March 2018 17
touching a grounded object has experienced
ESD. Figure 3A shows the IEC-61000- 4-2
ESD waveform used for testing devices that
will be subjected to static electricity. Pulse
voltages are typically around 2 kV but can
be as high as 15 kV! The various models and
tests are discussed in the STMicroelectronics
document DM00023467.pdf which is
Figure 3B shows a typical ESD test
pulse generator based on the Human Body
Model or HBM. The HV source is set to the
desired voltage and the 100 pF capacitor
representing the body is charged. A test
probe is connected to the Device Under Test
(DUT) and the switch is thrown, discharging
the capacitor through it. (This is the contact
discharge test. If the capacitor is discharged
through an air gap, that is the air discharge
Static electricity can certainly pack a punch — ask
any cat in the winter! The pulse can destroy sensitive
electronics and also upset their operation. Use of static-discharge equipment in your workshop and when working
on equipment is a great idea and not expensive.
Nevertheless, ESD doesn’t present a direct hazard to
humans. There simply isn’t enough energy to cause burns
or significant shocks.
High Voltage Components
When building high voltage circuits, be sure to select
components intended for that use. High voltage doesn’t
just result in higher power dissipation; there are special
stresses to account for.
Resistors have maximum voltage ratings aside from
their power dissipation rating. For example, a 1/4 watt
carbon-film resistor with axial (wire) leads has two primary
voltage ratings: maximum working voltage (MWV); and
maximum overload voltage (MOV). (See the datasheet for
Yageo CFR resistors at www.mouser.com.)
The first represents the “normal” maximum voltage the
resistor should experience on a regular or continuous basis.
The second is a temporary overload voltage for pulses
or transients. For these voltages, the resistor won’t arc
over or break down just from voltage effects. (It might be
dissipating too much power, but that’s a different issue.)
For the 1/4 watt Yageo CFR series resistors, MWV
is 250V and both MOV and VP are 500V. If your circuit
will have higher voltages — even if the resistor’s power
dissipation is not exceeded — an arc across the body may
Surface-mount resistors have very low voltage ratings
because they are so small. The typical voltage rating for
body size 0603 components is 75V, an 0805 can handle
150V, and 1206 components up to 200V. This applies to all
the components in SMT packages.
For voltages above a few hundred volts, you’ll need to
use resistors that are physically larger with a longer body
and more insulation from lead to lead. Compare the Yageo
HHV series of high voltage resistors to their lower voltage
cousins. They are longer and have broad end caps that act
similarly to the insulating discs on power-line insulators.
These resistors have MWV ratings starting at 1,600V.
You’re probably more familiar with capacitor voltage
ratings. These are generally working voltage ratings for
continuous use. Depending on the type, the capacitor may
also have pulse overload ratings or peak voltage ratings
that represent the absolute maximum voltage the dielectric
can withstand. Once the dielectric breaks down, the
capacitor is generally destroyed. In a high voltage power
supply, this type of failure is not subtle!
For typical tube circuits, ceramic and mica, plastic
film, and oil-filled capacitors are normally used, in order of
increasing capacitance. Disc ceramic and mica capacitors
are very common with values of 0.1 μF or less and ratings
up to 1 kV. Film capacitors are generally available with
ratings up to 600V and values up to 1 μF or so.
Oil-filled capacitors are commonly available with
capacitances in the 1 to 50 μF range and voltage ratings
up to several kV. Old units are available as surplus and are
often removed from old equipment. Be wary of old oil-filled
capacitors! Really old ones made in the 1960s and 1970s
often have PCBs (a carcinogen) in the insulating oil.
Energy-Storage or Filter Capacitor?
Browsing through websites or rooting through boxes of capacitors at a
flea market, you will encounter capacitors that have pretty high values and
voltage ratings — but they’re so small! What’s the deal?
These are probably energy-storage or photoflash capacitors for pulse
discharge use, like in a camera flash unit. They are intended to be charged
up, hold that energy, and then discharge it all at once.
Filter capacitors, on the other hand, are intended to have a continuous
AC ripple current through them as they store and release energy in a power
supply or other filter circuit. This continuous current creates heat in the
capacitor which must be dissipated.
An energy-storage capacitor is not manufactured to withstand that
stress and while it may work “for a while,” it’s best to use the physically
larger filter capacitor. Similarly, the filter capacitors may have too much
inductance to discharge as quickly as an energy-storage capacitor.
Even though the two types may have the same value and voltage
rating, they behave very differently. Know that difference!