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Author Topic: power supply design help  (Read 706 times)

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Offline aruna1Topic starter

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power supply design help
« on: December 10, 2011, 07:40:36 AM »
Hi
I'm trying to make a usb NiMH charger based on PIC 12F683 and a constant current source. (To charge 2*AAA NiMH batteries from laptop USB port)

12F683 is used to measure the battery voltage and battery temperature using its ADC pins.
I need to use the Vdd of 12F683 as the reference voltage of its AD converter. I cannot use separate Vref because I have use all other pins of the microcontroller.

Problem is When I change the current of constant current source voltage of USB port begins to drop. and when current is about 150mA it becomes about 4.1V. So using Vdd as reference to the ADC is not going to do any good unless I have fixed Vdd value.

Therefore I need to make some kind of regulator circuit that will deliver constant 4V or 3.5V output with about 40mA from varying input of 5V to 4.1V.

I tried zener regulator and TL431 but they didn't do much good, output of them tends to change as input voltage varies.

Can some one help me to make a voltage regulator that meets above requirements.

(I cant use any other PIC than 8 pin 12F683 because I need to finish it as small as possible. Only problem achieving this task is the voltage regulation problem.)

Thank you
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Offline Soeren

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Re: power supply design help
« Reply #1 on: December 11, 2011, 04:50:31 PM »
Hi,

An LDO regulator will do nicely. You can get a 3.3V LDO or a variable version - Micrel produce some very good LDOs (extremely low drop).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
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Offline aruna1Topic starter

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Re: power supply design help
« Reply #2 on: December 11, 2011, 08:13:53 PM »
problem is finding a part number of a LDO which I can obtain from local market.
I can find lot of LDO numbers from different manufacturers but only few of them is going to be in the local market. It would be nice if you know any common and popular LDO number
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Offline Soeren

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Re: power supply design help
« Reply #3 on: December 13, 2011, 06:40:49 PM »
Hi,

problem is finding a part number of a LDO which I can obtain from local market.
Well, you're the only one local to your local market and as such, the only one who can find out.
If you ask for an LDO complying with your voltage/current requirements, someone should be able to deliver.


I can find lot of LDO numbers from different manufacturers but only few of them is going to be in the local market.
How many different types do you need?


It would be nice if you know any common and popular LDO number
Short of LM2940 (National Semiconductor), which is one of the oldest and so should be "everywhere", I don't know what is considered standard in your hood.
If some of the local stores have online shops, you could try searching for "LDO" or just zip through their voltage regulator section.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline aruna1Topic starter

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Re: power supply design help
« Reply #4 on: December 13, 2011, 08:20:48 PM »
hi
i must say doing electronics in sri lanka sucks, coz there are no online shops, local shops dont have databases and most of workers dont know parts. lol
I cant take long list of LDOs to the shop and ask "hey do you have any of these parts?" they will ignore me.
Unless you know specific part number which is available, local shops are no good

I once searched for LM2940 couldnt find it a local shops said they are bringing new stocks about in week so come back in about week to buy it. But they never did :D

I'm going to look in to ebay for some LDOs.
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Offline Soeren

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Re: power supply design help
« Reply #5 on: December 13, 2011, 08:27:29 PM »
Hi,

The world is a strange place... A lot of semiconductors are produced in Sri Lanka :)

If you cannot find an LDO, you can always build one from an op-amp and a transistor.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline aruna1Topic starter

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Re: power supply design help
« Reply #6 on: December 13, 2011, 08:29:20 PM »
LOL
and we also export tea . top quality ones which never available in local market  :D ;D
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Offline Soeren

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Re: power supply design help
« Reply #7 on: December 13, 2011, 10:54:51 PM »
What... No nice cup of Ceylon Tea for you?  That's IS crazy - You should be able to get it for free!

Incidentally... In Denmark, LEGO is more expensive than everywhere else... And it's a Danish product. In Sweden, Norway, Germany and Great Britain (our neighbors), it's from 20% to 50% cheaper and since the Danish market only counts for a few percent of their sale and the fact that practically all Danes have had LEGO at some point in their childhood, I consider it an act of evil misplaced greed.

And they know they're doing wrong, as they hide the price info if you go to their US site from Denmark (and using proxies to counter that is unfortunately not common knowledge).

Strange world indeed!
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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