# Using a 12V 2A power supply direct and with 3V voltage divider?

Started by February 24, 2015
```I have forgotten everything I learned in the single electrical science class I took in college 25 years ago.  Please forgive my ignorance.

I have a single 12V 2A power supply.  I want to simultaneously power a 20W 12V fan and supply a 3V 2mA current via a voltage divider.

The fan would be connected direct to the +/- of the PS while a voltage divider with R1=4500 and R2=1500 would provide the 3V, 2mA current to a second device.

Since the PS can supply 25W total power, I don't see a problem.  Am I missing anything?

Thanks,

Steve
```
```<stkeith11@gmail.com> wrote in message
>I have forgotten everything I learned in the single electrical science
>class I took in college 25 years ago.  Please forgive my ignorance.
>
> I have a single 12V 2A power supply.  I want to simultaneously power a 20W
> 12V fan and supply a 3V 2mA current via a voltage divider.
>
> The fan would be connected direct to the +/- of the PS while a voltage
> divider with R1=4500 and R2=1500 would provide the 3V, 2mA current to a
> second device.
>
> Since the PS can supply 25W total power, I don't see a problem.  Am I
> missing anything?

I think you can forget the r2- 1500 ohm resistor.  The R1-4500 ohm resistor
would drop the 12 volts down to 3 volts at 2 ma.  If you parallel the 2 ma
load with 1500 ohms you will have less voltage.  YOu would drop 10.5 volts
across the 4500 ohm resistor and only have 1.5 volts across the load and
1500 ohm resistor.

If the load changes the voltage will change also.  While it will be bigger,
for just a couple of dollars you can get some buck voltage reducers from
China off ebay.  They are adjustable and seem to hold the voltage constant

```
```S Keith wrote:
>

> I have a single 12V 2A power supply.
> I want to simultaneously power a 20W 12V fan
> and supply a 3V 2mA current via a voltage divider.
>
> The fan would be connected direct to the +/- of the
> PS while a voltage divider with R1=4500 and R2=1500
> would provide the 3V, 2mA current to a second device.

** You need a 3V supply capable of delivering 2mA or more.

Connect 300ohms to +12V and 100 ohms to 0v and the mid point is 3V.

The source resistance is then 75ohms, so 2mA will only cause 0.15V drop.

...  Phil

```
```On Tue, 24 Feb 2015 14:06:41 -0800 (PST), stkeith11@gmail.com wrote:

>I have forgotten everything I learned in the single electrical science class I took in college 25 years ago.  Please forgive my ignorance.
>
>I have a single 12V 2A power supply.  I want to simultaneously power a 20W 12V fan and supply a 3V 2mA current via a voltage divider.
>
>The fan would be connected direct to the +/- of the PS while a voltage divider with R1=4500 and R2=1500 would provide the 3V, 2mA current to a second device.
>
>Since the PS can supply 25W total power, I don't see a problem.  Am I missing anything?

---
A voltage divider or just a single series resistor isn't a stable
supply since the input voltage to the load will vary with changes in
the raw supply voltage or changes in the load current, so you might
want to try something like a:

John Fields
```
```>"Connect 300ohms to +12V and 100 ohms to 0v and the mid point is 3V."

You are wasting 40 mA ! You Austrtalians think the world is made out of resources don't you ?

You're almost as bad as us.
```
```jurb6006@gmail.com pretended :
>> "Connect 300ohms to +12V and 100 ohms to 0v and the mid point is 3V."

> You are wasting 40 mA ! You Austrtalians think the world is made out of
> resources don't you ?

> You're almost as bad as us.

It will work though and is at level the OP can understand
Unlike JFs answer which ofers a data sheet to an IC that is most likely
way out of the OPS realm to understand much less build.
JF too often does this just to show off that he knows more than the
resrt of us. :-?

--
John G Sydney.
```
```"John Fields" <jfields@austininstruments.com> wrote in message
>> ---
> A voltage divider or just a single series resistor isn't a stable
> supply since the input voltage to the load will vary with changes in
> the raw supply voltage or changes in the load current, so you might
> want to try something like a:
>
>

It depends on how stabel the voltage needs to be.  If for only an indicator
light, probably not too stable.  Where a more stable voltage is needed, I go
to the China adjustable regulators from ebay such as this one for less than
\$ 2.50 shipped.

Ebay number 301506417461

```
```On Wed, 25 Feb 2015 19:10:27 +1100, John G <john.g@green.com> wrote:

>jurb6006@gmail.com pretended :
>>> "Connect 300ohms to +12V and 100 ohms to 0v and the mid point is 3V."
>
>> You are wasting 40 mA ! You Austrtalians think the world is made out of
>> resources don't you ?
>
>> You're almost as bad as us.
>
>It will work though and is at level the OP can understand
>Unlike JFs answer which ofers a data sheet to an IC that is most likely
>way out of the OPS realm to understand much less build.
>JF too often does this just to show off that he knows more than the
>resrt of us. :-?

---
Perhaps you give the OP too little credit.

In any case, it's only a baby step from the data sheet to Digi-key's
part selector where there's a part number for the fixed 3-volt
regulator in most any package you'd like.

Sorry if the leap was too much for you.

John Fields
```
```On Wednesday, February 25, 2015 at 9:39:54 AM UTC-7, John Fields wrote:
> On Wed, 25 Feb 2015 19:10:27 +1100, John G <john.g@green.com> wrote:
>
> >jurb6006@gmail.com pretended :
> >>> "Connect 300ohms to +12V and 100 ohms to 0v and the mid point is 3V."
> >
> >> You are wasting 40 mA ! You Austrtalians think the world is made out of
> >> resources don't you ?
> >
> >> You're almost as bad as us.
> >
> >It will work though and is at level the OP can understand
> >Unlike JFs answer which ofers a data sheet to an IC that is most likely
> >way out of the OPS realm to understand much less build.
> >JF too often does this just to show off that he knows more than the
> >resrt of us. :-?
>
> ---
> Perhaps you give the OP too little credit.
>
> In any case, it's only a baby step from the data sheet to Digi-key's
> part selector where there's a part number for the fixed 3-volt
> regulator in most any package you'd like.
>
> Sorry if the leap was too much for you.
>
> John Fields

All, thank you for your respective inputs.

John,

Thank you for going one step further and giving me some credit.  I successfully simulated the voltage divider circuit in Qucs and modeled the fan with a resistor.  The resulting change in the voltage and current at the divider was concerning.

My lack of experience is painfully evident; however, when I'm pointed down a path and there are resources available, I can usually work through it.

The linked component looks like it's exactly what I need, and the price is right.  The TO-92 package seems most practical.  Please confirm my interpretation:

PS 12V+ to IN
PS - to GND
LP2950 out + and GND provides 3V, 100ma provided input is > 600mV and < 30V.  I assume the wattages much match, i.e., 0.3W minimum.

This would allow me to use the same 12V PS to drive the fan provided it can supply the minimum watts and voltage to the VR.

How could I pull the current down to 2mA? A resistor would drop the voltage, right?

Thanks,

Steve

```
```On Tue, 24 Feb 2015 14:06:41 -0800, stkeith11 wrote:

> I have forgotten everything I learned in the single electrical science
> class I took in college 25 years ago.  Please forgive my ignorance.
>
> I have a single 12V 2A power supply.  I want to simultaneously power a
> 20W 12V fan and supply a 3V 2mA current via a voltage divider.
>
> The fan would be connected direct to the +/- of the PS while a voltage
> divider with R1=4500 and R2=1500 would provide the 3V, 2mA current to a
> second device.
>
> Since the PS can supply 25W total power, I don't see a problem.  Am I
> missing anything?

If you need a steady 3V, you probably want to use a voltage regulator.  If
you're willing to mail-order, any of the usual suppliers will have lots of
choices available.  Just make sure it can stand 12V in -- I can't imagine
you'll have any trouble finding one that can dissipate the 18mW caused by
drawing 2mA.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
```