Dumb DC Power Supply question (Garage Clock)

[HoosierBuddy]

SO,

I've got one of those Garage Clocks with the big neon ring around it. The Neon runs off of a 12V power Brick. The stupid clock on the thing (however) runs off a single AA battery. Oh...and it hangs 9-feet of the garage floor above a set of cabinets...meaning the clock battery goes dead and stays dead for weeks because I'm too lazy to drag a ladder out to change a battery when I have a watch, a phone, and like 50 other appliances with clocks on them to tell me the time if I really need to know what it is.

The question is this: What circuit would I need to build to allow me to tap into the 12V at the clock and reduce that to 1.5 Volts for the clock. Would it be as simple as 2 wires and a resistor?

At first I was thinking I'd buy a separate power adapter for the clock...but I'm not seeing any that are 1.5 volts. Even the switched ones only seem to go down to 3 volts.

I wish I remembered more from my circuits class in college. But I don't.

Phil

 

[sublimate]

A simple Voltage Divider will do it for you.

https://learn.sparkfun.com/tutorials/voltage-dividers

A voltage divider is a simple circuit which turns a large voltage into a smaller one. Using just two series resistors and an input voltage, we can create an output voltage that is a fraction of the input. [snip]

 

[CJ7VFR]

They also sell an AC Power pack that is used in place of a 1.5 volt DC AA battery.

Below is a picture of one. They cost about $9.00 or so.

Just another alternative for you to think about.

Jim

 

[404]

Solder an alkaline D cell to some wires, and solder those to the tabs on the clock. I replaced a 9V battery in my setback thermostat with a remote pack of 6 alkaline D cells and it cost less and is still working about 12 years later.

 

[HoosierBuddy]

THanks!

So looking something like this:


R1---------12V+
|
|
+ V out to clock
|
|
R2---------12V-


I'm thinking I can use a 2K ohm resistor for R1 and a 0 to 500 ohm mini trim Potentiometer for R2. If I adjust that R2 to be about 285 ohms, that would yield 1.5 volts at the "V out to Clock" tap.

Based on a battery lasting on the order of 2000 hours and a AA battery having roughly 2000 mAh capacity...I'm assuming the clock only pulls 1 mA

I THINK my numbers above give me a total resistance of 2285 Ohms across 12V yielding about 5 mA...but I'm getting a little fuzzy there. By that, I'm not exactly sure what connecting the clock motor to ground does to the above diagram and current flow.

I do know my power brick is rated at 1 Amp, but most of that is going to be required for the neon....although it seems to me that with these high R value, I'd still have 995 mA available for the neon.

ALSO...on the AA adapter idea, any source for that? Looking at Amazon, I didn't see it.

Phil

 

[HoosierBuddy]

Solder an alkaline D cell to some wires, and solder those to the tabs on the clock. I replaced a 9V battery in my setback thermostat with a remote pack of 6 alkaline D cells and it cost less and is still working about 12 years later.

Looks like an Alkaline D battery has about 8 times the capacity of a AA. That is not a bad idea either as it might last about 2 years on the clock.


ALSO it wouldn't have to be reset after a power outage. Hmmm.
Phil

 

[Mattlt]

Off track, but don't you have to crawl up there to set it twice a year for daylight savings time?

Indiana does that now, right?

 

[sublimate]

THanks!

So looking something like this:


R1---------12V+
|
|
+ V out to clock
|
|
R2---------12V-


I'm thinking I can use a 2K ohm resistor for R1 and a 0 to 500 ohm mini trim Potentiometer for R2. If I adjust that R2 to be about 285 ohms, that would yield 1.5 volts at the "V out to Clock" tap.

Based on a battery lasting on the order of 2000 hours and a AA battery having roughly 2000 mAh capacity...I'm assuming the clock only pulls 1 mA

I THINK my numbers above give me a total resistance of 2285 Ohms across 12V yielding about 5 mA...but I'm getting a little fuzzy there. By that, I'm not exactly sure what connecting the clock motor to ground does to the above diagram and current flow.

I do know my power brick is rated at 1 Amp, but most of that is going to be required for the neon....although it seems to me that with these high R value, I'd still have 995 mA available for the neon.

ALSO...on the AA adapter idea, any source for that? Looking at Amazon, I didn't see it.

Phil

I don't think you have a -12v (negative), that side is at 0v (essentially ground, but you might not actually ground it to earth).
The clock will be relative to that same 0v.

Your clock won't be very sensitive to voltage - AA batteries can range from 1.2v to 1.8v depending on the type. So the trim pot is overkill. Just use a fixed resistor that's close.
270 or 330 are both common and either should work fine.

 

[HoosierBuddy]

Off track, but don't you have to crawl up there to set it twice a year for daylight savings time?

Indiana does that now, right?

Yes.

Phil

 

[kand3g]

CJ, Do you have a link to the AC power pack pictured? Thanks.

 

[404]

Looks like an Alkaline D battery has about 8 times the capacity of a AA. That is not a bad idea either as it might last about 2 years on the clock.


ALSO it wouldn't have to be reset after a power outage. Hmmm.
Phil

This is cheap, but would need rewiring rework to be 2 cells parallel instead of in series. Avoids soldering on the batteries. Once in while spin the batteries in the holder (no need to remove) to break up the oxides that form on the contact ends.

http://www.ebay.com/itm/Black-Plast...Two-Wires-W-/201370936254?hash=item2ee2a4a3be

 

[rattle_snake]

Yes it is as easy as 2 resistors. The values you have (2k, 285) will work fine for standard 1/4 or 1/8 watt resistors.

 

[Alchymist]

So if you have to change the setting for DST, why not just change the battery at the same time? It's not like it's an expensive item.

 

[404]

Yes it is as easy as 2 resistors. The values you have (2k, 285) will work fine for standard 1/4 or 1/8 watt resistors.

Not that I have the energy to figure this out, but I wonder what the cost of electricity is with a wall wart, vs the cost of the battery..

 

[CJ7VFR]

CJ, Do you have a link to the AC power pack pictured? Thanks.

http://www.clockparts.com/aa-cell-uninterruptible-power-supply/c/gs/?gclid=COWfgey8lccCFYsRHwodwb4GLA

This is one place that sells them. It's called Clock Parts. This one costs about 8 bucks.

If you Google "AA 1.5 volt ac power" you will find them.

They were made originally to power clocks with the Quartz movements that required a single 1.5 volt AA battery.

So this falls right inline with what the OP was looking for.

Down side is that you need an AC outlet near your clock, thus negating any benefit of being able to hang a battery powered wall clock anywhere you want.

But if you have an outlet handy, these things work great!

Jim

 

[sublimate]

Not that I have the energy to figure this out, but I wonder what the cost of electricity is with a wall wart, vs the cost of the battery..

He's estimating the clock uses 1mA @ 12v, which seems about right.
That's .012 watts.
So over a year, that would be 0.1 kWh, which would cost around $0.01 (1 penny).

But that assumes a 100% efficient wall wart.
In reality a wall wart uses something like 0.5W which would be around 50 cents for the year.

 

[DHCrocks]

easyway out, just put in an energizer lithium AA in there, it should last a longer, you won't have to worry about leaking batteries and no modifications to the clock.

 

[kand3g]

Thanks for the link CJ.

 

[404]

He's estimating the clock uses 1mA @ 12v, which seems about right.
That's .012 watts.
So over a year, that would be 0.1 kWh, which would cost around $0.01 (1 penny).

But that assumes a 100% efficient wall wart.
In reality a wall wart uses something like 0.5W which would be around 50 cents for the year.

Thank you.

 

[doan]

Solder an alkaline D cell to some wires, and solder those to the tabs on the clock. I replaced a 9V battery in my setback thermostat with a remote pack of 6 alkaline D cells and it cost less and is still working about 12 years later.

I like your style!

 

[doan]

Yes it is as easy as 2 resistors. The values you have (2k, 285) will work fine for standard 1/4 or 1/8 watt resistors.

Since the clock is likely a constant load, just use one series resistor. Put a 10k pot on it and adjust until you get ~1.5v at the clock. Then replace with a fixed resistor, or just put a drop of glue on the pot.

 

[SLYDIT]

just get an LT1587 regulator chip. easy. it has 3 legs...input/output/ground couldnt be easier

 

[sublimate]

Since the clock is likely a constant load, just use one series resistor. Put a 10k pot on it and adjust until you get ~1.5v at the clock. Then replace with a fixed resistor, or just put a drop of glue on the pot.

That will not work well.

 

[imperialman67]

I have the same neon/AA battery clock setup. Maybe I'm lucky , but the rechargeable Enloop battery I use lasts until I change the clock for daylight savings. And this is in a garage that will have its temperature in the teens in January.

 

[HoosierBuddy]

Thanks for all the advice guys!

After considering everything said, my installation, level of desire to climb up on my ladder, then stand on my workbench to get to to the clock, AND the somewhat frequent power outages we have in the summer...

I decided to go with a D-battery.

I didn't have anything going on Sunday morning, so I rooted through my plumbing supplies and electrical leftovers to make a spring loaded holder. I soldered small alligator clips to the leads and clipped them right onto the +/- terminals in the clock.

Oh...and I cleaned about 5 year's worth of spider webs off the neon and rehung the clock. Works like a champ!

Phil