Using a 12V 2A power supply direct and with 3V voltage divid

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

 

[Phil Allison]

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

 

[Ralph Mowery]

<stkeith11@gmail.com> wrote in message
news:866d1360-e359-494e-8c5b-3a65cb615ebd@googlegroups.com...

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
under varing loads.

 

[John Fields]

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:

http://www.ti.com/lit/ds/symlink/lp2951-n.pdf

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.

 

[John G]

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.

 

[Ralph Mowery]

"John Fields" <jfields@austininstruments.com> wrote in message
news:50lqeadegqvub4gegmqhdf1ig2scgv0m0e@4ax.com...

---
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:

http://www.ti.com/lit/ds/symlink/lp2951-n.pdf

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

 

[S Keith]

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

 

[John Fields]

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

 

[Tim Wescott]

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

 

[S Keith]

On Wednesday, February 25, 2015 at 4:31:09 PM UTC-7, John G wrote:

S Keith used his keyboard to write :
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

JF, I rest my case. We don't know what the application is but the above
question reveals a basic ignorance of OHMS Law so :-?

--
John G Sydney.

Thanks John G. While my ignorance is vast, I know V=IR. I need to supply 3V, 2mA - not meet the requirement of a 3V, 2mA load.

I have a 12V PWM controlled fan. I don't know anything other than it takes 12V ~1.6A to run it and a 3V, 2mA signal to activate it. I'm trying to accomplish this. If I supply a higher current, I am concerned I might damage something.

Steve

 

[John G]

S Keith used his keyboard to write :

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

JF, I rest my case. We don't know what the application is but the above
question reveals a basic ignorance of OHMS Law so :-?

--
John G Sydney.

 

[Tim Wescott]

On Wed, 25 Feb 2015 13:19:33 -0800, S Keith wrote:

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.

The input voltage has to be equal or greater to the output voltage plus
the dropout. So, over the full temperature range, 3.6V or more. You've
got 12V, so that's not an issue.

I don't know what you mean by "wattages must match". The regulator will
burn up some insignificant amount that it needs for its internal
circuitry, plus (output current) * (input voltage - output voltage). In
your case that's about 18mW. Looking at the numbers for the TO-92
package, you'll see an (insignificant) 3 degree rise from case to junction.

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?

The current drawn by the 3V rail that you make will be determined by
whatever you hang off of it. If you've got something that consumes 2mA at
3V, that's what will get consumed.

For any two-terminal network, the current draw is a function of voltage --
you cannot set both current and voltage externally.

--

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

 

[Phil Allison]

S Keith wrote:


> Thanks John G. While my ignorance is vast, I know V=IR. I need to supply 3V, 2mA

** You are being far to literal.

3V at 2mA MEANS with a voltage supply of 3V, the LOAD draws 2mA.

I have a 12V PWM controlled fan.
I don't know anything other than it takes 12V ~1.6A

** That is a very big PWM fan.

High time you supplied a link to the thing.

to run it and a 3V, 2mA signal to activate it.

** PWM fans normally require a PWM signal to control them.

You are contradicting yourself not making sense.


.... Phil

 

[S Keith]

On Wednesday, February 25, 2015 at 10:15:00 PM UTC-7, Phil Allison wrote:

S Keith wrote:


Thanks John G. While my ignorance is vast, I know V=IR. I need to supply 3V, 2mA

** You are being far to literal.

3V at 2mA MEANS with a voltage supply of 3V, the LOAD draws 2mA.


I have a 12V PWM controlled fan.
I don't know anything other than it takes 12V ~1.6A


** That is a very big PWM fan.

High time you supplied a link to the thing.


to run it and a 3V, 2mA signal to activate it.


** PWM fans normally require a PWM signal to control them.

You are contradicting yourself not making sense.


... Phil

If I could link it for you, I would know more about it. It's the cooling fan for a Honda Civic Hybrid battery pack. It's a "squirrel cage" blower type fan. It blows air into the trunk thereby creating a vacuum in the sealed case, which then draws cabin air in through the battery pack and the related electronics.

I "discovered" that I can activate the fan with a multimeter that puts out a 3V, 2mA current. I wish to duplicate this to drive the fan as I thought it would be a simple solution compared to figuring out how to supply a PWM signal.

I do not know the components involved or their limitations. I only know the "experimental" input and outcome.

Steve

 

[S Keith]

On Thursday, February 26, 2015 at 10:46:10 AM UTC-7, John Fields wrote:

---
I don't know what you mean by: "the wattages must match."

You don't need to do anything but connect the regulator's input to
+12V and its output to your 3V load, and whatever current your load
wants - up to 100mA - the regulator will supply and keep the input
to the load at 3V.

John Fields

I meant that since the VR puts out 0.3W, I must meet that input plus a little for inefficiencies. With 12V/2.1A PS, that shouldn't be an issue.

It's probably been lost in the mess, but I'm trying to simulate what amounts to a PWM signal, so I need to cut the current. Reading up on computer fan PWM control suggests 3.3V/5-8mA max if this fan is analogous.

Steve

 

[S Keith]

On Thursday, February 26, 2015 at 10:59:15 AM UTC-7, John Fields wrote:

Steve, forget the regulator for the time being.

How did you use your multimeter to arrive at 3V 2mA?

I was probing the PWM/tach wires of the fan with my multimeter set to sense 200 Ohms. 12V was supplied to power, but PWM/speed were disconnected. To my shock, the fan spooled up. I checked the multimeter with another, and the output of the multimeter at that resistance setting was 3V/2mA.

 

[S Keith]

On Thursday, February 26, 2015 at 11:59:32 AM UTC-7, Ralph Mowery wrote:

I am guessing that you do not understand the voltage current relationship.

The load wihch in your case is 3 volts and 2 ma (assuming it is not
changing) is the equal of 1500 ohms. YOu do not need to limit the current
as that is automatically done if you only supply 3 volts. By simple math,
if you have 3 volts, then the only current that can flow is 2 ma. Also by
the same math if 2 ma is flowing only 3 volts will cause that.

Just plug in 1500 ohms for the 3 V, 2 ma load in the formulars you are
using.

That is why I said all you need is one resistor if the load is a constant 3
V, 2 ma. You are starting with 12 V so 12-3= 9. YOu must drop 9 V at 3 ma.
So 9/.002 = 4500 ohms.

I understand that much, but I got hung up on the concept of a voltage divider. Thanks for the clarification.

Steve

 

[John Fields]

On Wed, 25 Feb 2015 13:19:33 -0800 (PST), S Keith
<stkeith11@gmail.com> wrote:

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?

---
I don't know what you mean by: "the wattages must match."

You don't need to do anything but connect the regulator's input to
+12V and its output to your 3V load, and whatever current your load
wants - up to 100mA - the regulator will supply and keep the input
to the load at 3V.

John Fields

 

[John Fields]

On Thu, 26 Feb 2015 10:31:01 +1100, John G <john.g@green.com> wrote:

S Keith used his keyboard to write :
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

JF, I rest my case. We don't know what the application is but the above
question reveals a basic ignorance of OHMS Law so :-?

---
All it reveals is that he's confused and needs a little help, not
insults, to get on track.

In the same vein, it seems you're also confused about a series
resistor "working" as a ballast, especially with your admission of
being ignorant about the OP's application.