spring strip. Here, the high voltage is
due to the high dv/dt as the moving
spring strip suddenly opens the circuit.
Winding a secondary coil with many
more turns over the existing primary
coil would increase the output voltage
on the secondary by the turns ratio.
Placing the right value of capacitor
across the primary coil to tune it to
the self-resonant frequency of the
secondary would increase the voltage
/Tesla_coil and www.hvtesla.com/
tuning.html for more information.
#2 Go to www.teslacoildesign.com
by Kevin Wilson. It is very well done
and has a nice and brief explanation of
the theory of operation and how to
build it safely.
Mt Airy, MD
[#11133 - November 2013]
Filter Caps and Power Supplies
This has to do with electrolytic
aluminum filter caps for switching
No matter what type filter cap I
try, they blow out (become pregnant)
after months or a few years. I repaired
cable boxes for many years that had
the exact same problem.
This is only a three volt supply at
about two amps. Ten volt 1,000 mfd
caps are used in the stock supply. Also,
a three amp Schottky diode (burns up)
supplies the DC to a 15 amp logic
N-channel MOSFET with a heatsink. It
gets hot. Then, the output of it gets a
cap, a choke, and a cap. Nicely filtered
This is my final change-out and it is
lasting the longest. So far, no blow
outs, but it has only been seven
Now, the two five amp Schottky
diodes in parallel. Using only one still
gets super hot. Caps 25 volts at 1,000
mfd. I’m only using general type filter
caps at 20% 105° C. Why has this been
such a big problem?
The cap that usually blows is the
first one after the MOSFET. I see no
spikes on the output of the MOSFET
either. I could use a TO-220 pack with
dual diodes in it, but no room. The two
five amps in parallel work just great
and only get warm.
#1 The quick answer to your inquiry
is that you're using the wrong type of
From the description given of
the power suppy — three volts, two
amperes (six watts), and the heating
problems encountered — I surmise
that you have a small flyback switching power supply that is not operating
very efficiently. If the supply is rated
for a mains input range of 90-130 volts
AC and you're operating near the top
end of that range — 120 volts or so —
the ON time of the switch transistor
will be quite short relative to the OFF
time. All of the input power to the
flyback transformer must be delivered
during that short ON time, so the
switch current will be high. Similarly,
the flyback (secondary) current pulse
through the output diode will be high
because it must deliver all of its energy to the output capacitor in a short
time. Both of these conditions serve to
elevate the operating temperature of
the switch transistor and output diode.
Finally, that output current pulse is
dumped into the output capacitor. A
real capacitor can be visualized as an
ideal capacitor in series with a small
resistance; the latter is known as the
Equivalent Series Resistance (ESR).
You need to use capacitors having a
very low ESR value and a high ripple-current rating. I would expect that
your output capacitors are seeing very
high instantaneous ripple currents.
Capacitor heating is a function of the
ESR value and of the square of
the RMS value of the ripple current.
A suitable capacitor might be a
Panasonic EEU-FR1E102, available
from Digi-Key (part number P14424-
ND, $0.91 each). The ESR of this
device is 0.020 ohms and it will
tolerate over two amperes RMS.
As far as paralleling diodes goes,
I've had bad experience with that. You
cannot guarantee exactly when the
diode will switch from non-conduction
to conduction, so for a very short
instant one diode may be exposed to
the full current pulse. I can't visualize
why you have room to fit two five
ampere Schottky diodes but not room
for one 10 ampere TO-220 package.
The reason that the capacitor
nearest the MOSFET switch is the first
to be destroyed probably relates to the
board layout, and the fact that the
other capacitors have additional lead
inductance (including the etched
conductors) in series with them. If
possible, try to equalize distribution of
current from the switch to each of the
capacitors, and do the same for their
returns to the common bus.
I hope these suggestions help.
Peter A. Goodwin
#2 The likely cause is the high frequency ripple; aluminum electrolytic
capacitors don't deal well with high
frequencies (high loss factor) or a
large AC component of a waveform
(capacitor may be depolarized).
You could substitute a tantalum
capacitor, and/or parallel some ceramic
capacitors across the aluminum one,
e.g. 0.01 µF and 0.5 µF (or 10 nF and
500 nF, if you prefer), keeping the
leads of the ceramic capacitors short.
You could also use a small ferrite
choke in series with the capacitor to
decrease the AC component of the
A schematic of the power supply
would help to pinpoint the problem.
80 January 2014
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