I've got a problem that I've been wracking my brain over for a while now. I need a small (under 5 parts) 2-terminal circuit to drop 0.2 V at up to 100 uA.
The situation is that I have an old but high quality camera that used a
1.35 V mercury cell for the light meter. These cells are no longer available due to a world-wide ban on mercury cells. I had a few extras stashed away but those are now gone. :(
The closest thing I can find with similar discharge characteristics is a silver oxide cell at 1.55 V. Compounding the problem is the fact that the camera uses the metal body as the positive ground and getting access to the on-off switch would require far more disassembly than I'm comfortable with. Hence the need for a 2-terminal solution. The battery check circuit in the camera considers a cell to be "good" when the terminal voltage is between 1.27 and 1.35 V at 90 uA load.
A 3 V manganese dioxide lithium cell is also an option if paired with a
3-terminal positive-ground regulator with very low (
Are you saying that the 1.55 V silver oxide cell messes up the reading or functionality of the camera? Other than a resistor (if current load is relatively constant), theer is nothing that would give a 0.2V drop. A schottky or germanium diode will give about 300-350mV drop, depending on current and diode rating. Now using a 3-terminal regulator with a 3V source seems to be a solution, but you need one with two attributes: Low Drop-Out (or LDO in the trades) *and* low drain. Maxim advertises a shunt regulaor that supposedly works down to 1uA, but they can be completely discounted, as Maxim/Dallas parts are vaporware. However, Linear technology does have the LT1389 shunt regulator, available at 1.25V, 2.5V and 4.096V, speced to operate from 0.8uA to
2mA; curves show operation down to around 0.4uA. Then their LT1634 shunt regulator, available at 1.25V, 2.5V, 4.096V and 5V, speced to operate from 10uA to 2mA. Their LT1004 seems to have similar specs, but available only at 1.23V and 2.50V. The LM285-2.5 works from 20uA to 20mA, and there are adjustable versions made by National Semi. So, you could make your own 3-term regulator, as most of what is cited above is available in SOT-23 or SOIC-8. Idea: NPN pass transistor, base to cathode of adjustable, FB of adjustable to tap of voltage divider: 1.25 meg from FB to ground (anode) and 100K from FB to NPN emitter. Now all one needs is base drive from NPN collector that will run the regulator and the NPN. Say one allows a LM285 regulator current of 10uA and a base drive of
1uA (yes, that is high - so the extra also goes thru the LM285). The base will be 1.35V (ouput or emitter voltage) plus a Vbe of about
600mV, or about 1.95V; making the Vcb about 1.05V. Obviously, no constant current device (JFET or DMOS) is available, so a resistor "pullup" will have to do. E = I * R or 1.05(V) = 11(uA) * R(megohms); or roughly 100K. The resistors can be SMD 0805; easily available in 1% values to 10 megs; the NPN is available in SOT-23 so it can be done in a small profile PCB which could be 20mils thick if desired. That totals to the 5 parts max.
Perhaps you should try an LM10 opamp with 200mV voltage reference. Specs say it could work down to 1.1V. The 90uA output might keep the the saturation enough below 0.2V.
Of course you must be able to live with the quiescent current of about
300uA. But if it is similar to my old camera, then there is a button press needed to operate the light meter. So it is not on very often.
I just read your post more carefully and see the requirement of having the positive side to the camera body. Making it a 1.35V regulator might not work without external components. Right now I can't think of a simple solution for that.
But making it a 0.2V regulator is simple. Just connect the reference opamp output to the its input. When your battery gets more empty, then the opamp output drop voltage relative to the positive rail. This setup always 'takes 0.2V off', but requires no external components.
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Does it affect the light meter much? If only 10%, does it make a noticable difference?. Shutter and diapragm settings are made in large steps, and even one (half) step off isn't often a problem. Perhaps there is a calibration pot, that you could adjust.
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Thanks, Frank.
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The mercury cells were very stable over their life span. The selenium photo detectors used this 'calibrated voltage' to give stable results. The idea is to mimic the mercury cells with the replacement battery, ahla 0.200 v drop.
Maybe a 3v or 1.5v battery and a small switching regulator to 1.35v will work. The problem is where to put it.
LTspice says it draws 3.8 uA quiescent, so my battery should last a couple of years. I may do a bit more tweaking on the resistor values after further analysis but I think that this is a pretty good starting point.
That would probably work but I'd rather "do it right" since I'm going to loan the camera to my niece (a definite non-techie) once I get a new digital SLR. Plus, the higher voltage would invalidate the battery check setting, and I'm also not sure what non-linearities (if any) a higher voltage would introduce.
All in all, supplying the correct voltage is the best solution.
No, this circuit sets the "to meter" point at 1.35 V above the negative rail. The output needs to be referenced to the positive rail (body). Thanks for checking it over, though.
I don't have any nano-power design experience either, but it looks to me like your circuit has a shot at working. I think better values for the resistors are 1 meg and 80.6k, though.
Of course you will use 1% tolerance resistors, which will give you a variance of around 1% in the output voltage. If you want to, you could use a 100k trimpot instead of the 80.6k resistor. Then you can trim to the desired voltage. Or you could use your 68k resistor, then add a 20k trimpot, but then the parts count will be up to 6 parts.
I also haven't checked the deviations you will get due to leakage currents at the inputs, but the 1494 seems to have pretty good specs in that regard. If you use a trimpot, this probably won't matter anyway.
I was going to recommend the lm185 (or lm285), but this circuit seems to have that one beat in terms of current consumption. But if you did use the lm185, the part count might be as low as 3 parts. The current consumption would be right around 10 uA.
I'd use 1% resistors. If you move the 1.2M and change its value, you can make the 1.25V reference current independent of the battery voltage by running it from the 1.35V output. Then if you select a different opamp, you can run it from your 1.55V silver oxide cell.
No fiddling needed. Instead of 1 meg, use an 0805 1.24 meg 1%; Digikey cut tape price is an outrageous $0.80; and thus instead of 68K use an 0805 100 K 1%; same price. If the reference was exactly 1.24V, then the op-amp would be at 1.35V within 1% (the resistors in practice are inside the limits of the spec, so the average error of the output voltage works out to within 1%. Since the reference is 1.25V "exactly", the derived voltage will be slightly higher than 1.35V, and will be well within 1.4% (worst case) and probably better than 1%. Then, might as well change that 1.2 meg and use 1.24 meg 1% from the same cut tape to save money. Lay out a PCB and have a bunch made (Express PCB or similar), assemble them yourself and sell the solution!
BTW, in this circuit the opamp shouldn't necessarily be a RRIO type. In operation the input and outputs are both near the negative rail, an easy opamp spec (even tho low-power 1.5V operation is not). But to insure startup, you want an opamp whose input-stage works properly with CM voltages closer to the + rail than the output is at startup, or that pushes the output negative until the input CM is working. If you turn the circuit upside-down, and are working at higher power levels, many classic single-supply opamps meet these unusual specs. I'm not sure if any offered opamps will work in my 1.55V circuit...
I posted a note concerning startup. This type of bootstrap circuit start up fine if the opamp's output has more BJT saturation voltage than the input's offset (divided by the feedback-resistor ratio), so when the power is applied, the + input sees a bit more voltage than the - input and drives the output further toward the turned-on state.
There are two problematic issues. First, the feedback resistors to common tend to reduce the BJT saturation voltage. Using high-value resistors helps; e.g., here they draw a current of 30nA for say 30mV of output-transistor saturation. We note that 30nA is probably much less than the base-drive current for the PNP output transistor.
Second, when the output is only modestly-higher than the reference, the "offset voltage divided by the feedback-resistor ratio" parameter becomes painful. Here we would divide our estimated 30mV by 12.5 to get a 2.5mV max offset-voltage spec, which might be hard to meet.
Assuming a good-quality reference, this two-resistor bias scheme has little advantage over one resistor to the battery, unless a variety of battery voltages is expected and power conservation is very important.
Assuming the anode of the zener and the 1Meg output resistor are earthed V(+) = V(-) at the opamp inputs would be = Vee. If we had a opamp configuration like a 741 the input transistors would be cut off as would the darlington stages. That would force the opamp output into a positive direction . With other Opamps any other simple solution would do.
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