Convert 12 VDC batteries to 8.4VDC

[even]

i want to convert my 12VDC Battery 7.5 A to 8.4V because i want to use it to my camcoder pls.
I was thinking about using voltage divider but can cause drop of voltage
and i was thinking about using lm317 but the lm317 Ampere is 1.5A pls help me

[Guest Alun]

Use the LM317 and add a few transistors to boost the output as shown on the datasheet.

http://www.angelfire.com/electronic/hayles/downloads/lm317.pdf

These transistors will heat up a lot and it won't be very efficeint to I recomend you a switching regulator, there are many chips availble:
http://www.st.com/stonline/books/pdf/docs/4269.pdf
http://www.stanford.edu/class/ee122/Parts_Info/datasheets/LT1170.pdf
http://cache.national.com/ds/LM/LM2594.pdf

[audioguru]

A camcorder doesn't draw 7.5A, the battery is capable of providing 7.5A to a load that draws that much. An LM317 by itself with a small heatsink is fine.  ;D

[Guest Alun]

;D Silly me, I knew that I just mis-interpreted the post I shall try to read posts more carefully from now on.

[audioguru]

Hi Alun,
Wait a minute. Maybe the camcorder has a really bright 50W light on it. ;D ;D

[even]

the camcoder output adaptor is 8.4V 1.5A

[even]

sorry my electrical is poor i need help to design this project

[even]

how about this schematic

[audioguru]

Hi Even,
Are you sure that your camcorder has a powerful bright light so draws 7.5A?

I thought that you wanted 8.4V for it. Your circuit with a 2N3055 emitter-follower boosting the current of a 7808 will have an output voltage of only about 6V, because the base-emitter voltage of a 2N3055 could be as high as 2V at 7A. Your circuit's output voltage would increase maybe to 7.2V when you turn off the light.
This circuit will provide 8.4V at up to 7.5A or more:

[Guest Alun]

This ciruit will require a small mimimum load in the region of 100mA or else the voltage will rise off load.

[ante]

Hi Guys,

I would go for a switcher for this one.  8)

[audioguru]

Hi Alun,
The MJ3055 has a max leakage current of only 0.7mA with 30V across it, lower leakage with this application and the LM317 has plenty of output current through the 120 ohm resistor and pot. Why does the circuit need more minimum load current? ???

Hi Ante,
I don't think the video and audio circuits in a camcorder will like the noise from a switcher. If the car batt is 14.4V, with a 7.5A load this linear reg circuit will dissipate 45W. Good to keep Even and his camcorder nice and warm in winter. ;D

[Guest Alun]

My mistake, strike my comments from the record. ;D

audioguru,
Switchers are often used for battery powerd audio circuits these providing they're properly screened and filtered the noise isn't that bad. My PC has a switching power supply and the and the sound card still sounds very good. ;D

[audioguru]

Hi Alun,
1) We don't know if the camcorder has enough filtering to be used with a switching supply.
2) This will take care of your comments:

[ante]

Hi Audioguru,

I don’t know about you but I always use output filtering on my PSU:s! Could be fine or coarse but a filter for the project that is to be feed.
BTW, I have two camcorders (Sony & Panasonic) and both have switcher type supplies and I can’t remember ever seen any linear ones for any video device yet!   

[Guest Alun]

I have a sneeky feeling that this camcorder will work of anywhere between 7.5 and 15V so it might be ok to just connect it straight to the 12V supply but I'd read the manual and maybe look on the website before trying this.

[even]

hi thankyou so much. i think there is misunderstanding my camcoder not need 7.5 A but it is a capacity of battery like motorcycle battery. i use itu because i want using my camcoder for long time, my camcoder battery is only 1.1A and it can be approx. 80 minutes so i choose to convert my 12V 7AH battery to voltage level of camcoder.

my camcoder general spec:

        power requirements : 8.4V (1.5A)
        power consumtions on recording (LCD) : 5.9 W

my question is :
  is it enough if i only use adjustable voltage regulator with lm317T like this schematic

[audioguru]

Hi Even,
It makes a big difference that your camcorder doesn't need a 7.5A power supply circuit. ;D
An LM317 is rated between 1A and 1.5A so might not be enough. An LM350 is rated at 3A and will work fine. It will dissipate about 9W so will need a medium-size heatsink.

[ante]

Don’t burn your fingers on it!  ;D ;D ;D

[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

[ante]

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)

[audioguru]

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.

[audioguru]

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

[ante]

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)

[Guest 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