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Old 11th October 2006, 07:48 PM   #1
bigphil is offline bigphil  Canada
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Default Voltage Regulation from a battery

Hey guys, I know this isnt audio related but I think that a few of the EEs around could answer my question in 5 seconds.

I'm trying to put together a project for school. I have a 12V (nominal) battery, and need to power 5V motors with the least amount of wasted energy possible (obviously). I know that a 5V regulator can do the job, but I have also learned about step-down dc-dc converters and have found a few chips that would do the job as well. Example: LT1108. DOes anyone know what would be my better option? the regulator would be powering a PWM setup with mosfets. Seems simple enough, but there are phDs written on the topic. Thanks in advance.
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Old 11th October 2006, 07:59 PM   #2
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If efficiency is your main goal, then DC to DC conversion method will win by quite a margin, esp. when you realise a 12v lead-acid is about 13.8v fully charged. Even a crude switched-mode supply will better 80% efficiency, whereas the linear reg cannot improve upon (5/13.8)*100, or 36%. HTH.
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Old 12th October 2006, 04:36 AM   #3
CBS240 is offline CBS240  United States
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Perhaps maybe a step down buck converter might be simpler and acomplish the same goal? How much current is involved here?
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Old 12th October 2006, 04:46 AM   #4
johndiy is offline johndiy  Greece
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regulator
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Old 12th October 2006, 05:12 AM   #5
bigphil is offline bigphil  Canada
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around 500mA I think... its for a small rover.
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Old 12th October 2006, 05:46 AM   #6
clem_o is offline clem_o  Philippines
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Default Re: Voltage Regulation from a battery

Quote:
Originally posted by bigphil
the regulator would be powering a PWM setup with mosfets. Seems simple enough, but there are phDs written on the topic. Thanks in advance.
If that's the case, don't bother with the regulator. You can always set the PWM to a maximum of 5V/12V = 41.67%.

That will give you maximum efficiency at the same time. Such limiting can be built into the code of the microcontroller (if it's to be servo-controlled by such).

Just make sure that the PWM frequency is fairly high so that the motor inductance naturally smooths out the pwm pulses...

Cheers
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Old 12th October 2006, 06:24 AM   #7
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Why don't you check Simple Switcher from National. You'll need only a few parts.
http://www.national.com/pf/LM/LM2675.html

The efficiency approx. 90%, not extremely high but better than a linear regulator.
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Old 13th October 2006, 05:23 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
Clem has hit it straight off in post6.

Don't bother with intermediate voltage control.

Let the PIC do the whole job and limit the mark:space as suggested.
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Old 13th October 2006, 11:24 PM   #9
clem_o is offline clem_o  Philippines
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You can even go beyond 46% for short intervals, giving you extra 'grunt' when accelerating/stopping... I can guarantee it works - students in mobile robotics do it regularly.... :-)

Cheers!

Clem
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Old 14th October 2006, 03:44 PM   #10
bigphil is offline bigphil  Canada
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Thanks guys, I will give it a try.
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