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'[EE] Selecting a Power Supply'
2006\01\04@155604 by Mauricio Jancic

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Hi,

           Recently, a customer asked me to make him a product. It's a gas
sensor that's connected to the wall wart in the user's house, so, the input
voltage is 220VAC.



           Some of the product was already there and working so, the
customer, asked me just to change the MCU and digital part of it, leaving
the PSU alone. The PSU is a simple center tap transformer (which will
remain) + a half wave rectifier + a LM78L05 (TO-92). The transformer is a
9+9 Vac.



           The thing works fine. It also has a temp sensor to monitor the
temperature inside the case. Now, the customer wants to be able to power the
device with 12Vdc, 24Vdc or 220Vac (only one of them at a time) so the PSU
needs to be modified.



Initially he made the modification by adding a LM7812 on the input (only
when using 24Vdc) but the result was an over heated case because of the 1.68
Watts on the TO-220.



           Oh! Input current is max 250 mA at DC input, 60~80 mA of those
250 mA are used on the 5V branch. I use part of the circuit at 5V and the
rest at 12V. The circuit can work with only 5V. I was using part of the
circuit at 12V to decrease the current at the 78L05, not that I need 12V.



           Some of the main components that need to be supplied:

-          PIC

-          2 Leds

-          1 LCD + Backlight

-          1 gas Sensor (~ 150mA average with peaks of 800mA )

-          1 12V relay (I can use a 5 Volt relay)

-          1 Buzzer with buit in oscilator



           Now, I *think* I can make a SMPS and that would be better than
the linear regulators, but I would like to hear you opinions about it.



*        What's cheaper?
*        What's better?
*        I think I could use a buck regulator, is that correct?
*        Olin, on his quick proto, uses a 10F to generate de 5.5V and then an
LDO to get the 5V. Why did he (you, if you are reading) use the LDO?
*        Can't you just use the 10F??



           Any other comments or references will be very appreciated.



Regards,



Mauricio Jancic

Janso Desarrollos

Microchip Consultant Program Member

spam_OUTinfoTakeThisOuTspamjanso.com.ar

http://www.janso.com.ar

+54 11 4542 3519



2006\01\04@164641 by olin piclist

face picon face
Mauricio Jancic wrote:
>             Now, I *think* I can make a SMPS and that would be better
> than the linear regulators, but I would like to hear you opinions
> about it.

Sure sounds like it.

> * Olin, on his quick proto, uses a 10F to generate de 5.5V and then an
> LDO to get the 5V. Why did he (you, if you are reading) use the LDO?
> * Can't you just use the 10F??

You mean just use the buck regulator directly?  Yes, you can if you just
want to run a PIC.  I used an additional LDO for better accuracy, less
noise, and better transient response.  I figured some applications will use
the PIC A/Ds, and with a good clean and accurate 5V supply you can get away
without a separate A/D reference voltage much of the time.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

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