged manually with a
shorting wand or equivalent before handling the caps. If there are no
bleeder resistors on the MMC array, then to be absolutely safe, each
individual cap should be discharged manually as well.
I highly recommend adding resistors across caps in an MMC as one of several
human safety precautions. Cap protection is achieved by using caps of all
the same type and having an ample voltage margin, e.g. run the caps at 1/2
or 1/3 of their DC ratings.
From: tesla-bounces at pupman.com [mailto:tesla-bounces at pupman.com] On Behalf
Of Ray von Postel
Sent: Tuesday, August 19, 2008 2:41 AM
To: Tesla Coil Mailing List
Subject: Re: [TCML] mmc cap protection by spark-gaps-any ideas?
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
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
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
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.
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