Figure 1 shows the
details of measuring the breakdown voltage of a small AC plug style
transformer of the type often used with small electronic equipment. In
this case, a Fischer F-65
current probe was used to measure the waveshape of the breakdown
current, but this is not necessary to measure breakdown voltage.
Many ESD simulators work by charging up a storage capacitor, often
on the order of 150 pF, to the desired high voltage and then switching
the charged capacitor to the tip of the simulator. Unfortunately, ESD
simulators that work this way cannot be used to measure breakdown
voltage accurately and many of them have digital controls that also
complicate matters. What is needed is a simulator that keeps the
storage capacitor connected to the tip at all times and charged through
a low current, high voltage power supply.
The KeyTek MiniZap ESD simulator by Thermo Scientific is such a device.
The storage capacitor is connected to the tip at all times and is
charged by a low current, high voltage supply. The digital display is
actually a voltmeter reading the tip voltage in real time. The
MiniZap's analog controls (read that as "knobs") facilitate the
breakdown voltage measurement.
The method is as follows:
- Connect the two nodes for the breakdown measurement between the tip of the MiniZap and its ground cable.
- Using air discharge mode, slowly raise the voltage setting of the
MiniZap remembering that the display on the MiniZap is actually reading
the DC voltage stress being applied to the circuit or device under test.
- At some point, the MiniZap fires and turns off the high voltage supply, signaling that a breakdown has occurred.
- The last reading on the display just before the MiniZap fired is the breakdown voltage of the circuit or device under test.
Figure 2 shows another example of a breakdown test on another small AC
plug style transformer. It is probably best not to have your fingers on
the circuit during the actual test, lest you measure your
Figure 2. Test Setup for Measuring DC Voltage Breakdown of a Second Small AC Plug Style Transformer
Using an ESD simulator, like the KeyTek MiniZap, one can measure
breakdown voltage up to 15,000 Volts. If you don't have an outright
breakdown but just a leaky path, you will notice the device will load
down the reading on the MiniZap, possibly making it impossible to reach
the desired voltage.
Here is an example of how measuring breakdown voltage this way proved
useful and time saving. I was working on a small embedded controller
that used an electromechanical relay to operate a 240 VAC 60 Hz motor
that rotated a sizable drum. The problem was that when the equipment
was subjected to a 6 kV ringwave lightning surge test, the
processor IC was often destroyed (burnt to a crisp).
The processor IC controlled a discrete transistor that operated the
electromechanical relay which in turn applied the 240 VAC mains to the
motor, so I suspected breakdown of the relay. I connected a MiniZap, on
the test bench, from the contacts of a relay to its coil and slowly
raised the voltage. The relay was rated at 6 kV, but at 5200 to 5400
Volts breakdown occurred between the coil and contacts! So the relay
was not meeting its published specifications and was allowing the
lightning surge to be applied directly to the processor circuit with
Summary: Some, but
not most, ESD simulators can be used to measure high voltage breakdown
in circuits and devices. The KeyTek MiniZap is one such device. The MiniZap will measure breakdown voltages to 15,000 Volts, probably more than most uses require.
I am writing this from my new office in Boulder City, NV!!! I am doing morning 5 to 10 minute podcasts on
technical topics most mornings I am in the office. These podcasts will
appear on the home page of http://CircuitAdvisor.com
by late morning except for days when I am not in the office. The first
one has been posted under the headings of Free Audio, General Interest.
Equipment used in this Technical Tidbit:
- Thermo Scientific KeyTek MiniZap Electrostatic Discharge Simulator