How to cut down 12 volts to 1.5 volts

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I know one can use a LM317 to cut 12 volts dc to 1.5 volts but can one use resistors? I have a wall clock that runs a small motor to wind spring and it powered by a c battery you can run on more voltage without out damage to any thing as it only run for 1 second every 2 minutes. I tried a 30 ohm resistor but it still must be to much voltage as when it winds up the spring it it goes to far causing it not to release the switch when unwound. Do these small brush type dc motors draw a constant load from startup thru full speed running? can't imagine these motors draw many amps running off a c battery. I was also wondering when you use a resistor to cut voltage down do you also lose amps?
 

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  • #2
jim hardy
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Do these small brush type dc motors draw a constant load from startup thru full speed running?
No.
Current is in proportion to torque and vice-versa.
Speed is in proportion to voltage.

I was also wondering when you use a resistor to cut voltage down do you also lose amps?
Yes you do.

Are you using this in a car ? Or using a "Wall Wart" ?
If the latter look for a 3 or 5 volt phone charger. It won't overspeed your motor so severely.
upload_2018-6-21_6-8-50.png
 

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  • #3
davenn
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I know one can use a LM317 to cut 12 volts dc to 1.5 volts but can one use resistors?
as @jim hardy said, no

do as he suggested and use a 3V wall adaptor and you could put a couple of 1N4001 diodes in series with the positive wire and that will drop ~ 1.4V and you will be close to spot on for your 1.5V requirement

1.5V PSU.GIF





Dave
 

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  • #4
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I seemed to got it to work just went up to 60 ohm resistor used 10 watt only thing I am not sure of is should one put oil on these wire brush as this would make them wear less but probably get dirty faster is this not right? I am running this off house solar power that is why I wanted to do this as have lots of free solar power.
 
  • #5
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If you have 12V then a neat solution would be one of those buck converters that plug into the cigarette lighter and drop the 12V down to 5V USB. You could then feed this into the LM317. Less wasteful than just resistors.
 
  • #6
davenn
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Less wasteful than just resistors.

As already noted, you cannot use "just resistors" anyway :wink:

the OP is going to find that the voltage will not be stable with his resistor
 
  • #7
CWatters
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How much current does the motor draw?

There are plenty of DC to DC converters on eBay that will turn 12v to 1.5v.
 
  • #8
berkeman
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I am running this off house solar power that is why I wanted to do this as have lots of free solar power.
Solar power and running off of the grid is all about efficient conversion of energy. The advice you are getting about switching DC-DC power converters is good. This is a good project to learn more about those, IMO. :smile:
 
  • #10
russ_watters
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I am running this off house solar power that is why I wanted to do this as have lots of free solar power.
I know this is a different topic, but since you brought it up, that's a very surprising thing to say. Didn't you pay for the solar system? A lot? Generally solar systems are very expensive so maximizing efficiency, as @berkeman said, is typically critical to the economic success of the system. We'd be delighted to help find ways to maximize the economics of your system if you want.
 
  • #11
jim hardy
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Ahhh, solar. So that's why 12 volts...

You might take apart one of those solar yard lights.
They typically have a single rechargeable AA cell .You'll relocate that to the rear of the clock. AA and C are same length so you can make it fit , or get a new holder..
Run small wires out to the photocell that you've salvaged and ingeniously mounted where it gets full sun. Doorbell wire will be perfect, telephone wire would work too.

It's low voltage so there's not a code issue, just keep it away from any AC mains wires. Don't share conduits or pass-throughs with them.

EDIT - And don't make an unintended lightning rod of it ...

old jim
 
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You might take apart one of those solar yard lights.
Great suggestion. You can buy those things for at little as $1 each.
 
  • #13
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When they said resistor is not going to work that sure was right I had to go back to the way it was using carbon c battery only thing I added that seems to work is a put a 10 watt 600 ohm resistor in between 12 volt dc and battery it only raises battery voltage .01 of a volt I think this should be perfect as battery voltage with 12 volts applied thru resistor was at 1.61 volts. I tried 1000 volt resistor didn't seem to raise battery voltage and 300 ohm raises battery voltage .02 of a volt. Once in a while it stops I am always thinking it is the motor brushes not making contact but this does have a magnetic reed switch I might have to put a LED on motor terminals so when stops running motor so I can tell if power is getting thru switch to motor cause just because you hear switch snap does not mean switch has closed. Is a magnetic reed switch a lifetime type of switch or can contacts go bad?
 
  • #14
Tom.G
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Is a magnetic reed switch a lifetime type of switch or can contacts go bad?
They are pretty good as to lifetime, but a motor load is rather nasty. If the clock still reliably works on that single "C" cell the switch is OK.

If you do run into switch life problems, try this: Wire a 0.1 uF capacitor in series with a 100 Ohm resistor then wire the combination across the switch contacts. For this use, any cheap capacitor, like a disc ceramic, will do and the resistor can be quite low power, ¼ Watt and ½ Watt are the most available.
 
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Seems to work this way
 

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I know this is a different topic, but since you brought it up, that's a very surprising thing to say. Didn't you pay for the solar system? A lot? Generally solar systems are very expensive so maximizing efficiency, as @berkeman said, is typically critical to the economic success of the system. We'd be delighted to help find ways to maximize the economics of your system if you want.
Have 465 watt solar panels
Have 300 watt inverter
Have 26 car batteries
Have spent $500
 
  • #18
russ_watters
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Have 465 watt solar panels
Have 300 watt inverter
Have 26 car batteries
Have spent $500
Ok! A quick calc tells me that depending on where you live you might get the money back in 3-6 years (if you don't waste it). That's quite good.
 
  • #19
sophiecentaur
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might get the money back in 3-6 years
Very much depending on Latitude and average Cloud Cover. My 40W (Nominal) Panel, spends most of its daylight time delivering around 6W or so.
 
  • #20
Using a step down transformer 12 V can be cut down to 1.5 V. In a step down transformer number of turns in secondary windings is less than the number of turns in primary windings.
 
  • #21
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Ok! A quick calc tells me that depending on where you live you might get the money back in 3-6 years (if you don't waste it). That's quite good.
Just in time to replace those 26 car batteries! :)

Off grid is expensive, and has ongoing costs. If there is no alternative, OK. But as a means to save money? probably not.
 
  • #22
berkeman
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Using a step down transformer 12 V can be cut down to 1.5 V. In a step down transformer number of turns in secondary windings is less than the number of turns in primary windings.
That only works for AC. The OP seems to be asking mainly about DC power. That's why we have suggested a DC-DC "Buck" converter. :smile:
 

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