# How to get 1.5 volts out of a wall-wart power supply's 3 volt output...I am new to this group I believe anyways...Nice to meet you all!

Started by May 28, 2013
```Hi, I have recently been trying to get 1.5 volts (or anything other than th=
e power supply's rated 3 volts for that matter) out of a wall-wart 3 volt p=
ower supply. The wall-wart power supply is dc in output at 3 volts and even=
if I connect a 300 ohm resistor in series with the wall-wart power supply =
and the load (a 1.5 volt clock, old style analog clock)and when i measure t=
he voltage with the voltmeter in place of the load (but remember the resist=
or is still in series) I keep getting 3 volts dc measured. I even put a 20 =
ohm resistor in parallel with the 300 ohm resister and still no change in m=
easurement. I bought a older style wall-wart with a 1.5 volt setting and it=
measures at 3 volts with the voltmeter in series with the output leads. I =
can't get 1.5 volts (or anything other than 3 volts for that matter) no mat=
ter what I do and if with the older style wall-wart power supply I measure =
at the output leads with that power supply set to 12 volts, etc. the measur=
ed voltage is very high (19 volts for the 12 volt setting). Now, I know tha=
t without a load certain power sources don't measure the same as with a loa=
d but if I measure the voltage (I wish i didn't have to do that as the volt=
age measured was 3 instead of the 1.5 the clock is rated for, I might have =
done tiny damage to the clock for all I know), if I measure the voltage acr=
oss where the battery would normally be it measures at 3 volts! SO I had to=
have been taxing the clock with it running at 3 volts instead of 1.5! Anyw=
ays - can someone shed some light on this? I am at the minimum above averag=
e in electronics skills and this problem I have been having has been drivin=
g me bonkers. The math shows that dropping 3 volts to 1.5 and allowing a cu=
rrent of .08 amps requires 18.75 ohms and I provided that like said above (=
300 ohms in parallel with 20 ohms equals 18.75 ohms) and it still measures =
at 3 volts dc! any help appreciated before I tear my hair out...Thanks, I t=
hink!
```
```"Daniel"
>
Hi, I have recently been trying to get 1.5 volts (or anything other than the
power supply's rated 3 volts for that matter) out of a wall-wart 3 volt
power supply. The wall-wart power supply is dc in output at 3 volts and even
if I connect a 300 ohm resistor in series with the wall-wart power supply
and the load (a 1.5 volt clock, old style analog clock)

** Does this clock normally use a 1.5V, AA cell ?

Does the cell last about 1 year -  assuming Alkaline ?

The math shows that dropping 3 volts to 1.5 and allowing a current of .08
amps requires  ...

** How did you get the 0.08 amps figure ?

A 1.5V, AA cell would last about 1 day at that rate.

FYI:

Typical battery wall clocks consume about 150 micro amps -  or  0.00015
amps. Two resistors of say 1000ohms wired in series ACROSS the 3V supply
will divide the voltage in half.

Add a 47 microfarad, 10 to 25volt, electro cap across the clock and you have
it done.

...  Phil

```
```Bottom posted.

On Monday, May 27, 2013 9:55:24 PM UTC-7, Daniel wrote:
> Hi, I have recently been trying to get 1.5 volts (or anything other than =
the power supply's rated 3 volts for that matter) out of a wall-wart 3 volt=
power supply. The wall-wart power supply is dc in output at 3 volts and ev=
en if I connect a 300 ohm resistor in series with the wall-wart power suppl=
y and the load (a 1.5 volt clock, old style analog clock)and when i measure=
the voltage with the voltmeter in place of the load (but remember the resi=
stor is still in series) I keep getting 3 volts dc measured. I even put a 2=
0 ohm resistor in parallel with the 300 ohm resister and still no change in=
measurement. I bought a older style wall-wart with a 1.5 volt setting and =
it measures at 3 volts with the voltmeter in series with the output leads. =
I can't get 1.5 volts (or anything other than 3 volts for that matter) no m=
atter what I do and if with the older style wall-wart power supply I measur=
e at the output leads with that power supply set to 12 volts, etc. the meas=
ured voltage is very high (19 volts for the 12 volt setting). Now, I know t=
hat without a load certain power sources don't measure the same as with a l=
oad but if I measure the voltage (I wish i didn't have to do that as the vo=
ltage measured was 3 instead of the 1.5 the clock is rated for, I might hav=
e done tiny damage to the clock for all I know), if I measure the voltage a=
cross where the battery would normally be it measures at 3 volts! SO I had =
to have been taxing the clock with it running at 3 volts instead of 1.5! An=
yways - can someone shed some light on this? I am at the minimum above aver=
age in electronics skills and this problem I have been having has been driv=
ing me bonkers. The math shows that dropping 3 volts to 1.5 and allowing a =
current of .08 amps requires 18.75 ohms and I provided that like said above=
(300 ohms in parallel with 20 ohms equals 18.75 ohms) and it still measure=
s at 3 volts dc! any help appreciated before I tear my hair out...Thanks, I=
think!

Hi Phil! Yeah, the clock operates on one AA alkaline battery for about 1 ye=
ar at 1.5 volts. I accidentally lost track while typing and combined two pr=
ojects, one powering a 1.5 volt AA mp3 player with a wall-wart power supply=
and of course the mentioned analog old style 1.5 volt alkaline wall clock.=
The mp3 took .08 amps and as you mentioned the all clock uses a lot less c=
urrent than that. I don't follow your math for the solution but I am very h=
appy with what you said so I can complete this project soon - thanks a mill=
ion! Again, thanks a million Phil! By the way =96 I previously solved the m=
p3 issue by powering it with a wall-wart power supply that has a usb power =
source socket so I am happy with that. Thanks!
```
```<videoman@ccountry.net>
Daniel wrote:

Hi Phil! Yeah, the clock operates on one AA alkaline battery for about 1
year at 1.5 volts. I accidentally lost track while typing and combined two
projects, one powering a 1.5 volt AA mp3 player with a wall-wart power
supply and of course the mentioned analog old style 1.5 volt alkaline wall
clock. The mp3 took .08 amps and as you mentioned the all clock uses a lot
less current than that.

** Only about 530 times less.

** It's really simple.

You need a 1.5V supply that can deliver 150 microamps average. The clock
actually takes short pulses of about 10 times that current every second,
hence the 47uF electro to handle the pulse current.

The two 1000 ohm resistors just split the voltage coming from the wart and
have low enough resistance so the average load current is not enough to make
any difference to this. You can connect the clock & electro across either
resistor, long as the polarity of both is right.

....  Phil

```
```<videoman@ccountry.net>

Yeah, the clock operates on one AA alkaline battery for about 1 year at 1.5
volts.

** Changing to a wall wart will cost much more in electricity than using AA
cells does.

PLUS  if the AC power ever goes off for a while,  the clock will stop and
thereafter show the wrong time.

Till you figure that fact out.

Wot a dumb idea.

...  Phil

```
```On Mon, 27 May 2013 21:55:24 -0700 (PDT), Daniel
<videoman@ccountry.net> wrote:

>Hi, I have recently been trying to get 1.5 volts (or anything other than the power supply's rated 3 volts for that matter) out of a wall-wart 3 volt power supply. The wall-wart power supply is dc in output at 3 volts and even if I connect a 300 ohm resistor in series with the wall-wart power supply and the load (a 1.5 volt clock, old style analog clock)and when i measure the voltage with the voltmeter in place of the load (but remember the resistor is still in series) I keep getting 3 volts dc measured. I even put a 20 ohm resistor in parallel with the 300 ohm resister and still no change in measurement. I bought a older style wall-wart with a 1.5 volt setting and it measures at 3 volts with the voltmeter in series with the output leads. I can't get 1.5 volts (or anything other than 3 volts for that matter) no matter what I do and if with the older style wall-wart power supply I measure at the output leads with that power supply set to 12 volts, etc. the measured voltage is very
high
>(19 volts for the 12 volt setting). Now, I know that without a load certain power sources don't measure the same as with a load but if I measure the voltage (I wish i didn't have to do that as the voltage measured was 3 instead of the 1.5 the clock is rated for, I might have done tiny damage to the clock for all I know), if I measure the voltage across where the battery would normally be it measures at 3 volts! SO I had to have been taxing the clock with it running at 3 volts instead of 1.5! Anyways - can someone shed some light on this? I am at the minimum above average in electronics skills and this problem I have been having has been driving me bonkers. The math shows that dropping 3 volts to 1.5 and allowing a current of .08 amps requires 18.75 ohms and I provided that like said above (300 ohms in parallel with 20 ohms equals 18.75 ohms) and it still measures at 3 volts dc! any help appreciated before I tear my hair out...Thanks, I think!
Put three or four silicon rectifiers in series with the plus lead?
(Depending on the measured voltage output of the wall-wart)  Drop.6
volts per series diode under practically any load.  Signal diodes are
good for ~100 milliamps (and smaller cheaper than rectifiers)

Or you can use the forward voltage drop of an led - something I use
when driving a clock from a processor running at 5 volts (cheap and
dirty way to check long time delays) - in that case the brightness of
the led changes with power draw - blinks once a second with a quartz
analog clock as the load.  Red leds drop ~2 volts, green ~3, white and
blue ~3.4...  load is limited to 20 milliamps with garden variety
leds, and quite a bit higher with high power leds.
```
```Daniel wrote:
> Hi, I have recently been trying to get 1.5 volts (or anything other than the power supply's rated 3 volts for that matter) out of a wall-wart 3 volt power supply. The wall-wart power supply is dc in output at 3 volts and even if I connect a 300 ohm resistor in series with the wall-wart power supply and the load (a 1.5 volt clock, old style analog clock)and when i measure the voltage with the voltmeter in place of the load (but remember the resistor is still in series) I keep getting 3 volts dc measured. I even put a 20 ohm resistor in parallel with the 300 ohm resister and still no change in measurement. I bought a older style wall-wart with a 1.5 volt setting and it measures at 3 volts with the voltmeter in series with the output leads. I can't get 1.5 volts (or anything other than 3 volts for that matter) no matter what I do and if with the older style wall-wart power supply I measure at the output leads with that power supply set to 12 volts, etc. the measured voltage i
s very high (19 volts for the 12 volt setting). Now, I know that without a load certain power sources don't measure the same as with a load but if I measure the voltage (I wish i didn't have to do that as the voltage measured was 3 instead of the 1.5 the clock is rated for, I might have done tiny damage to the clock for all I know), if I measure the voltage across where the battery would normally be it measures at 3 volts! SO I had to have been taxing the clock with it running at 3 volts instead of 1.5! Anyways - can someone shed some light on this? I am at the minimum above average in electronics skills and this problem I have been having has been driving me bonkers. The math shows that dropping 3 volts to 1.5 and allowing a current of .08 amps requires 18.75 ohms and I provided that like said above (300 ohms in parallel with 20 ohms equals 18.75 ohms) and it still measures at 3 volts dc! any help appreciated before I tear my hair out...Thanks, I think!

The wallwart most likely isn't a regulated type and it most likely has
a cap in there, which gives you a higher reading.. But putting a load
on the supply normally brings things down..

If  you start with a 1.5 RMS (AC internally), that equates to ~2Volts
after rectified and filled wit out load. So lets try something different.

For what you're trying to do does not require anything very special.

Using 2 silicon diodes in series will drop your voltage ~ 1.2 volts

for every Si diode in series there is an ~650 mv drop.

use diodes that can handle the current rating..

Jamie

```
```On Mon, 27 May 2013 21:55:24 -0700, Daniel wrote:

> Hi, I have recently been trying to get 1.5 volts (or anything other than
> the power supply's rated 3 volts for that matter) out of a wall-wart 3
> volt power supply. The wall-wart power supply is dc in output at 3 volts
> and even if I connect a 300 ohm resistor in series with the wall-wart
> power supply and the load (a 1.5 volt clock, old style analog clock)and
> when i measure the voltage with the voltmeter in place of the load (but
> remember the resistor is still in series) I keep getting 3 volts dc
> measured. I even put a 20 ohm resistor in parallel with the 300 ohm
> resister and still no change in measurement. I bought a older style
> wall-wart with a 1.5 volt setting and it measures at 3 volts with the
> voltmeter in series with the output leads. I can't get 1.5 volts (or
> anything other than 3 volts for that matter) no matter what I do and if
> with the older style wall-wart power supply I measure at the output
> leads with that power supply set to 12 volts, etc. the measured voltage
> is very high (19 volts for the 12 volt setting). Now, I know that
> without a load certain power sources don't measure the same as with a
> load but if I measure the voltage (I wish i didn't have to do that as
> the voltage measured was 3 instead of the 1.5 the clock is rated for, I
> might have done tiny damage to the clock for all I know), if I measure
> the voltage across where the battery would normally be it measures at 3
> volts! SO I had to have been taxing the clock with it running at 3 volts
> instead of 1.5! Anyways - can someone shed some light on this? I am at
> the minimum above average in electronics skills and this problem I have
> been having has been driving me bonkers. The math shows that dropping 3
> volts to 1.5 and allowing a current of .08 amps requires 18.75 ohms and
> I provided that like said above (300 ohms in parallel with 20 ohms
> equals 18.75 ohms) and it still measures at 3 volts dc! any help
> appreciated before I tear my hair out...Thanks, I think!

Your error comes when you misinterpret the difference between allowing a
current and actually having that current flow.

A 18.75 ohm resistor that has 80mA actually flowing through it will drop
1.5V.  But connecting that resistor, waving your hands over it, and
saying "you may now flow 80mA" won't make current flow in the resistor.

What you're seeing is a voltage source, with a resistor connected, and
practically no current flowing.

All of Phil's practical comments are spot-on, and he hasn't even started
cursing and foaming at the mouth, so he's a useful USENET poster today.

An alternative would be to get a red LED from Radio Shack that's rated
for 1.4 or 1.5V, connect it up with a 150 ohm resistor in series to make
a light, and connect the clock in parallel with the LED.  This will
provide some rather sleazy voltage regulation as well as a visual
indication that the clock is powered (and it'll be yet another
perpetually glowing LED adding to the light pollution in your house at
night -- we all need that).

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
```
```"Tim Wescott"

> An alternative would be to get a red LED from Radio Shack that's rated
> for 1.4 or 1.5V,

** Huh  ???

Red LEDs have forward voltage fro 1.7 to 2.2 volts - depending on current
flow.

The 2 R and 1C solution I posted is the only one certain to work.

But " work"  means increased running cost and far less reliable time
display.

...   Phil

```
```> All of Phil's practical comments are spot-on, and he hasn't even started
> cursing and foaming at the mouth, so he's a useful USENET poster today.
>

Yep, Daniel dodged a bullet.
His Usenet life could have been ruined!

Mikek :-)

```