Convert 12 VDC batteries to 8.4VDC

[even]

what is difference between 3A adjustable regulator (image above) and this picture (i found it in datasheet LM350) thankyou very much.. ;D

the 3A adjustable regulator use R 120 and Rtrim 2K even the C1 and C2 is different with this image. sorry my english is bad....

i wanna know pleasseeee...... thankyou very much thankyou

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[ante1]

Have a look here: http://pdfserv.maxim-ic.com/en/ds/MAX724-MAX726.pdf

Why convert almost half the energy from the 7.5Ah battery to heat? On page one is a suitable circuit (Typical Operating Circuit), just set the voltage divider to the desired output voltage. Can you do this simpler or with lower thermal loss or lower part count?
You can get he MAX 724 free as sample from Maxim!  8)

 

[audioguru2]

Hi Even,
The premium LM117 and LM150 cost much more than the LM317 and LM350. Their max operating current (5mA) is half the amount of the cheaper ones (10mA) so can use resistors that are twice the value, without the regulator's output voltage rising without an external load.

I changed the 1uF tantalum output capacitor to a 47uF electrolytic one on my post for the LM350 circuit because tantalum capacitors aren't reliable and are expensive. National says in the datasheet that an electrolytic output capacitor of at least 25uF is the equivalent to a 1uF tantalum one.

Without a capacitor across the trimpot, the ripple at the output of the regulator is low.

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[audioguru2]

Hi Ante,
It is odd that Maxim don't give line, load and mains ripple rejection spec's for their switching regulators. Maybe they aren't very good and they don't want anyone to see them.

A voltage regulator circuit would operate cooly and have excellent spec's if a switching regulator is used as a pre-regulator then followed by a linear regulator. ;D

 

[ante1]

Hi Audioguru,

I think you may be exaggerating the ripple and noise problems with switchers. I don’t see the problem with filtering, it has worked fine since early VCRs. Anything you buy today in VCRs DVDs and any kind of video cameras all have one thing common, they all get their power from switchers! Only cheap wall warts have (best case) linear regulation and most of them non what so ever. From TV-sets to cellular phone chargers they all runs from switched mode power supplys. The only equipment I can think of that uses the transformer/rectifier PSUs (50/60Hz) are audio amplifiers and even then there are exceptions. OK maybe some odd alarm clock radio too. ;D ;D
BTW. The switched pre regulator you describe is commonly used and works fine in laboratory power supplys and other applications as well. 8)

 

[Alun]

Hi Ante,
It is odd that Maxim don't give line, load and mains ripple rejection spec's for their switching regulators. Maybe they aren't very good and they don't want anyone to see them.

You've forgotten this is been run off a battery so mains ripple rejection isn't a factor. ;D

If you're worried about ripple on the output add a low EST tantalum    10uf capacitor followed by a 10uH inductor series and a 100nf capacitor in paralell.

A voltage regulator circuit would operate cooly and have excellent spec's if a switching regulator is used as a pre-regulator then followed by a linear regulator. ;D

Yes, that will solve both of the problems with a small loss in efficiency,  the MIC29150/29300/29500/29750 range has only a 370mV dropout voltage at full load, you could set the switcher to 8.9V and the linear to 8.4V, and you'll have a very good regulator but it won't be cheap.  ;D

 

[ante1]

Hi tnk2k,

They are alternatives if you don’t have the energy to do it yourself! ;D

 

[Alun]

I agree with you tnk2k.

I'd just like to make a point about switching regulators, they are often cheaper than linear regulators because you don't need a large heatsink which is often the most expensive part of a linear regulator.

 

[ante1]

Hi Alun,

That’s a good point. Another thing about switchers is that they just need a few external parts and the complete circuit is very small in size and weight. A good example is a PSU I recently purchased, 13.8V at 20A which is one third the size of the one it replaced. The old one was a linear 13.2V at 15A and 6.5kg the new 1.4kg and it runs cool too.  8)