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

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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 R145%00 and R215%00 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

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

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

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
S Keith wrote:

** 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

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

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.

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
snipped-for-privacy@gmail.com pretended :

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.

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

---
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

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
On Wednesday, February 25, 2015 at 9:39:54 AM UTC-7, John Fields wrote:

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

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
S Keith used his keyboard to write :

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.

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
On Wednesday, February 25, 2015 at 4:31:09 PM UTC-7, John G wrote:

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

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
S Keith wrote:

** You are being far to literal.

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

** That is a very big PWM fan.

High time you supplied a link to the thing.

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

You are contradicting yourself not making sense.

...  Phil

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
On Wednesday, February 25, 2015 at 10:15:00 PM UTC-7, Phil Allison wrote:

ply 3V, 2mA

If I could link it for you, I would know more about it.  It's the cooling f
an for a Honda Civic Hybrid battery pack.  It's a "squirrel cage" blower ty
pe fan.  It blows air into the trunk thereby creating a vacuum in the seale
d case, which then draws cabin air in through the battery pack and the rela
ted 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 th
e "experimental" input and outcome.

Steve

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
On Thu, 26 Feb 2015 06:39:34 -0800 (PST), S Keith

---
Steve, forget the regulator for the time being.

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

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
On Thursday, February 26, 2015 at 10:59:15 AM UTC-7, John Fields wrote:

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 t
he output of the multimeter at that resistance setting was 3V/2mA.

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
S Keith wrote:

** Figured it was something weird and automotive.

How do you know it is a PWM fan ??

** OK - so we FINALLY  get to know where your 3V, 2mA nonsense came from.

The spec sheet for a DMM.

And it does not mean what you assume.

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.

** You do not know what is needed.

So neither do we.

....  Phil

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
On Thursday, February 26, 2015 at 4:28:35 PM UTC-7, Phil Allison wrote:

Phil,

Since you're clearly not willing to read what I write as evidenced by your response (I said nothing of any DMM spec sheet), I'm going to disregard any of your future responses.

I'm not sure what you're getting from your participation in a group labeled "basics".  You remind me of why I stopped using Usenet back in the day (started in the late 80s).

Simply put, you're an arrogant asshole. I've seen it time and time again in dozens of groups covering as many topics. Take comfort in your false sense of superiority.  I'm far kinder to those ignorant of my areas of expertise.

Steve

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

** No, I am simply not willing to BELIEVE anything what you write.

Cost it is obviously all crap.

** No, they all show I read what you posted very carefully.

** Looked just like you were quoting from one though.

Why the secrecy ?

Why refuse to post where those two numbers came from ??

You must be some sort of anal nut bag.

** No you won't.

** To find an shoot down trolling nut cases like you - buddy.

> > Simply put, you're an arrogant asshole.

** Fraid the REAL arrogant ASSHOLE is YOU  !!

I've seen fuckwits like YOU, time and time again in dozens of groups covering  many topics.

Instead of posting all the facts ( so others would know what they actually have and what they are really doing ) smartarse anal nut cases like YOU hide all the facts.

They foolishly imagine doing this is the way to control the discussion and make it go their way.

Got news for you pal - it don't work like that.

No poster can control the discussion on such a public forum.

However, that is just what TROLLs all try to do.

Consider yourself thoroughly outed.

...  Phil

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

He is an arrogant asshole but he knows WTF he is talking about. Some people jusr want others to do well in electronics.

Indeed, why is he here ? He certainly needs little help if any. Think about that.

Well I mean help in electronics. Psyche on the other hand... ... ...

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
On Friday, February 27, 2015 at 6:34:04 AM UTC-7, snipped-for-privacy@gmail.com wrote:

beled "basics".  You remind me of why I stopped using Usenet back in the da
y

le jusr want others to do well in electronics.

ut that.

Indeed. I'm not contesting his expertise. I'm questioning his motivation. G
iven the expertise, I see only two real motivations in this situation, 1) h
e wants to contribute his knowledge, or 2) he wants to demonstrate his supe
riority. The bulk of the data thus far supports #2.

His type is pervasive. They are lacking something in either themselves or t
heir lives and they fill the hole with, "Oh yeah, I'm smarter than you!"

Thanks again to all who have helped.

Steve

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

** Why are YOU here ???

Your idiot Q is way off topic, it's got nothing to do with "basic electronics" and is not answerable.

Only one explanation is possible - you are here to amuse yourself and that means you are TROLLING.

What you don't have the brains to see is the ONLY person currently on this NG who is NOT trolling is me.

....  Phil

Re: Using a 12V 2A power supply direct and with 3V voltage divider?
On Thu, 26 Feb 2015 17:19:18 -0800, S Keith wrote:

Steve, an essential USENET skill (any-"NET" for that matter) is to just
ignore the trolls.  Phil oscillates between being a valuable resource on
audio circuits and an incensed troll.  No amount of talk will get him down
from the trees when he's chosen to go there, so you need to just ignore
him.  Or bait him on purpose, if it makes you feel better.  But trust me
-- many people have tried talking sense to him, and all it does is get him
worked up.

He's:

--> very trustworthy on audio stuff

--> less reliable on regulatory and "that'll burn your house down" stuff

--> not at all reliable when you can tell that there's spittle hitting
the computer screen.

--

Tim Wescott
Wescott Design Services
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