Yes, I used to use the 1N34a, then Mouser discontinued in and replaced it
with the other number.
John Hermanson
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----- Original Message -----
From: <HI100@xxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Friday, January 19, 2001 3:23 PM
Subject: Re: [OM] OM1 conversion diode
| I originally replied to Chip off list not realising he posted this query
| generally.
| Here is a slightly edited version of the reply:
|
| Chip,
| I have a document (attached) with some sketchs I made on this, I
sent
| to all the people who asked me about this when we had a long discussion on
| the list before. I actually do a different conversion on my own OM1's that
is
| much better in that the meter linearity is not changed in anyway and you
| don't need to recalibrate at all. (sometimes the diode conversion is a
little
| off near max EV) The dowside is you have to remove the top cover unlike
the
| diode conversion where it is easy to remove the baseplate. The document
has a
| lot of stuff to do with the other conversions and OM2 circuit theory but
at
| the end is the part about diode conversion.
|
| I looked at a variety of diodes, schottkys and Ge. The large
schotkys
| are a bit leaky while the smaller schottky diodes like the 1N5700 series
from
| HP are a little too high in forward drop. The common point contact 1n34A
| available from Radio Shack for about $1.20 for 5 seems close and that is
what
| John Hermanson mentioned previously he had used, to the list. If I were
using
| a diode I might try to select a small Ge transistor connected as a diode
| since they have better leakage characteristics than point contact diodes
like
| the 1N34A. Unfortunately these are hard for amateurs to get particularly
in a
| small enough package to fit. Similarly Ge junction diodes are also not so
| easy to find.
|
| You can improve accuracy if you have a DVM and electronic skills: Buy a
| couple of sets of diodes and select a diode by connecting it in series
with
| the camera (AgO) battery and the camera and measuring the diode forward
drop
| at maximum allowed EV (something around 18EV or so from memory) using your
| fastst lens (at least f1.8). A slightly less accurate way would be to
measure
| the forward drop with about 0.5mA through the diode (approximate typical
max
| EV current of meter with fast lens) . The mercury cells initial voltage is
| around 1.35V but drops to about 1.3V if left on for a long time
similarly
| with the silver battery it starts out around 1.55V-1.57V and drops a
little
| with age.I have a tedious long discussion of these voltages in the
archives
| from some yeas back. Bottom line you need something of the order of
| 0.2V-0.21V drop at around 0.5mA. The forward drop varies very
approximately
| 20mV/octave or 60mV/decade of current leading to some modest meter
| non-linearity with the diode.The drop also varies with temperature leading
to
| a slightly reduced accuracy over temperature. A rough approximation is
| %voltage erors translate directly into 0f EV value. that is why errors
are
| only really significant at higher EV.
|
| Battery spacer : I just used a piece of carefully cut foam to stop
the
| battery rattling. You can (carefully) bend the spring to increase the
battery
| holding force a little too.
|
| Regards,
| Tim Hughes
| >>Hi100@xxxxxxx<<
|
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