AW: AW: [Leica] voltage dependence and regulators (was gossen, ba tter ies etc..)

[Michael Dienert <MDienert@foq.de>]

> -----Ursprüngliche Nachricht-----
> Von:	A.H.SCHMIDT [SMTP:horst.schmidt@actek.com.au]
> Gesendet am:	Montag, 21. Juni 1999 15:24
> An:	leica-users@mejac.palo-alto.ca.us
> Betreff:	Re: AW: [Leica] voltage dependence and regulators (was
> gossen, batter	ies etc..)
> 
> 
> 
> Michael Dienert wrote:
> 
> >         hello everybody,
> >
> 
> Snip:
> 
> >         most light-meters using CdS cells use a simple electronic
> > (bridge-) circuit to transform the light dependent resistance of the
> CdS
> > cell to a proportional voltage which is measured with an analog
> meter.
> > thus the meter's readout is dependent on the voltage one uses to
> power
> > the circuitry.
> 
> I'll have to disagree with you there. If a bridge circuit is used,
> then the
> battery voltage is not important. The meter of the bridge will show a
> zero
> position, when the light reading is correct. In other words, when the
> bridge
> is in balance. No matter what voltage is used. The notable difference
> with a
> 
> lower or a higher voltage is the amount the meter deflects away from
> the
> zero (balance) position , when the meter is out of balance. The
> voltage
> independence is the big advantage of the bridge.
> 
> ....
> 
> Regards, Horst Schmidt
> 
> 
> [Michael Dienert]  hi horst,
> 
> you are absolutely right. a balanced bridge is independent on its
> supply voltage and i must admit, that i didn't analyse many light
> meter circuits, yet.
> 
> i have some experiencies with the leica cl's lightmeter and that
> circuit surely is not a balanced bridge and therefore the readout is
> dependent on the battery type. and i am pretty sure that the
> lightmeter of the leicaflex sl is rather similar.
> 
> to overcome the problem of different battery voltages and decreasing
> voltage over discharge time someone suggested a voltage regulator.
> unfortunately there is no easy-to-use three-terminal regulator
> available which operates with input voltages less than 2V.
> 
> maybe a switching (boost-) dc-dc converter can be used. there are some
> types operating at voltages as low as 1V. if someone is interested i
> can design an apropriate circuit which must be mounted INSIDE the
> camera or the lightmeter (between the battery terminals and the supply
> input of the lightmeter).
> 
> michael
> 
> >