# 3.7v to 3v?



## Sickie Ickie (Jun 20, 2006)

Quick question that I know has a simple answer from someone.

I have an adapter for a wallwart that outputs 3.7v. I need 3v .

I use a resister to down the voltage, right? What type of resister do I need? Does it matter which polarity I put it on?

Thanks guys!


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## pshort (May 6, 2008)

What is it? A Nokia phone charger?

Some time back I looked at the output of an ACP-7U charger. It turns out that this had an unregulated output, not even an output filter cap if I recall correctly.

The circuit that you need depends on the type of wall-wart, as well as what type of power you need (AC or DC, regulated or unregulated, how much current you want, etc).


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## Dr Morbius (Sep 21, 2004)

I'll bet it's for a Boris? Use the 3V output from an ATX computer power supply, you'll need the current for the other skulls.


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## Sickie Ickie (Jun 20, 2006)

wallwart input to 3.7v dc output. Yes, it's an ACP-7U phone charger.

It's driving a motor that I can get to run on 2 AA batteries (3v), and a quick set up test showed that it can run just attached to this charger without any problem, however I want to slow it down to the 3v.

LOL Nope doc, you lose on this one. Not for this quartet. This is just to drive a small motor for a toy.


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## pshort (May 6, 2008)

Hard to say, since the 3.7V is more or less just a number they throw out, not anything that you can hang your hat on. But as a starting point, find out how much current the motor takes, and divide that into .7V, and use that as the resistor. If the motor is then going too fast, increase the resistor, and decrease it if it's going too slow. As an example, if the motor is drawing 50 mA, you might start with a 15 Ohm resistor, and adjust it up or down as needed.


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## Sickie Ickie (Jun 20, 2006)

does it matter which polarity to put it on?


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## pshort (May 6, 2008)

You mean the resistor? They don't have any polarity, so it doesn't matter.


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## scourge999 (May 4, 2008)

If I remember right, it's bad to underpower a motor. Are you saying that the .7ma is speeding up the motor?


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## Scottzilla (Jun 13, 2007)

The .7v (not ma) is the amount over the voltage rating for the motor. The motor would indeed run faster if powered by 3.7v instead of the 3.0v it is rated at, and depending on how sensitive the motor is it could burn it out eventually. Underpowering is usually better than overpowering. If he uses an even bigger resistor and underpowers it a little bit it won't hurt it.


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## Sickie Ickie (Jun 20, 2006)

Scottzilla said:


> The .7v (not ma) is the amount over the voltage rating for the motor. The motor would indeed run faster if powered by 3.7v instead of the 3.0v it is rated at, and depending on how sensitive the motor is it could burn it out eventually. Underpowering is usually better than overpowering. If he uses an even bigger resistor and underpowers it a little bit it won't hurt it.


yepper. you got my reasoning in a nutshell.


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## randyaz (May 26, 2006)

if you want a truely regulated 3v's... get a voltage regulator. search digikey or mouser electronics for 3v linear voltage regulator. Should cost around $1.50.

also keep in mind that any unregulated supply/wart will at times output much higher voltage than its rateing.


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## Sickie Ickie (Jun 20, 2006)

Thanks, Randy.


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## Scottzilla (Jun 13, 2007)

Here's a 3v 1A regulator at DigiKey:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=MCP1826S-3002E/AB-ND
(Part number MCP1826S-3002E/AB-ND in case the link doesn't work)


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## Sickie Ickie (Jun 20, 2006)

good to know for the future. 

I have to make it as cheap as possible. $20 secret Santa thang. With shipping it will be more, so a resister is the way to go ATM.


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## Koumajutsu (Aug 9, 2006)

just use a diode in series with your load, they drop .5 to .7 volts on average. that should make a pretty decent voltage regulator


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## Sickie Ickie (Jun 20, 2006)

what's a diode?


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## time2dive (Dec 29, 2007)

A diode is basically o one way road sign, it allows voltage to flow in one direction. They also happen to drop .7 volts which should give you your 3 volts. You could also use a variable resistor and adjust the voltage to what ever you want.


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## Moon Dog (Oct 3, 2006)

Koumajutsu said:


> just use a diode in series with your load, they drop .5 to .7 volts on average. that should make a pretty decent voltage regulator


Beat me to the answer... guess that's why you're the train driver and I'm just the tech...


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## Sickie Ickie (Jun 20, 2006)

Thanks guys. Maybe I'll get a chance to get a diode today...


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