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500V DC Source and femtoamp resolution

Discussion in 'Electronic Design' started by David Monaghan, Jul 21, 2004.

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  1. Now that Keithley Instruments have announced their 500V DC Source with
    femtoamp power supply, I would like to announce my Power Supply Beta
    Version that was published on the WEB on January 2004, and in other forms
    prior to that date.

    The public are invited to download the acrobat circuit and all comments are
    welcome. The design works well. But I look upon the project as an open-
    design to be improved and to be available freely to all who have a use for
    this sort of instrument.

    David Monaghan
  2. Interesting.
    Any intentions to actually build a series ?

  3. David Monaghan wrote...
    A nice effort so far, but your site needs work: there are missing
    web-page drawings (empty boxes show), broken links, no description,
    e.g., question: no constant I mode?

    Your drawings need work: unlabeled connectors, unlabeled switches,
    missing dots (or too small), etc. 5V ref drawing missing entirely.

    And your design needs work (and/or the drawing is in error?),
    e.g., U1b and U2b fight each other during current limit events
    (LMC662 opamp's have identical source/sink specs)? U2a drives
    the FET's source, and U1b the gate (so the FET's gate-charge
    current goes through U1b and U2a)? 500V is obtained across 64k
    with 1mA? Only 1mV signal across critical spots at 1pA? What's
    the voltage across the 1k trim pots (the divider resistor values
    seem wrong)? Hmm, I don't like the look of the 0.1uF across the
    BUZ350 MOSFET, all set to force any HV supply noise onto fragile
    DUTs. And why is the 100-ohm damping resistor a 3W part? What
    happens to the floating 500V supply's AC main leakage currents,
    when you're making pA measurements? Ditto for the 12V supply's
    leakage currents? Floating I and V meters are awkward, hard to
    power, impractical to get any analog outputs, etc. Say what,
    500V max, and *no* safety interlock? Is there a separate safety
    voltage limit to protect parts from inadvertent knob adjustments?
    How about a soft start for the voltage source? What's the green
    LED for?

    That's OK, keep trying - I'll stick with my trusty Keithley 487
    for now. When they say 500V they mean 500V, and more impressive,
    when they say 10 femptoamps, they mean 10 femptoamps. :>)

    - Win

    (email: use hill_at_rowland-dot-org for now)
  4. I read in that Winfield Hill
    Probably not. I recommend extracting the p. (;-)
  5. John Larkin

    John Larkin Guest

    Looks like the output is opamps driving a fet source follower, which
    can't deliver more than maybe +9 volts. Am I missing something here?

  6. John Woodgate wrote...
    ROFLOL. We'll just say fA and be done with it.

    - Win

    (email: use hill_at_rowland-dot-org for now)

  7. There are 14 decades of constant I control, with the max current of each
    decade (FSD on I meter) set with VR3. (See Current Sense circuit PP50a)
    VR3 can be disabled by removing JP3, and constant current/current limit can
    controlled from zero to FSD of each decade with a 5k pot on the front

    Yes. You have to zoom it to see the connection dots. The 5V
    reference on my prototype ended up being the 1ppmDegC Maxim 6350

    The LMC662 sinks 21mA and sources 22mA. Say the load in dynamic, and
    goes lower resistance, and the consequent increase in load current
    exceeds the Constant Current/Limit point. U2b output goes sufficiently
    low to limit the DUT current (by dropping gate voltage), U1b output(gate
    drive) goes high(but
    now has no effect on the gate), and the Red LED (current control) goes on.
    It seems to work quite well.

    U2a drives
    Yes. Is there a problem with this? It may be easier to think of U2a
    functioning to position the 12v rails in relation to the FET source so that
    the fixed voltage (in relation to the 12V rails) at the output of U1a
    is precisely at the same level as V+out on the DUT.
    So, saying that again.... As V+out varies, then so do the 12V rails.
    The importance of this will be understood when the instrument is set
    to zero source voltage, and used as a femtoamp meter ...for DUTs that
    generate their own current. Note also that this is a zero impedence
    femtoamp meter when used this way. Have you seen the projects
    that this Power Supply facilitated?

    500V is obtained across 64k

    Yes some confusion there. :) This particular circuit is limited
    to 64 volts output. It is probably a more practical range. Another
    three front panel microswitches, and you get to 500volts.

    Only 1mV signal across critical spots at 1pA?



    The 1K trim pots are to assure that the voltage at +input of U1b (ie top
    of the resistor column) is the same as the same as the
    Guard Voltage on the output of U1a. What I will do on the site
    is to make a simplified circuit of the principle of operation of the
    PS. It seems so simple to start with...then you have to take into
    input offsets and all sorts of protection. I had to blow a few chips
    along the way to learn what precautions to take. The PS will now
    take inductive kickbacks from running and switching on and off
    DC motors.

    Hmm, I don't like the look of the 0.1uF across the

    Ah....I wondered that too. But does not the series RC serve to
    dampen any inherent tendency for the BUZ350 to oscillate?
    I thought it was common practice.

    And why is the 100-ohm damping resistor a 3W part?

    Because once while prototyping, I was amazed to have a 1/2 watt
    100 ohm resistor burn out. I understand your point. It is probably only
    necessary until one gets the circuit right.
    It has been my experience that the challenge of this type of circuit is to
    assure its stability against oscillation.


    I have only ever floated the DUT along with the 500V(to take that example)
    rails. Thus the DUT is tied to the Earthed negative rail and the same
    earth and neg rail is attached to a faraday cage around the DUT. For small
    current measurements,
    the result will be messy without a faraday cage enclosing the experiment.

    12V supply leakage does not matter, because the 12V rail is locked onto
    1mA pulling it negative, and U2a output to FET source pulling it positive.
    The 1mA constant current column is made up in part of any current leakage
    through the FET during small current measurements.

    Ditto for the 12V supply's

    They work well in practise here, although protection needs to be built
    the Current Meter which is actually calibrated as a 0-1 volt meter.

    Say what,

    A parallel protective device would be handy. I remember once, my boss
    imported an expensive X-Ray tube from Italy. He tested the filament out
    nonchalantly with
    one of my supplies. I was sweating. A badly soldered joint, or a
    intermittant pot, and the tube would be history.
    But for a supply like this, one could adapt it for a purpose, rather than
    build it to do everything.

    The green LED indicates VOLTAGE CONTROL. The interesting thing is that
    if both the Red and Green LEDS dont light, it indicates that their is
    insufficient power to regulate either.

    And when they say $3000 they mean $3000 :)

    Thanks Winfield for taking the trouble of checking the circuit out.
    I agree with you...there is a long way to go.

    David Monaghan
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