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RE: [beam] couple-o-questions

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  • Wilf Rigter
    The all important parameters for our applications are xx Rds(on) @ xx Vgs. The Rds on is the effective source to drain series resistance at a given source to
    Message 1 of 4 , May 21, 2001
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      The all important parameters for our applications are xx Rds(on) @ xx Vgs.
      The Rds on is the effective source to drain series resistance at a given
      source to gate voltage Vgs. Note that beam circuit output voltage that
      would be applied as Vgs are generally in the range of 3V-5V.

      According to the current Digikey catalog guide (p299-p300), for the Zetex
      TO92 packaged 2N7000P-ND Rds = 5 ohm @ Vgs=10V and for the ZVN2106A-ND Rds =
      2 ohm @ Vgs = 10V. Those are not particularly good specs for Beam circuits
      and a 74ACxxx (advanced complimentary mosfets) device can give better
      performance at low voltage.

      A better choice at the same price of $1.02 would be the Zetex ZVN4206A-ND
      Rds = 1.5 ohm @ Vgs = 5V or at $2.10, the ZVN4306A-ND looks good with Rds =
      0.45 ohm at 5V. There is a TO92 P-channel mosfet ZVP4105A-DS with Rds = 10
      ohm @ Vgs = 5V. I am not aware of any!! TO92 complimentary mosfets that have
      excelent low voltage like those readily available in tiny SMD packages For
      example, look at the two complementary devices in one 8-MSOP package
      ZXMD63C02XCT-ND with Rds=0.2ohm(N) and 0.4ohm(P) @ Vgs = 2.7V and it costs
      only $2.28! Now all we would need is two of these dual transistors and two
      resistors (shown per attached), mounted on a small carrier board with 0.1
      inch terminations and you are looking at a nearly ideal 4 transistor
      non-smoking h-bridge for our beam applications.

      Regarding the second question, power is an important consideration but
      threshold symmetry, frequency stability and spurious oscilations are other
      parameters. Some 74ACxxx devices have thresholds that are almost perfectly
      at Vcc/2 over a range of Vcc. That means that time constants will be less
      sesnitive to Vcc variations. 74ACT/HCTxxx has relatively fixed threholds
      offset from Vcc/2 at Vcc = 5V. Note however that the threshold will be
      symmetrical near between 2.5-3V. The upshot of this is that for 74HCTxxx
      bicores/ucores the time constant increases when the Vcc drops from 6V to 4V
      which can help compensate for the slower motor speed as the battery voltage
      drops, thereby maintaining a slower but equal length stride. Finally, the
      74ACxxx devices require fast input transistions, short supply leads and good
      Vcc filtering in order to avoid spurious output oscillations. I suspect that
      many beam designs based on 74ACxxx oscillators violate FCC rules by
      radiating RF in the FM band something that can be easily verified by
      scanning the band with a portable FM radio. In addition, these high
      frequencies are conducted via common supply conductors through out the Beam
      circuit and account for the tendency of multiple 74ACxxx bicores to
      synchronize which is only good if you need that.



      -----Original Message-----
      From: Jean auBois
      To: beam@yahoogroups.com
      Sent: 5/20/01 4:39 AM
      Subject: [beam] couple-o-questions

      First: I notice that Solarbotics sells the ZVN2106 FET. From the name,
      can infer that it is an n-channel device. Another device, the ZVP2106
      exists which is, naturally enough, a p-channel device. Not only does
      Solarbotics not carry it, but when I went looking elsewhere
      (www.future-active.com, to be precise) I found that they require that
      call them to get a quote on price/availability.

      So my question is: how can the ZVN2106 be used as such a
      device? For example, I haven't seen a bridge yet that doesn't require
      PNP and NPN transistors...

      Second question: so HCT parts use less power and can sink/source less
      current than AC parts, however, the latter are obviously preferred in
      Solarbotics catalog. Presume you've got an infinite power source so
      doesn't enter into your decision: when making Nv neurons and both kinds
      bicores and all the rest of this stuff, of -all- of the possible chip
      technologies, which has the best overall characteristics? Which
      characteristics make the difference?

      Thanks bunches,


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