I wont comment on isolation gaps, but I have a few other tips that may
be of use.
I'm assuming that you are creating a voltage divider (say 2.94M +
49.9K), and the 5V will go to the high impedance input of an op amp
configured as a buffer, so you get good accuracy under normal conditions
(when there are no HV spikes).
1. Many op amps and other IC's have diodes or other protection circuits
that shunt current to the power supply or ground if an input goes
outside the supply rails. These circuits typically can tolerate 5 to
50mA continuously without damage. However, correct circuit operation
under such conditions may not be guaranteed. Read the datasheet for the
device to find out.
With the above voltage divider, 15KV would flow 5mA into the op amp
input.
2. Be wary of using zener diodes, transorbs and the like. They leak a
substantial amount of current before they reach the breakdown voltage.
That throws off the accuracy of the voltage divider, if the divider is
high impedance. The current is temperature dependent, of course, so you
can't just "calibrate it out".
3. If you need to use a lower impedance voltage divider for some
reason, you can still protect an op amp used as a buffer with this
elegant trick: put diodes (parallel, opposite directions) between the
+ and - inputs of the op amp, then protect the - input with diodes to
the power supply rails. Since the + and - inputs are at the same
voltage (they differ by Vos, a few millivolts), the current flow through
the protection diodes is essentially zero. The - input is driven by the
op amp output, so its impedance is very low and is not bothered by some
leakage through the protection diodes to the power rails. Having
explained that, it should be apparent that in the case of an overload on
the input that exceeds the op amp's output current drive ability, the
input can go two diode drops above or below the supply rails, which may
be bad for the op amp. So, you really need to put a small resistor
between the + input and the protection diodes to limit the current into
the + input in that case.
The same idea can also be used in the case of an inverting amplifier,
where the + input is tied to GND. Put protection diodes between the -
input and GND.
4. Instead of cutting slots in a PCB to prevent current flow from high
voltage traces to sensitive, high impedance circuitry, consider
separating the HV and the sensitive circuits with a ground trace on the
top and bottom layers of the PCB. Any leakage current then flows into
the "guard" trace and doesn't affect the circuits behind the guard
trace.
Cheers,
- Jan
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-----Original Message-----
From: Dave Mucha [mailto:dave_mucha@...]
Sent: Wednesday, September 29, 2004 10:07 PM
To: Homebrew_PCBs@yahoogroups.com
Subject: [Homebrew_PCBs] board isolation cut-outs
Hi all,
I'm looking to measure a 300 volt line and drop it to 5 volts. One
of the many problems is that there is also the potential of a 15kV
spike on the line.
I would appreciate any help finding parts that can withstand the high
voltage. And any links to information on what to watch out for.
But, for this group. Does anyone have any ideas on how to determine
the slices in a PCB around chips ? I would like to isolate the 15kV
pulse from the rest of the board. A second board is a very good
possibility.
VERY open to suggestions.
Dave