Any adjustable voltage regulator ?...

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Externet
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Any adjustable voltage regulator ?...

Post by Externet » Mon Jul 13, 2009 6:38 am

Hi.
Can anyone direct me to a 0.0 to 1.0 Volt adjustable voltage regulator, 0.5 or more Ampere capable ? LM317 does not go under 1.2v. :sad:
Hmmm... could be a 2N3055 circuit instead of an IC...
Miguel
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Dave Dixon
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Re: Any adjustable voltage regulator ?...

Post by Dave Dixon » Mon Jul 13, 2009 6:44 am

Cant' think of any off hand. Is there room available to make a -1.5 volt rail? If you are using batteries, just pick off midstream for ground? Or else I think Maxim makes some IC's to get a negative supply. Sorry I wasn't more help.

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Re: Any adjustable voltage regulator ?...

Post by CeaSaR » Mon Jul 13, 2009 7:34 am

If you have a circuit that takes the LM317 to 1.2V and feed that through
a silicon diode to the base of a 2N3055, wouldn't that put you at 0 volts?
Then when you adjust the LM317 up the 2N3055 will track the voltage.

Right??

Of course you would only need to feed the '317 with a max of 2.3V (avg.
silicon diode is 0.7V, I think). Of course you'd have to experiment with the
needed resistance, min-max, for the adjust circuit.

Just the basics, I'm sure others have a better understanding of how to
make such a circuit work correctly.

CeaSaR
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MrAl
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Re: Any adjustable voltage regulator ?...

Post by MrAl » Mon Jul 13, 2009 1:51 pm

Hi Miguel,


To get the LM317 to work down to zero volts all you have to do is supply a negative
reference to the bottom of the lower resistor that usually connects to ground.

For example, using the usual 220 ohm resistor from output to ADJ terminal and using
a 200 ohm resistor from ADJ to a reference voltage of minus 1.25v, we get a voltage
approximately equal to 1.1v output. If the 200 ohm resistor value is reduced (aka
a 200 ohm pot instead of fixed resistor) then the output adjusts from zero volts up
to 1.1 volts.

If you dont have or can not include a negative reference, then as someone said you can
connect two 1N4001 diodes in series to drop around 1.4v and that will give you 0v output
when connected in series with the output, but the voltage will vary with load and with
temperature so this is only good when you dont need good regulation.

The minus reference voltage technique still gives good regulation. The reference
voltage source must be capable of handling around 6ma or so.
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sofaspud
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Re: Any adjustable voltage regulator ?...

Post by sofaspud » Mon Jul 13, 2009 2:52 pm

The Nat Semi LM117/LM317 datasheet (DS009063 from July 2004), page 15 shows a 0-30V regulator circuit. You
don't need the 35V input or the 10V negative reference, but it should be fairly easy to modify it to work the way
you want. A ICL7660/MAX1044 or maybe even a 555 are possible sources for the negative reference, but you'd
probably want it regulated also.

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MrAl
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Re: Any adjustable voltage regulator ?...

Post by MrAl » Mon Jul 13, 2009 8:42 pm

Hi sofa,

Yes, the negative supply need to be regulated.
The typical technique is to use any negative source (unregulated is ok as long as it is filtered)
and use a voltage reference diode of 1.25v. This gives -1.25v which is just enough to pull
the output to ground (0v) when the adjustable resistor is at zero ohms. The 220 ohm
feedback resistor is still needed though.
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sofaspud
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Re: Any adjustable voltage regulator ?...

Post by sofaspud » Tue Jul 14, 2009 6:42 am

Yes, I think the physics demands that a negative voltage be included. I don't know what the primary
voltage source is so I listed some converter options off the top of my head. The negative voltage
reference will, by definition, provide the regulation.

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Dave Dixon
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Re: Any adjustable voltage regulator ?...

Post by Dave Dixon » Tue Jul 14, 2009 6:56 am

What choices do you have for your power supply? Wall warts, batteries, or on board with 110AC?
If batteries - a piece of cake.
On-board PS with 110 - another piece of cake.
Otherwise - do you have room for something like the MAX739. It doesn't take many external components and handles .3 Amp.
good luck,
Dave

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Externet
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Re: Any adjustable voltage regulator ?...

Post by Externet » Tue Jul 14, 2009 10:32 am

Hi Dave.
+5V DC supply. The wanted adjustable regulator is meant to simulate a galvanic cell.

Tried the diodes after the LM317 with no success ¿? -but I may have made some mistake, will try again later.
Miguel
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Re: Any adjustable voltage regulator ?...

Post by MrAl » Tue Jul 14, 2009 10:45 pm

Hi again Miguel,


Here is a full schematic of a 0 to 1v regulator. It uses a shunt reference diode but
doent require a negative voltage supply. The three diodes D1 D2 D3 are 1N4001 or equivalent.
Make R1 adjustable from 0 to 180 ohms to get 0 to 1v output.
The input source being 6v seems to be enough for 0 to 1v output, but if you want higher
output you'll have to raise this accordingly. 0 to 2v output would require a 7v input.
Considering an unregulated dc input of 6v with some ripple however, the dips would go
lower than 6v so a higher voltage unregulated supply would be needed. 9v would do it.


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