One of the current topics in my AP Chem class is how the changing molarities of ions in galvanic cells make the voltage trend toward 0. Then the cell/battery is dead. If you put a voltmeter on a new AA cell, you get 1.5V, then it decreases by its nature as the cell is used. How do the devices use the cell despite this during the voltage decay? Just now I’m imagining that they don’t need the size of the electron group to be a constant size to represent a 1 in digital information. Is there a different real reason? How did an old Walkman compensate to keep the motor running the cassette tape the same speed? It’s a pretty dumb device. How does my wall clock use an AA cell for a year and keep the hands running at the rate?
Many/most chips will accept a range of supply voltages—look through the datasheet for specifications. (E.g., it says it is recommended to operate a a Texas Instruments CD4013B between 3 and 18 Volts.) Furthermore, if necessary for stable operation the voltage or current can be regulated.
Battery voltage fluctuates all of the time, most electronic devices use regulators, converters, capacitors, etc to ensure a steady supply of the actual voltage and current required, up to a point of course.
Thank you. This seems on point.
However, it seems unlikely that my wall clock has much going for it in this way. Does anyone know how a “dumb” device like the clock or a Walkman manage?
With regards to the Walkman, if I recall correctly, tape players didn’t do anything at all to regulate voltage–the tape would run slower and slower as the batteries wore down.
There’s a scene in Apollo 13 where they have a tape player playing C&W music floating around in the cabin, then further along in the incident they showed the same tape player with the music playing very slow and the singer’s voice super low.
Modern battery operated wall clocks have quartz crystal oscillators which control the motor speed. Such oscillators are largely independent of power supply voltage, so as long as the battery has enough oomph to keep the motor going the clock will keep accurate time.
Old batteries will still show the correct voltage.
I’ve tested them for many years with a multimeter.
The voltage on a old battery drops under a current load.
Try attaching a 220 ohm resistor across a new battery. It will still show 1.5v.
Old batteries will drop voltage significantly.
Modern chips usually don’t draw very much current and can work with batteries for a long time. Depends on the circuit. A voltage divider can draw significant current.
A flashlight bulb is a heavier current load and requires fresher batteries.
Digital electronics have varying degree of tolerance for voltage droop, but at some point the voltage will drop to the point that logic levels become unreliable and circuit operation shuts down.
Devices in which a constant input voltage is critical may emloy a switched-mode power supply:
This is basically the same thing that’s in “wall wart” chargers these days. They’re pretty efficient, and and they can take a wide range of source voltages and deliver a constant output voltage that may be higher or lower than the source voltage.
The Walkman motor was not a dumb motor connected to a battery. It used a servo loop to lock the motor speed to a tunable RC oscillator. IOW, the motor speed was independent of supply voltage as long as the voltage was high enough to keep the motor and servo electronics happy. If the voltage fell below that point the motor probably just stopped. Here is an example schematic:
The RC is implemented by C605 and a combination of R601-605.
Older battery operated tape players did in fact noticeably slow down as the batteries aged. This was especially apparent before alkaline batteries since the old zinc carbon batteries had a much more gradual discharge curve.
I’ll point out that the traditional chemistry AA batteries had a fairly linear voltage decay as the charge is consumed.
One of the big advances in battery tech starting with NiCads 30 years ago and up to the latest and greatest Li-ion cells is that the voltage does not decay (much) as charge is consumed.
E.g. using made up but plausible numbers … A traditional non-rechargeable AA will drop to 80% of nominal output voltage at 75% remaining charge. A modern battery will drop to 80% of nominal output voltage at 15% remaining charge.
The good news with modern batteries is that voltage drop due to charge depletion is not much of an issue. The bad news is that pretty much as soon as the voltage does start dropping, the battery is darn near dead. So there’s little warning of imminent failure. The “remaining charge” indicators on modern electronic devices are often mostly guesswork, not a direct reading of actual charge state like the fuel gauge on a car.
The Walkman would be different from typical tape drives if it did that. The tape passes over a spindle under tension that controls the speed of the motor(s) so that tape is passing over the head at a constant speed.
I couldn’t notice a difference in the music from a Walkman for quite a while, and Ni-Cd batteries had a characteristic of maintaining a good speed until they suddenly failed badly, which made them a better choice than even alkalines, because the music would slow to a bad speed over a much longer time.
Thanks. I suspected that someone was going to tell me that the quartz crystal made it so the lower voltages didn’t change the time “calculation”.
That reads sorta backwards to me, but maybe I misunderstood.
All tape drives (reel to reel, cassette or even 8-track) had a drive capstan and pinch roller system where the drive capstan spindle rotated at a constant regulated RPM to pull the tape across the head at a constant linear speed. Meanwhile in play (or record) mode the pinch roller was spring-loaded against the back side of the tape at the drive capstan to give the capstan spindle traction. Separately the supply and take-up reels had tension motors pulling just hard enough to keep the tape flow smooth and taut, but not opposing the drive capstan excessively.
For FF and rewind, the pinch roller disengaged, the drive capstan quit turning, the relevant take-up reel tension motor went into high speed while the opposing reel tension motor pulled gently the other way to keep the tape taut.
As described upthread, Walkman itself had a pretty sophisticated speed regulation system that was fairly voltage tolerant. Some cheaper tape players did not and ran perceptibly slowly once the batteries got low-ish…
Your description is correct. I didn’t provide good detail, just wanted to point out that the tape travels at a constant speed, but the motors don’t.
The capstan motor does turn at a constant speed. It’s regulated to do exactly that to move the tape past the head at a constant linear speed.
The reel motors don’t; they’re just tensioners and their resultant RPM is driven by the diameter of the tape pack on the reel. Which RPMs both change constantly but in opposite directions as tape is paid out or taken up.
I just was teaching the basic idea of the Nernst Equation today, which lets us calculate the current voltage based on the metals involved and the molarities of the ions at the current time. What created the ability to drastically change the decrease in voltage over the life of the cell? We can change the metals involved, but even then this equation just gets a different set of starting values, and I would expect the V decrease to be similar by the percentage of the life of the cell. The only other thing I can think of is somehow manipulating the molarities present in the working part of the cell, but as these don’t have moving parts, I don’t know how to do that.
Battery chemistry is a black box to me. I’m just repeating what I’ve read from what I think are reliable sources.
IIRC we have / had somebody here who really was a professional in the battery design game. Not recalling who though or I’d page them.
The big environmental variable for a crystal oscillator is temperature. Temperature compensation/frequency drift is a major problem for transmitter and receivers. One technique is to put the crystal in a chamber that intentionally heats it beyond ambient, an oven. Keep the xtal at a warm 200 F body temperature and it will wiggle nice & steady.