C
Christoph M. Wintersteige
Guest
Hi there!
I'm facing the following Problem: I own a 70's Camera (Praktica LTL,
but the problem applies to a whole lot of brands and models) which has
a battery powered light meter built in. The light meter used to be
powered by a mercury battery (PX625), although those may not be sold
anymore because of the environmental problems with the mercury in
them. There are some substitutes, but those are all no option for me,
as they are quite expensive and nowhere near as good as the mercury
batteries were. The reason for this is, that the mercury batteries had
a very constant voltage throughout their lifetime (1.35V in this
case), and the light meter is very sensitive to changes in the supply
voltage. It's calibrated for 1.35V; Thus an alkaline battery that has
a changing voltage of something like 1.5V downto 1.2V during its life
won't be very accurate for most of the time. There are no voltage
regulators built into the camera.
Now, I was thinking that it must be possible to build a voltage
regulator that fulfills the following needs:
- It must be small (QUITE small) as there is not a lot of space in the
camera. (May be SMD)
- A Voltage of 1.35V must be held very constant throughout the
lifetime of the power supply, and may not have significant ripple.
- I plan on using a rechargeable battery built into the camera that's
mounted on a PCB and has a charge plug.
- The whole circuit may not draw a lot of power while not in use; It's
only needed while the button for the light meter is pressed.
Now I hope that some of the experts in here can help me finding
answers to a few questions:
Are there any ready made ICs that do (most of) this job? I found the
ON Semi NCP1501, which incorporates a PWM Modulator, but can be
switched to LDO Operation - Will there be a lot of ripple if I use a
chip like that? And I found the ST LD3985 Series, which look quite
promising to me - Any real life experiences?
For this kind of application, would it be better to use a Ni-Cd or
Ni-MH or other kind of battery? Would it be best to decide that upon
the size of the required charge circuit? And are there probably any
readymade ICs for charging batteries?
Would it probably be a better idea to regulate the voltage using
MOSFETs instead of ICs?
How could a shutdown circuit be implemented so that the circuit
doesn't draw too much current while not needed? (The ICs I mentioned
have shutdown pins).
And can anyone think of a circuit that cuts off when the battery is
down to, say, 1.3V? Can it even be made this accurate on the small
space available?
Thanks a lot for reading, and in advance for any answers,
CM Wintersteiger
I'm facing the following Problem: I own a 70's Camera (Praktica LTL,
but the problem applies to a whole lot of brands and models) which has
a battery powered light meter built in. The light meter used to be
powered by a mercury battery (PX625), although those may not be sold
anymore because of the environmental problems with the mercury in
them. There are some substitutes, but those are all no option for me,
as they are quite expensive and nowhere near as good as the mercury
batteries were. The reason for this is, that the mercury batteries had
a very constant voltage throughout their lifetime (1.35V in this
case), and the light meter is very sensitive to changes in the supply
voltage. It's calibrated for 1.35V; Thus an alkaline battery that has
a changing voltage of something like 1.5V downto 1.2V during its life
won't be very accurate for most of the time. There are no voltage
regulators built into the camera.
Now, I was thinking that it must be possible to build a voltage
regulator that fulfills the following needs:
- It must be small (QUITE small) as there is not a lot of space in the
camera. (May be SMD)
- A Voltage of 1.35V must be held very constant throughout the
lifetime of the power supply, and may not have significant ripple.
- I plan on using a rechargeable battery built into the camera that's
mounted on a PCB and has a charge plug.
- The whole circuit may not draw a lot of power while not in use; It's
only needed while the button for the light meter is pressed.
Now I hope that some of the experts in here can help me finding
answers to a few questions:
Are there any ready made ICs that do (most of) this job? I found the
ON Semi NCP1501, which incorporates a PWM Modulator, but can be
switched to LDO Operation - Will there be a lot of ripple if I use a
chip like that? And I found the ST LD3985 Series, which look quite
promising to me - Any real life experiences?
For this kind of application, would it be better to use a Ni-Cd or
Ni-MH or other kind of battery? Would it be best to decide that upon
the size of the required charge circuit? And are there probably any
readymade ICs for charging batteries?
Would it probably be a better idea to regulate the voltage using
MOSFETs instead of ICs?
How could a shutdown circuit be implemented so that the circuit
doesn't draw too much current while not needed? (The ICs I mentioned
have shutdown pins).
And can anyone think of a circuit that cuts off when the battery is
down to, say, 1.3V? Can it even be made this accurate on the small
space available?
Thanks a lot for reading, and in advance for any answers,
CM Wintersteiger