Alt-BEAM Archive

Message #04428



To: beam@sgiblab.sgi.com, davidperry@mail.geocities.com
From: Bruce Robinson Bruce_Robinson@bc.sympatico.ca
Date: Sat, 12 Jun 1999 08:54:12 -0700
Subject: [alt-beam] Re: bypassing a nv


davidperry@mail.geocities.com wrote:
>
> I need to know how to bypass a nv gate using a switch.

OK, nice & simple. Assuming you have a typical Nv with a resistor going
to ground at the input, all you have to do is use your switch to short
the resistor.

The resistor is gradually draining the capacitor in the Nv, so when your
switch closes it will finish draining the cap instantly.

> u may remember my walker not working, so while i'm waiting for parts i
> decided to try and fix it. I think its a combination of the h-bridges
> the the motors, they require 1 amp but will run with 1 or 2 AA (about
> 600ma, 3volts). What are my options (im using rich's 4 transistor
> h-bridge), 74hc245 driver, 6 transistor h-bridge, better motors?

Better motors is always the preferred solution, for those can get them
and don't mind the price. I've been using a 6 transistor h-bridge with
good results. The one Ian shows at BEAM online

http://www.beam-online.com/Robots/Circuits/circuits.html

can handle 100 mA. I substituted the following transistors to handle up
to 500 mA:

Replace the TOP row of (PNP) 2N3906 transistors with PN2907
Replace the BOTTOM row of (NPN) 2N3904 transistors with PN2222
Middle row stays the same unless ... (see below)

The ones I have are exactly the same physical size as the ones they
replace. Pinout orientation is also the same. My Motorola datasheet
actually gives these a maximum rating of 800 mA, but recommends 500 or
less. You should be able to use even bigger transistors if necessary.

What configuration are you using for your Nv loop(s)? Master-slave
bicore? Microcore with a 4 Nv loop? Reason I ask is this may affect
which type of transistor you use in the middle row of Ian's diagram.
Also, if you're using a 4 Nv loop you may need some protection to
prevent both sides of an H-bridge from turning on at the same time.

Regards,
Bruce

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