Alt-BEAM Archive

Message #07723



To: "'beam@sgiblab.sgi.com'" beam@sgiblab.sgi.com
From: Wilf Rigter Wilf.Rigter@powertech.bc.ca
Date: Mon, 15 Nov 1999 18:41:31 -0800
Subject: [alt-beam] 74xx240 problems and solutions




I discovered these problems while playing around with some new 74ACT240
chips that were not behaving the way they should. It is possible that
problems reported by others having difficulty in getting the PS head circuit
to work may be related. These problems are common to all beam circuits, so
the solutions can provide insight in how to deal with such problems.

Problem with high frequency oscillation in 74AC/ACT240 chips.

74AC/ACT logic has high output current characteristics and is recommended
for motor driver applications. But AC/ACT is also much faster logic and
there is a possibility that a circuit using AC and ACT240 (but not HC or
HCT) chips will oscillate uncontrollably at high frequency (HF) because of
the way that BEAM designs use these digital chips for quasi linear
applications. HF oscillation causes high dynamic current consumption. This
instability is a feedback problem caused by a combination of circuit layout,
stray capacitances and slow waveforms. It creates a feedback path for phase
shifted HF signals in the linear region. The HF problem has also been seen
odd number Nv cores like the tricore and in Nu circuits that do not use
Schmidt triggers such as the reversing circuits of HC240 walkers that use a
Nu connected directly to the tristate enable pins. For the PS head in
particular, the problem is more likely to occur when C1 and C2 are connected
series and when the Nv/Nu output oscillates the output to input stray
capacitance couples back through the series caps to the HLO input. Luckily,
for the Nv/Nu circuit the location of R2/C2 is interchangeable and swapping
positions of these two components breaks the feedback path. During my
investigation of HF oscillation I also discovered that a 74ACT240 chip made
by National Semiconductor sometimes had one output=input instead of
inverting it!!!! probably caused by an internal latchup initiated by HF
oscillation.

Solution

The solution to both problems was to swap R2/C2 and adding a 1K damping
resistor R3. So if you have a problems with 74xx240 high frequency
oscillation or "non-inverted" inverting logic try adding damping resistors
between stages to reduce or eliminate the problem.

Problem with input threshold of HCT/ACT chips.

When using HCT or ACT logic family, the assumption of an input switching
threshold value of nearly +V/2 does not hold. Instead this logic uses a
threshold measured at 1.2V with Vcc=5V. Since the symmetry of the HLO
outputs depends on the threshold being near 1/2V+, this can result in a head
that continues turn in one direction unless the bridge is greatly unbalance.


Solution

This problem can be largely compensated for as long as the photo bridge uses
a +V reference of 2 times the threshold. Adding R4 and the 2 LEDs connected
to the HLO outputs is a unique way to form a 2V shunt regulator which is
sufficiently close to the required photo bridge reference voltage to make
the HLO symmetrical and has a big bonus besides.

Bonus

Connecting the 2 leds to the HLO outputs cause them to blink back and forth
like the FLED to attract attention when the SE has triggered and to show the
direction of head rotation when hunting a light. The penalty is a slight
increase in standby current rising from 2ma to about 5ma.

A winky head for all seasons

This neat SE ready/HEAD direction LED indicator circuit can be adapted to
the AC/HC logic version without connecting LDRs to the LED common point as
shown but connect the LDR straight to the +V/0V lines as was shown in the
original circuit .

Attached is a schematic for the PowerSmart head V2.0 , designed specifically
to address problems that may occur associated with using chips other than
the 74HC240. It also shows an example of using more commonly available 1M
trim pots and larger caps with no bad effect on performance.

The bottom line is that there is not one PShead design but a family of
flexible designs providing similar results while utilizing various available
resources. I will add these new mutations of the PS head circuit to the
tutorial together with some ideas on Siamese heads, soon.

Wilf Rigter mailto:wilf.rigter@powertech.bc.ca


<>

> -----Original Message-----
> From: Wilf Rigter [SMTP:Wilf.Rigter@powertech.bc.ca]
> Sent: Saturday, November 13, 1999 6:17 PM
> To: 'beam@sgiblab.sgi.com'
> Subject: RE: SUCCESS (Sort-Of) Solar Head Project
>
> I'll look at this and reply tomorrow if no one else has answered by then.
>
> regards.
>
> Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
> tel: (604)590-7493
> fax: (604)590-3411
>
> > -----Original Message-----
> > From: SG [SMTP:sparkyg@seark.net]
> > Sent: Thursday, November 11, 1999 2:00 PM
> > To: beam@sgiblab.sgi.com
> > Subject: SUCCESS (Sort-Of) Solar Head Project
> >
> > well, i have foolishly NOT dismantled my breadboarded
> > SolarPShead circuit. I thought i might tinker with it some more before
> I
> > took the solar engine off and went to batteries, assebmbling it step by
> > step LIKE WE ALL SHOULD DO!
> > (forgive me wilf, i will prob do it after today)
> >
> > i did replace the .01 cap on pins 13 and 15 with a .022uf and ALSO
> > replaced
> > the 74HC with a 74AC.
> >
> > old results where: not enuff power from motor outputs
> > new results: pager motor spins for about 2 seconds (yup! jus two!).
> >
> > this IS progress! (heh heh) also this is with a .047F gold Cap
> > for power storage.
> >
> > Now the questions i gotta answer is -- i wunder what is eating all my
> > power? what is draining that cap so quickly?
> >
> > I think it has sumpthin to do with the resistor values and until the 5M
> > pots come in i can't do much.
> > so far its exactly like darrell's circuit, except for the .022uf cap and
> > the AC chip, except pins 6 & 8 are grnded.
> >
> > but, hey! progress!
> > -Sparky



Attachment: PSHEADV2.gif

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