Heath Bebout <dhb@engr.uark.edu> wrote:

>I am working on using a ultrasound transducer to measure a distance to a
>surgical pin in the human body.  I am going to use a microcontroller for
>the control and analysis of the signal.  The question I have is how do I
>genrate a 150 to 250 V spike to intiate a signal from the transducer.
>The spike must be of a very short duration, approximately 20 - 40 ns.
>Thanx for any help you can give.

Standard method for generating an impulse drive is charge a capacitor
to desired voltage thru a current limiting resistor (R1), discharge
capacitor (C1) with a SCR (SCR1) or other fast switching device.  This
is the standard method used in driving transducers for ultrasonic
non-destructive testing (these are 1 to 5 MHz units with really low

         R1       C1        D1       D2
B+ o---/\/\---+---||---+---|<|---+--|<|---+---o  output to xdcr
              |        |         |        |
              -        /         /        /
       SCR1   V        \ R2      \ R3     \ R4
              -        /         /        /
      C2     /|        |         |        |
in o--||--+-/ |        |         |        |
          |  gnd      gnd       gnd      gnd
          \ R5

R1: 220k, use 1 watt carbon composition, not film if possible
R2: 1.0k, 1/2 watt carbon composition, not film if possible
R3: 2.2k, 1/2 watt carbon composition, not film if possible
R4: 4.7k, 1/2 watt carbon composition, not film if possible
R5: 100 ohm, 1/4 watt
C1: 1.0 nF, many volts, polyester (mylar) or polycarbonate
C2: 10 nF
SCR1: Something that can handle 50 to 100 amps at voltage
      you require. Motorola MCR729 is a hefty part (expensive)
      which is used in these types of amplifiers.
      You might try a MCR265 and see if that works.
D1: 1N4608  (you can try a 1N4148)
D2: 1N4608  (you can try a 1N4148)

You want to use a big size capacitor for C1, preferably made for high
current pulse applications.  Wima has some very nice units.  You might
consider a 2000 volt unit so you can carry the current pulse.

Best to use carbon composition resistors for R1-R4.  This is a pulse
circuit which means that you will be applying very short, high stress
pulses to the parts.  Carbon film resistors don't handle high voltage
pulse well.  Carbon film resistors work fine for experimenting, but
don't use them for the final design.  You might have to adjust the
value of R1 to adjust the charge rate of C1.  Depends on your pulse
rep rate.

SCR1 must be able to handle high current.  You will be dumping close
to 100 amps thru the SCR for a few nanoseconds.  You might also try
using a power FET.  I have a funny feeling that a FET won't work as
good as a big SCR.

You may have to adjust the value of R4 to make sure that you do not
load your receive signal from the transducer too much.  Be sure that
your T/R switch can handle the high voltage pulse.  You will have to
build some sort of preamplifier which needs to be protected from the
high voltage pulse.

D1 and D2, I like using the 1N4608 (handles lots more current than the
1N4148), but it is hard to find sometimes.  1N4148 will do if you
can't find the 1N4608.

The input (marked "in") will require a positive going pulse of at
least 5 volts.  The circuit will fire on the positive edge of the
trigger pulse.  Be sure to use a somewhat healthy drive to the gate.

The "B+" input is connected to your high voltage power supply.  You
can use anything up to about 800 volts (make sure your SCR and C1 can
handle the voltage).  If your high voltage supply is noisey (you
typically will be using a small DC-DC converter from Endicott Research
Group), then you can put an RC filter at the output of the HV power
supply (10k and a 47nF as a starting point).

You want to keep all leads really short.  Be sure to use a copper
plane for ground.  You need to keep inductance to a minimum to keep
the rise time fast.  Remember that you are switching 100 amperes
(about 10 toasters), thus keep everything tight.

Above all, beware of HIGH VOLTAGE.  250 volts DC is enough to hurt you
really bad, if not from the direct shock, then from hitting yourself
against some sharp object as you fly across the room.  Trust me, I've
banged myself up using less voltage.

Mark Chun  |  mark@reson.com  |  Santa Barbara, CA

Date: Thu, 25 Jan 1996 07:07:03 GMT

Original Subject: Re: impulse generating for Ultrasound Transducer

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