# Re: [Rollei] Meter calibration...

• Subject: Re: [Rollei] Meter calibration...
• From: Don Williams <dwilli10 >
• Date: Mon, 04 Nov 2002 13:41:41 -0800
• References: <20021105072029.A351 1@icarus>

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Hi,

contributions with this message.  It's really getting off-track.

I would, however, be interested in hearing from someone with a Rollei
manual about whether there is a resistor in the circuit between the
selenium cell and the meter.  It might be in the meter housing and not be
shown as a separate part, however.  If there is a resistor, what would the
approximate value be?

DAW
=======================================
At 07:26 AM 11/5/2002 +1030, Richard Urmonas wrote:
>I can assure you it is a current source.  Photon in = electron out.
>The current source behaviour has a simple, logical explaination.
>There is no similar explaination for a voltage source.

I guess we could debate this till we both give up.  Let's try the following
line:

1.      Is a photo-voltaic cell a current source if nothing is connected to
the output terminals?

2.      If a photo-voltaic cell is a current source, how would you define a
battery?

3.      If you say neither of them is a current source, what would
constitute a current source?

4.      What would you call a device whose measured output voltage varies
almost linearly with incident light?

From an engineering point of view, there is a term
"constant-current-source", which means that the output current is
independent of the load.  That means that regardless of the resistance of
the circuit the current is constant.  This means that the voltage of the
source varies inversely with the load resistance.

====================
You say the following:

"Wrong. The photocell is a current source, so it determines the current in
the circuit. Try connecting an ammeter across a photocell."

I would reply by saying I would need to know the internal resistance of the
photocell, knowing that the resistance of an ammeter is near
zero.  Alternatively, I could vary the series resistance in the circuit and
cause the output current to vary.

By the way, Einstein got his Nobel Prize for "Photon in = electron out",
not for any of his writings on relativity.

===============
You say the following:

"I am at home with limited Internet access, otherwise I would try to find
some references which clearly present the relationship between light and
current for photocells."

I have already quoted from a web site the following:

"The selenium photocell is usually linear [Here they are speaking of
voltage] if the output is under 150 millivolts and the impedance
(resistance) of the load is at least 100,000 ohms." [This means they are
measuring the open circuit voltage of the cell.  When this is read by a
meter in series with a relatively large resistor in series, the current
will be proportional to that voltage.]

The relevant website was given as:

http://www.eta-is-opacity.com/tech3.htm

Anyhow, if you want to continue we should go off-list.  I don't think any
of this really matters to Rollei Users.

Arguing with an engineer is like mud wrestling with a pig.
You soon realize they both enjoy it.

Regards,

Don Williams

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<html>
Hi,<br><br>
contributions with this message.&nbsp; It's really getting
off-track.&nbsp; <br><br>
I would, however, be interested in hearing from someone with a Rollei
manual about whether there is a resistor in the circuit between the
selenium cell and the meter.&nbsp; It might be in the meter housing and
not be shown as a separate part, however.&nbsp; If there is a resistor,
what would the approximate value be?<br><br>
DAW<br>
=======================================<br>
At 07:26 AM 11/5/2002 +1030, Richard Urmonas wrote:<br>
<blockquote type=cite class=cite cite>I can assure you it is a current
source.&nbsp; Photon in = electron out.<br>
The current source behaviour has a simple, logical explaination.<br>
There is no similar explaination for a voltage source.</blockquote><br>
I guess we could debate this till we both give up.&nbsp; Let's try the
following line:<br><br>
1.<x-tab>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</x-tab>Is a photo-voltaic
cell a current source if nothing is connected to the output
terminals?<br><br>
2.<x-tab>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</x-tab>If a photo-voltaic
cell is a current source, how would you define a battery?<br><br>
3.<x-tab>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</x-tab>If you say neither
of them is a current source, what would constitute a current
source?<br><br>
4.<x-tab>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</x-tab>What would you call
a device whose measured output voltage varies almost linearly with
incident light?<br><br>
From an engineering point of view, there is a term
&quot;constant-current-source&quot;, which means that the output current
is independent of the load.&nbsp; That means that regardless of the
resistance of the circuit the current is constant.&nbsp; This means that
the voltage of the source varies inversely with the load
resistance.<br><br>
====================<br>
You say the following:<br><br>
<font color="#FF0000">&quot;Wrong. The photocell is a current source, so
it determines the current in the circuit. Try connecting an ammeter
across a photocell.&quot;<br><br>
</font>I would reply by saying I would need to know the internal
resistance of the photocell, knowing that the resistance of an ammeter is
near zero.&nbsp; Alternatively, I could vary the series resistance in the
circuit and cause the output current to vary. <br><br>
By the way, Einstein got his Nobel Prize for &quot;Photon in = electron
out&quot;, not for any of his writings on relativity.&nbsp; <br><br>
===============<br>
You say the following:<br><br>
&quot;I am at home with limited Internet access, otherwise I would try to
find <br>
some references which clearly present the relationship between light and
<br>
current for photocells.&quot;<br><br>
I have already quoted from a web site the following:<br><br>
<font color="#FF0000"><i>&quot;The selenium photocell is usually linear
</font><font color="#0000FF">[Here they are speaking of voltage]
</font><font color="#FF0000">if the output is under 150 millivolts and
the impedance (resistance) of the load is at least 100,000
ohms.&quot;</font><font color="#0000FF"> [This means they are measuring
the open circuit voltage of the cell.&nbsp; When this is read by a meter
in series with a relatively large resistor in series, the current will be
proportional to that voltage.]<br><br>
</i></font>The relevant website was given as:<br><br>
<a href="http://www.eta-is-opacity.com/tech3.htm"; eudora="autourl">http://www.eta-is-opacity.com/tech3.htm</a><br><br>
Anyhow, if you want to continue we should go off-list.&nbsp; I don't
think any of this really matters to Rollei Users.<br>
<x-sigsep><p></x-sigsep>
Arguing with an engineer is like mud wrestling with a pig.<br>
You soon realize they both enjoy it.<br><br>
Regards,<br><br>
Don Williams<br>
</html>

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