Hey,
I need a dc source of about 250mv at about 250ua. I can't seem to find any way to regulate a dc source down that low and it remain stable. My input is 5vdc. Anybody got any ideas.

 

If it's a steady load, a simple resistive divider from a stabilized 3.3V would work, as long as losses were acceptable.

 

A typical follower should work:
A 1k pot fine tunes the output voltage.

 

Thanks Michal, I'm going to have to source some of those parts before I can try it. Is that "10p" as in pico over the capacitors?

 

Thanks Michal, I'm going to have to source some of those parts before I can try it. Is that "10p" as in pico over the capacitors?

Both caps are 10u. You can use higher values like 47u since you deal with small voltages.

 

Got it, thanks.

 

The TL431 gives 2.500 V reference to a basic resistor divider having R1 and R2
The divider R1 = 25.42k
R2 = 2.80k + 10Ω adj gives 250 mV or ( approx 2.83K)
Voltage Divider Calculator - Engineering Calculators & Tools (allaboutcircuits.com)
The op amp voltage follower itself should cover 250uA

 

Let’sbe clear here. The proposed circuit does not limit the current, in case that is what the TS wants.

 

I'm going to have to source some of those parts before I can try it.

If you choose a different op-amp, look for one what works with the inputs near the negative supply voltage. In the attached picture you can see the inputs work from Vcc- to 2V below Vcc+. You could also use and op-amp that is labeled "inputs R-R" Which indicates the inputs were designed for work from Vcc- to Vcc+.

I would look for a low voltage part. There are many parts that love a 5V supply.
I think sparky 1 is right. You do not need the transistor on the output. The op-amp will work without any help from the transistor.

 

I'm going to have to source some of those parts before I can try it.

You don't need the 1k base resistor. You can substitute a low voltage zener diode for TL431.

If you have NPN and PNP transistors, you can try this (from Motorola):

You don't need the Darlington. This circuit can be modified to not use the differential amplifier.

 

The 5V supply is stable why not use it as a reference. R1&R2 divide down using standard 1% values.
If you don't trust your 5V supply them add a TL431.
I just picked a R-R op-amp without looking at its values.

If you do not trust the 5V, then add a TL431 and resistor to make a good 2.54V.

 

I ordered the parts before I saw these other options, so I'm gonna go with Michal's schematic, but thanks to everyone.

Let’sbe clear here. The proposed circuit does not limit the current, in case that is what the TS wants.

I'm not trying to limit the current.

 

Hello again,
I'm back on this project. Using Michal's original schematic, can I use a CA1558 in place of the LM358?

 

Hello again,
I'm back on this project. Using Michal's original schematic, can I use a CA1558 in place of the LM358?

No. Read the datasheets. CA1558 can't operate from a 5V supply.

 

Hey,
I need a dc source of about 250mv at about 250ua. I can't seem to find any way to regulate a dc source down that low and it remain stable. My input is 5vdc. Anybody got any ideas.

When you say, "about", just how "about" is that? Ask yourself what the min and max acceptable voltage actually is?

What is the min and max current draw, including peak transient currents that you need the voltage to stay within min and max?

How important is efficiency in your DC source? If it draws 25 mA, but meets your output requirements, is that acceptable? If not, what is the maximum current draw that is?

How stable is your 5 VDC input? What is the min and max that it can fall between?

The answers to these questions largely dictate how simple or how complicated your circuit needs to be. It's quite possible that a simple two-resistor voltage divider, perhaps with one or two capacitors, will meet your needs just fine. Or you might need to use a high quality voltage reference and a feedback regulator circuit to get the needed performance.

As always, the devil is in the details of what is actually needed to solve the actual problem at hand.

 

I'm back on this project.

You still haven't provided enough detail.

You can design a supply to provide 250mV or one that will supply 250uA of current, but you can't control both.

What is the load? Does it draw a constant current? What are your tolerances for voltage and current?

If the load doesn't vary, a simple voltage divider will work if your 5V source is well regulated.

To elaborate on my previous post (I didn't have a photo editor when I posted earlier):
With a 5V supply, that CA1558 isn't guaranteed to function.
The guaranteed input voltage range is NIL.
The guaranteed output voltage range is NIL.

It was designed to operate from +/-15V supplies.

 

The Circuit on brown paper used 1K on the TL431 it was followed with an LM358 comparator
I simulated the circuit found it was sort of larger source modified but capable higher scale. Some of the questions seem legitimate.
What about a zener? Why not a TL431? 2.5V ok I 'll try Since it is a good question, try to leave a circuit for reference.

TL431 as low current source. R2 is about 515Ω which allows sufficient current. Another circuit can draw 250uA from it.
The comment : " not constant current" is valid statement for this also. The nature of this zener like configuration.
The circuit can be current adjustable, The output voltage is constant over the range 250uA asked for.
The Voltage output could be adjusted with R6 while R2 is limiting to about 250uA (adjusting R6 means adjusting R2)
One application might be to vary the low current for a specific transistor, oscillator, sensor of that range and scale.
Many times a higher current will cause excess heat, This could be the beginnings of something precise. An LM7805 might help.
The circuit would allow someone to work at 250mV or below, easily modified to supply sub mA current needs. 250uA or less

 

It may also work to use a shunt regulator, which in this case wold be the forward drop of schotky diode at some current level.