best voltage and current regulator

[steev]

hello all
iam trying to design a voltage regulator ckt which gives output of 3V and 500-700ma of output current.
input is from a led-acid battery rated as 12v-2.5Ah

on net i found below circuit which is for battery charger.
circuit no-4
http://agendawiki.com/cgi-bin/aw.pl?ChargingCircuits

but regrading this voltage regulation some one said

The biggest problem I see is that the efficiency of such a scheme would be terrible. Assuming the full 700ma of current drawn from the battery, the linear regulator would dissipate 6.3 watts while only 2.1 watts would go into the 3 volt load. The overall efficiency would only be 25%. A switching regulator would give you more like 85% efficiency. There are lots of switching regulator designs posted with 3v and 3.3v outputs.

and he shown me this link
http://www.rowan.sensation.net.au/electronics/stepdown.html

and another one suggested L200 based voltage regulator.

iam really confused between these.
can u help me.

 

[ante1]

Hi steev,

This is definitely a switch regulator application. I think you should go for the MAX 724 / 726 that is simple and reliable. With only a few external parts, high efficiency and low quiescent current it’s a good choice.

 

[steev]

thanks ante

iam also planning to design switching regulator only.but most them are using inductors.is there any inductorless regulator.(as i want to keep it as simple as possible)
and i am unable to find schematic for MAX724
iam new to these circuits.
can u help me with the circuit diagram.

(i need to control (lead acid battery)12v 2.5amp to 3v 200-500mah)

thanks once again
steev

 

[ante1]

Hi steev,

http://pdfserv.maxim-ic.com/en/ds/MAX724-MAX726.pdf
On the first page of this data sheet you will find a circuit “Typical Operating Circuit”, you need to change the resistor values of the two resistors in the voltage divider to get the desired output voltage.
If you can’t find the MAX724 perhaps you could get the LT1074 that is almost identical. Here : http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1003,C1042,C1033,P1005
The circuit you see here is very simple and I can’t see why a small inductor would be a problem. 

 

[steev]

thanks ante ur too good man.

one more help i needed here.
iam connecting this power supply (3v 200ma from a 12v 2.5A lead acid battery) to a RF transmitter.is it ok
imean as this circuit uses inductor this might effect the RF.
so what is ur openion.is it ok to go with MAX724 or LT1074
or linear type regulators imean lm317,L200 etc

 

[audioguru2]

A 3V transmitter won't draw much more than only 20mA, an FM stereo one about 30mA. Use an LM317 linear regulator.

 

[steev]

then how can i limit the current if i use lm317 regulator.
as the source current is 2.5amp and transmitter draws 20-30ma(as u said).i think current shld be limited or else it might destroy my small transmitter.
2.5amp to 30ma

 

[audioguru2]

Steev,
An electronic circuit takes only as much current as it needs. The transmitter won't draw more than 20-30mA unless you short its power supply wires.

Your battery is probably rated for 2.5A/Hrs, not just 2.5A. So it can probably provide 25A for a few minutes to a short or very low resistance.

 

[y01jpowell]

NO ;D > ??? :

 

[audioguru2]

NO ;D > ??? :

No what?

The battery is a lead-acid type that has an extremely low internal resistance, so it can supply many amps of current to a short or very low resistance.

The transmitter operates from only 3V so its current is very low. If the transmitter operated from 100V or more then its current would be much higher.

 

[steev]

thanks audio guru
i got it.it takes exactly the amount needed.
so i'll try lm317(as amount of current used is very low.i hope chip will stay coll.but i'll still use a heatsink)

on more thing abt the step down dc-dc converter.it uses(most of them) inductors.so u think this cause any effect on the circuit we r connecting at the output.
imean  if the output is to a fm transmitter....

 

[audioguru2]

so i'll try lm317(as amount of current used is very low.i hope chip will stay coll.but i'll still use a heatsink)

An LM317 won't get warm powering a 3V transmitter from 12V.
Please post the schematic of the transmitter. What are you going to use it for? Have you tried it?

the step down dc-dc converter.it uses(most of them) inductors.so u think this cause any effect on the circuit we r connecting at the output.
imean

 

[steev]

this looks ok to me.i'll replace R1 and R2 with 120,18Ohms resistors.(no VR1).
then i think i'll get 3v output.
what wattage do i need for those resistors?

The LM317 is a linear voltage regulator. It is not a switching DC to DC converter.

yes i know that.iam talking abt those maxim dc-dc converter chips.

 

[audioguru2]

The resistors in the LM317 circuit have hardly any current in them so their power is also very low. I think a 3V LM317 circuit should be like this:

View attachment 38593

 

[steev]

thanks for the schematic.
do this ckt need any overvoltage protection.imean diode across the output.

 

[audioguru2]

Your transmitter isn't going to be a generator so the LM317 doesn't need a protection diode.

 

[steev]

thanks audio guru and ANTE.
u both helped me a lot thanks once again.


i don't want to start another thread for my friends quiry.so iam posting here only.
i hope u will solve his problem also.

he is working on a ckt which drives 6 LED remotely.imean using a RF transmitter(35Mz) and receiver.function is like when transmitter  pressed on 6 LED's mounted on receiver side will glow.receiver uses 3AA cells and transmitter 2AA.
he connected 6 LED's in paraller.so as voltage is above 3v.they r glowing with enough brightness.now the thing is they consuming power(current) quickly.
so he want to know is it possible to boost current like voltage using some charge pumps.so that the LED's can be driven for long time.

i'll be waiting for u r replies
thanks

 

[audioguru2]

Each LED might draw 20mA to 25mA. Therefore 6 will draw up to 150mA which is a lot of current for AA cells. The current would be less if the LEDs were in series or if they are blinking like in my Ultra-bright Chaser projects.

 

[steev]

series means more voltage.thats why we choosed parallel connection.
so is it possible to boost current?