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PICList Thread
'Optoisolators or MOV's'
1997\05\29@183743 by Wayne Bennett

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I'm monitoring five external lines for change in voltage level with the 74's
ADC but have to consider the effects of induced voltage from lightning
strikes.  I was thinking of using either MOV's or optoisolators but I'm not
very familar with either.

A couple of points I need to consider is that I need precise mesurements
from the ADC so devices that affect the reading are not desirable and the
amount of induced voltage and current from a nearby lightning strike (in the
magnitude of thousands of volts).


Any pointers to information or suggestions would be appreciated.

Thanks
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Wayne Bennett
Griffith University
Brisbane Australia
spam_OUTw.bennettTakeThisOuTspamsct.gu.edu.au

1997\05\30@054429 by STEENKAMP [M.ING E&E]

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> I'm monitoring five external lines for change in voltage level with the 74's
> ADC but have to consider the effects of induced voltage from lightning
> strikes.  I was thinking of using either MOV's or optoisolators but I'm not
> very familar with either.
I don't know what voltages are on your
lines, but MOV's are normally associated with rather high voltages.  I
don't think you get 5V MOV's.  You could try using a zener diode
instead.
Optoisolators can also work quite well as far as isolation is concerned,
but they will affect you voltage measurement.  You would have to bias the
transistor so that you get exactly a unity gain across the device - and
that would proably be temperature dependant, so it may drift.

Niki
.....steenkmpKILLspamspam@spam@firga.sun.ac.za


'Optoisolators or MOV's'
1997\06\02@025558 by Didier JEANJEAN
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----------
> From: Wayne Bennett <w.bennettspamKILLspamSCT.GU.EDU.AU>
> To: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
> Subject: Optoisolators or MOV's
> Date: jeudi 29 mai 1997 23:47
>
> I'm monitoring five external lines for change in voltage level with the
74's
> ADC but have to consider the effects of induced voltage from lightning
> strikes.  I was thinking of using either MOV's or optoisolators but I'm
not
> very familar with either.
>
> A couple of points I need to consider is that I need precise mesurements
> from the ADC so devices that affect the reading are not desirable and the
> amount of induced voltage and current from a nearby lightning strike (in
the
> magnitude of thousands of volts).

There is two point of view :
1) No destruction
2) Always precise mesurements

I think MOV (or tranzorb or zener) are really good solution for protection
but really poor for precise mesurement because they only limit the voltage.
The solution for precise mesurement are filter because they limit the
variation of the voltage in the time. So I place always a tranzorb to
protect and limit the voltage and then a filter to have a precise
mesurement under induced voltage.

Optoisolators are really poor for induced voltage because of the internal
capacitor between the input and the output. They are only good for
isolation of low and middle frequence or low voltage but induced voltage
are some times 4000V with a rise time of 50nS.

Didier

1997\06\02@034839 by Steve Hardy

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> From EraseMEowner-piclistspam_OUTspamTakeThisOuTMITVMA.MIT.EDU Mon Jun  2 16:58:10 1997
> X-Msmail-Priority: Normal
> X-Priority: 3
> From: Didier JEANJEAN <Didier.Jeanjeanspamspam_OUTONTONET.BE>
> [cut]
> Optoisolators are really poor for induced voltage because of the internal
> capacitor between the input and the output. They are only good for
> isolation of low and middle frequence or low voltage but induced voltage
> are some times 4000V with a rise time of 50nS.

How about Nuclear EMP: 10KV/1ns?  Lucky the cold war's over.

OI's can be made as safe as you like.  True, the standard type in a DIP
cannot withstand much common mode dv/dt.  The solution to this is to
increase the distance between the LED and the photodiode, or add a
grounded shield between the same.  Increasing the distance is taken to
extremes with optic fibres, but you can get good results with an
ordinary IR LED (narrow angle output) and a large area photodiode,
separated by a few cm.  For screening, use brass mesh since it doesn't
cut down on transmission too badly.  E.g., I have been using the
following design quite successfully:  Take one IR LED for remote
control usage.  Take one BPW50 photodiode and position it so it butts
up against the lens tip of the LED.  Reverse bias the photodiode with
>= 5V.  You will find that the current transfer ratio is about 2.5%.  I
have also used 5mW IR lasers (e.g. Sharp LT021).  In this case the
average CTR is only 1-2%, but the differential CTR (near the laser
threshold) is 4-5%.  One advantage of the laser is that you can
separate the circuits by many cm.  Another advantage is an incredible
speed increase.  An IR LED only has about a 1MHz bandwidth, and thus
limits the data transfer speed since a photodiode has a risetime
limited only by capacitance.  The laser, on the other hand, can be
modulated at >= 100MHz.  Contrary to popular belief, near IR SS lasers
are quite linear between their threshold and their maximum output --
unfortunately the range is rather small (2:1) and if you exceed the
maximum for only a ns... goodbye magic smoke!

Oh, and yet another plus for IR lasers: some models have an independent
photodiode at the 'non-business' end.  This can be used in a feedback
circuit to linearise the output.  I can't wait to try this out, but
unfortunately Sharp (in their questionable wisdom) connected the PD
in a completely awkward arrangement, with the WRONG PIN commoned on the
3-pin case.

But I have digressed.

Regards,
SJH
Canberra, Australia

1997\06\02@052155 by Keith Dowsett

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At 08:55 02/06/97 +0200, you wrote:
{Quote hidden}

The other problem with using opto-isolators without MOVs is flashover. When
E-fields in excess of 1kV/m can be generated along a 10m wire the eensie
weensie gap across the pins of an opto-isolator doesn't count for much. I
would go with MOVs to ground every time.

If your equipment is at risk from direct strikes (someone I met designed a
golf course watering system which got one every few years) you need to think
about grounding. If your network offers a low resistance to ground it might
take a significant fraction of the strike. Hence it is better to install a
local earth (metal plate and thick copper strap) to provide a lower
resistance path to ground than your network.

This won't protect the station which gets hit necessarily, but it should
minimise the damage to the surrouding stations.

Just my thoughts

Keith.
==========================================================
Keith Dowsett         "Variables won't; constants aren't."

E-mail: RemoveMEkdowsettTakeThisOuTspamrpms.ac.uk
  WWW: http://kd.rpms.ac.uk/index.htm

1997\06\02@182414 by Steve Smith

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Look at using Transorbs these devices are like zener diodes and impose no
load on the line until a excess of voltage is seen they need only a small
restance in the line to be supressed and are capable of killing lots of
joules of noise energy. SGS Thomson make some and I have used these devices
sucessfully to dispurse the high voltage transients induced into control and
data lines as defined by current EMC compatibility regulations for domestic
and commeceral equipment.

Steve.....

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