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

 

[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

 

[audioguru2]

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

 

[Alun]

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

 

[audioguru2]

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

View attachment 37067

 

[audioguru2]

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:

View attachment 37068

 

[Alun]

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

 

[ante1]

Hi Guys,

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

 

[audioguru2]

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

 

[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

 

[audioguru2]

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:

View attachment 37076

 

[ante1]

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!   

 

[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

View attachment 37090

 

[audioguru2]

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.

View attachment 37091

 

[ante1]

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