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 under varing loads.

** 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

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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

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.

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. :-?

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John G Sydney.

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

--
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

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

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.

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.

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

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

** 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

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

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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

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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.

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

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

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

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.