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Volt regulator & Battery


steveggz
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Hi. I would like to know to be on the safe side, is there any problem with having a fixed voltage regulator after a battery?
I connected 11 AA NiMH 2500mAh batteries in series to get a higher voltage but I would like a different fixed voltage. If I connect a voltage regulator (connected with the recommended capacitors) after the battery would there be a problem?

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Think about it, why do you think it could cause a problem?

How could it possibly cause a problem?

A regulator is just a load (like anything else) so it can only do one thing and that's discharge the battery, the only thing I strongly recommend is you put the power switch before the regulator because otherwize it'll flattern the battery even when the power's off due to it's quesant current.

Oh and don't use a LM317 on a battery circuit, if you can use a low dropout reglulator you can by ICs or build your own.

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I would strongly recommend the LT1038 over the LM338 because it's a low dropout regulator, if you can't get hold of one then I'll show you haw to make your own with a P-channel MOSFET and a few other componants that'll have an even lower dropout. ;D

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I would strongly recommend the LT1038 over the LM338 because it's a low dropout regulator, if you can't get hold of one then I'll show you haw to make your own with a P-channel MOSFET and a few other componants that'll have an even lower dropout. ;D


Alun, if you have an equivalent circuit, please post. There are many members here on this forum who do not have access to many of the newer or more sophisticated chips. I think it would be helpful to many.

MP
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I bought and assembled the parts yesterday. I used a "mfr. part# LD1085V12" for the voltage regulator. It has low dropout and is rated for 3A 12V.
I used the recomended capacitors and a switch before the regulator.
It works great. I have 11 nimh batteries in series that are rated 2500ma. They now output a constant 12V. My Magnavox Camcorder requires 12v 2A so im in good shape.
The camcorder runs a lot longer than it did with the older lead acid battery.
I also bought a nice battery charger to charge the rechargeable batteries.

When I ordered these parts online I also bought a "mfr. part# LD1085V". Its also a low drop out, 2.85v to 30V 3A adjustable. I would like to make another battery pack for another older camcorder I have.
I would like to use the "LD1085V" to make the required voltage for that camcorder. It uses two resistors in the diagram to get the fixed voltage.
I posted a picture of the diagram. Can someone please tell me what "Vref" means?

post-13145-14279142468941_thumb.gif

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Hi Steve,
It was difficult to spot the Vref in the datasheet but it is 1.25V just like an LM317. Therefore the resistor to ground is 1032 ohms for a 12V output.
The max dropout of an LD1085 is 1.5V at 3A, or 1V at 2A. Your 11 cells will have a voltage of about 13.2V so the regulator just barely makes it. ;D

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index.php?action=dlattach;topic=3462.0;i

Vref can be a zener diode connected to +V via a resistor or a bandgap reference like the LM113.

The op-amp could be a 741 though I'd recommend something faster like the MC34071 for improved transient response.

TR1 can be any P-channel enhancement MOSFET like the IRF9530, IRF6215, IRF9131, the dropout voltage will depend on the on resistance.

post-0-14279142469016_thumb.gif

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Thanks for correcting my typo, I forgot the all important 0, I have now edited it. ;D

I also forgot to say that when you're powering this circuit from a a higher voltage than 20V you need to connect an 15V to 18V zenner between the gate of the MOSFET to +V to protect. I know the gate voltage should never exceed 20V anyway because the op-amp should stop it but you never no what it'll do when you turn on the power.

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