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Simple problem: 0.20 V drop

Hi all,

This is a well-known problem (for those using old cameras...):

Mercury oxide cells have a pretty flat 1.35 V open circuit voltage. They have however been banned in many countries. The best replacement would seem to be silver oxide which has a voltage of 1.55 V.

the current drawn (by the exposure meter) is low.

So, I'd like to drop 0.20 V along the way somewhere.

a) Zener diodes: don't go down to 0.2 V

b) Schottky diodes: supposed to have a forward drop of 0.15 to 0.45 V at 1 mA (according to wikipedia) but the drop is current dependent and varies between samples.

c) Germanium diode?

d) voltage divider with a FET follower? a JFET ?

Is there a standard / best way to do this (with few, smd components)?

Thanks,

colin

Reply to
Colin Howarth
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Do you REALLY need this precise of a voltage? wouldn't your application accept 0.20 extra voltage, especially considering it is from a battery?

Reply to
OBones

On a sunny day (Thu, 18 Jan 2007 15:28:01 +0100) it happened Colin Howarth wrote in :

If the current is constant, then a simple resistor + decoupling capacitior will do. Germanium drop is also a bit current dependent, maybe 150mV or so at low currents (but less in the uA range).

Reply to
Jan Panteltje

Film exposure is so logarithmic, and so arbitrary, you'll never notice the difference.

John

Reply to
John Larkin

Today's instant ASCII design:

. Micropower low-voltage LDO regulator: . drop 13% = 200mV at 1.55V (if k=0.87) . . Vi___________________pnp Q3 . | | | E C ----+-- OUT . R1 R3 | B | Vo . | | | | | . | +---- | -----? | . | Q1 | | Q2 npn | . | C C | . +---B B -----------? . | E--+--E k = Vo/Vi = R2/(R1+R2) . | | R4 = (kVi-0.55)/Iq . R2 R4 R3 = 2*0.60/Iq . | | choose Iq >= Iout/25 . gnd gnd choose R1+R2 = 10 Vi/Iq

--
 Thanks,
    - Win
Reply to
Winfield Hill

I'm afraid so 0.2 / 1.35 is about 15 %. That changes the meter reading significantly.

colin

Reply to
Colin Howarth

decoupling capacitor? This is a dc voltage.

confound it.

I'm tending towards the voltage divider with a fet follower...

colin

Reply to
Colin Howarth

On a sunny day (Thu, 18 Jan 2007 16:08:24 +0100) it happened Colin Howarth wrote in :

If you increase Ri of the battery with a series resistor (and that must be a high value say in case 10uA and .2 V, 20 kOhm) then you perhaps need to 're-create' a low dynamic impedance with for example a tantalum cap. For some things this will not be needed, if pure DC, OK. Is that a simple LDR with a meter?

Reply to
Jan Panteltje

Well thanks again, Winfield. It's great that you're still active here. Now I just have to have a quick look at your book again to see what it's doing. :-)

BTW, is there a new edition coming out any time?

colin

Reply to
Colin Howarth

Mainly in the US. AFAIK, you can still buy them in Canada, and in a few less 'ethical' retailers in the US. Comparing the amounts of mercury in these and in the millions of fluorescent tubes sold in the USA makes one wonder what the problem really is. Also, what happens to all of the non-mercury batteries discarded in the US, those with lithium or cadmium for example?

Reply to
Homer J Simpson

IIRC you can make a speed adjustment to the film and zero out almost all of the error.

Reply to
Homer J Simpson

On Thu, 18 Jan 2007 16:00:39 +0100, Colin Howarth wrote in Msg.

That's 1/4 stop. Totally insignificant. If this worries you you can set the film speed a bit lower on your camera.

robert

Reply to
Robert Latest

15% meter error will be a small fraction of one f-stop, or of one standard 2:1 shutter speed step. And light meters aren't very accurate to start.

John

Reply to
John Larkin

They're banned in the EU too. One's not allowed to throw used batteries away in the normal rubbish. But because people do, I think it's ok to ban them.

One's supposed to get rid of these in an "environmentally friendly" way too.

Mercury is particularly nasty. I believe it accumulates in the body - and gets concentrated the higher up the food chain you go... Lithium and Silver are fairly harmless.

Reply to
Colin Howarth

Someone is still using film ?:-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

I suppose one can do this - but it's not ideal. One can buy adapters - but they're moderately expensive. So, I thought I'd like to make my own...

colin

Reply to
Colin Howarth

That's like using a micrometer to measure slices of bread.

Reply to
Homer J Simpson

This particular camera uses a CdS metering cell. What if the meter reads 0.3 V instead of 0.1 V ?

colin

Reply to
Colin Howarth

Nah. You wouldn't know when to stop turning the screw.

I suppose you could try measuring the capacitance, using bread as the dielectric - then calculate the thickness from that ? ;-)

colin

Reply to
Colin Howarth

Oh yes! There are hundreds of us! :-)

colin

Reply to
Colin Howarth

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