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clock movement battery

notny41

Apr 24, 2023
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Hi all,

First post here.

I have a clock project that I am working on. The movement takes a AA battery, like most analog clocks do.

For this project I am also powering a DC motor with 12v. The power supply is a 12v dc wallwart. I bought a buck converter that transforms 12v down to 1.5v to power the clock so I don’t need a battery.

I am wondering if I could install some sort of rechargeable battery in the clock and run the 1.5v to it - which would power the clock as well as keep the battery charged and in case of a power outage, the clock would continue to run off the rechargeable battery.

I’m sure it couldn’t just be that easy. just hoping it might be accomplished somehow. I know rechargeable AA batteries are usually 1.2v instead of 1.5v so it would probably get overcharged.

Any ideas on something like that?

Thanks
 

davenn

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Sep 5, 2009
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I am wondering if I could install some sort of rechargeable battery in the clock and run the 1.5v to it - which would power the clock as well as keep the battery charged and in case of a power outage, the clock would continue to run off the rechargeable battery.

you could, but you would need the appropriate charger circuit suitable for the battery
putting 1.5V strait across the battery will kill it
 

Delta Prime

Jul 29, 2020
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I would like to help but I like to make sure I got this right. :)You would like a DC to DC converter designed to power devices requiring 1.5 volts from a 12 volt source, including battery powered devices that require one alkaline cell?
Your 12 volt source is also powering a 12 volt motor with no Specifications ?
 

davenn

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You would like a DC to DC converter designed to power devices requiring 1.5 volts from a 12 volt source, including battery powered devices that require one alkaline cell?

almost ;)

he wants to swap the 1.5V battery with a rechargable one (that of course is 1.2V)
then wants to run the clock off the 1.5V supply and at the same time charging the 1.2 rechargable battery
 

Kiwi

Jan 28, 2013
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The battery is only for backup should there be a power outage.
Why not use a normal 1.5v battery powering the clock through a Schottky diode. The ~0.3v drop across the diode should still leave sufficient voltage to run the clock for a very long time.
The output from the buck converter could also run through a diode to the clock. Adjust the buck convert voltage after the diode to be slightly higher than the voltage from the battery. The converter will power the clock.
The battery will only power the clock when there is a power outage, so will remain fully charged.
 

AnalogKid

Jun 10, 2015
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I think Kiwi has the right idea. A simple alkaline cell will provide years of intermittent backup service.

If you really want a rechargeable in there, go with a nickel-cadmium (NiCd) cell rather than a lithium. A single resistor can trickle-charge it forever. It may take a day or two to come up to full charge the first time, but after that a very small current will keep it topped off without a complex charging circuit.

ak
 

crutschow

May 7, 2021
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Seems like a lot of work just to avoid having the replace the clock battery every year or so.
You could extend the battery life by going to an alkaline C or D cell.
A D cell has about 5 times the capacity of an AA so should last at least 5 years before needing replacement.
 

Kiwi

Jan 28, 2013
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Now we all wait with bated breath for the second post.
 

notny41

Apr 24, 2023
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I saw the first response about killing the rechargeable battery and kind of gave up on the idea. Then today I was thinking that maybe a normal alkaline battery would be a better idea since it is the same voltage as my buck converter is putting out. Then came out to this site again and was going to propose this - then I saw that someone already suggested that. LOL

So if I wanted to do that, did you say I need a diode in there somewhere?

Thanks everyone!
 

AnalogKid

Jun 10, 2015
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So if I wanted to do that, did you say I need a diode in there somewhere?
Yes. A normal alkaline battery is *not* rechargeable.

Put a small Shottkey diode in series with it. This prevents current from the buck converter going "backwards" through the battery.

Next, adjust the output of the buck converter so that it is approx. 1.8 V. This is the 1.6 V of a fresh battery plus 0.2 V to make sure the diode is reverse-biased (off).

Battery + terminal -> diode anode
Diode cathode -> clock mechanism battery holder + terminal
Buck converter + output -> clock mechanism battery holder + terminal

ak
 

crutschow

May 7, 2021
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So if I wanted to do that, did you say I need a diode in there somewhere?
Probably will need one in series with the supply output since the supply may otherwise sink current when it's not powered if the battery is directly connected to its output.
Then adjust the supply to give no more that 1.5V at its diode output when connected to the clock and battery.

Applying 1.5V to the alkaline should not be a problem, since that's very near the open circuit voltage of a new battery.
You might place a small resistor (e.g. 1kΩ) in series with the battery (which won't affect it operation to provide the very low clock current) to avoid any excessive currents when the power supply comes on if the battery has powered the clock for some time.
Alkalines can tolerate a few milliamps of charge current and can even be somewhat recharged by this current.
 

notny41

Apr 24, 2023
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quick question: Is a Shottkey diode a special type of diode? I have a bag of different diodes. They are mostly the black ones with a silver stripe around it.
 

AnalogKid

Jun 10, 2015
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Yes, it is a special kind of diode.

The color has nothing to do with the diode type. Most signal-level and low power level diodes are either black epoxy or clear glass. The stripe, or band, is an indicator of the cathode end of the diode,

Without diving into the device physics, generally speaking a Shottkey diode has a lower forward voltage (Vf), higher leakage current, and a faster response time than a standard diode.

ak
 

notny41

Apr 24, 2023
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is there any way to tell a Shottkey diode from a non-Shottkey?
 

notny41

Apr 24, 2023
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Crutschow, to answer your question, the part where the battery goes is obstructed so not convenient to be pulling a battery out and putting a new one in. Was hoping I could install one and leave it in there indefinitely and have it be just used for when the power goes out.
 

73's de Edd

Aug 21, 2015
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is there any way to tell a Shottkey diode from a non-Shottkey?
Pull out ye olde tyme digital Dee Vee Emm and place it in its DIODE test mode.

Place the meter probes across the diode and observe the forward voltage drop of that diode that will be a read out in millivolts, if no luck in getting a reading initially, reverse the leads and try again.
Expect a voltage in the 600-800 mv range for silicon diodes or down in the 500's if you have found yourself a fast switching silicon diode.
WHAT you are looking for is one with a reading down in the 200 +-50 mv range.
A remote possibility is the finding of a glass en cased . . . . ole' generation germanium diode with a like low Vf . . . but their back voltage /leakage makes them undesirable for this application.

LOOK under the rocks and expect Shottkey ( spell check insists that its Hot key ! ) diodes to be found and used in switch mode power supplies and 1990's up vintage TV chassis.
Of course this also includes easily available junked computer power supplies.
Also if identified by their markings, , you can then look up their types and specs.

73's de Edd . . . . .
 

AnalogKid

Jun 10, 2015
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If your DVM does not have a Diode mode, try this.

Put the diode in series with a 1 K resistor, connect the string to a 9 V battery, and measure the voltage across the diode. With the battery connected one way you will read 9 V. with the battery connected the other way you will read the forward voltage (Vf) across the diode. In very round numbers, above 0.5 V is a standard silicon diode, and below 0.5 V is a Shottkey.

Note that high power Shottkeys can have the same Vf as low power regular diodes. For equivalent power levels (estimated by physical size), the Shottkey Vf always will be lower.

ak

Edd is right about the spell checker. Annoying, so I added Shottkey and Shottkeys to the dictionary.
 

ramussons

Jun 10, 2014
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Why not put a 1 farad capacitor floating at 1.4 volts? 2 x In4007 in series gives 1.4 volts. The current drawn by the clock is very small.
My idea calls for a 1.0 F 5 Volts and 0.1uF/400 VAC cap, 3 1N4007 diodes, 220K and 220 Ohm 1/4 watt resistors.
 
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