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Author Topic: Power supply for PIC projects  (Read 5796 times)
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embromation
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« on: September 06, 2008, 02:38:04 02:38 »

What is a good circuit of power supply for projects with PIC?

To use a LM317 regulated for 5Volts?
To use 7805?

Recently I mounted a circuit with the PIC16F628A and one display LCD 16x2 in 7805, but when I tie the backlight of the display, 7805 warms up very much. (even with the heat sink - 100mA of the circuit)

He would like knowing of an economically viable solution.

Thanks. Wink
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DINESHjp
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« Reply #1 on: September 06, 2008, 02:55:52 02:55 »

u can use a 22 ohms to 33 ohms  register for back light
the vcc for backlight can be given directly (unregulated) using a suitable value register
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Silent_Thunder
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« Reply #2 on: September 06, 2008, 09:54:25 09:54 »

use 7805 regulator for supplying the whole circuit, and as DINESHjp said supply the back light b4 the regulator, and even if it is not stable it wont hurt the back light Smiley
and be sure to place a .1uf along with every IC's power and as close as u can,and one for sure right after the regulator's output.
Regards
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j0k3r
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« Reply #3 on: September 06, 2008, 12:11:32 12:11 »

What is the voltage on the 7805? Remember, High Vin->hot problem->less current.

I suggest a Switching Regulators they are in many forms, fixed or adjustable, from 100mA to 3Amp, or more.........
I use sometimes in my projects the LM2576. Give a look on manufactures page, they have more.....

Cumps.....
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Walkura
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« Reply #4 on: September 06, 2008, 08:49:09 20:49 »

When you also use the analog inputs its advisable to use the lm317.
At work i abandoned the 7805 cause of the poor regulation .
Although you can set the reference voltage for the pic processor you will always lose 1 input with this setup .
With a small to220 radiator that 100 or even 200 mA shouldnt be any problem .
(that is when the voltage you put in aint very much higher then the output 8 to 12 volt would be fine)
Below ~7.5 or 8 Volt you would get a ripple increasingly bigger with the current cause of the to low voltage difference .
If you use a higher input voltage a switcher would be more efficient .

Good luck Smiley
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MAXPAYNE
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« Reply #5 on: September 07, 2008, 01:00:35 01:00 »

Thank you, Grin
however would you have a circuit that uses few components efeciente for power suplly 5Volts?

I need convert 13Volts to 5Volts with few components, withou problem of hot.


Have a look at MAXIM. They have tons of switcher......
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TomJackson69
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« Reply #6 on: September 07, 2008, 02:04:54 02:04 »

Thank you, Grin
however would you have a circuit that uses few components efeciente for power suplly 5Volts?

I need convert 13Volts to 5Volts with few components, withou problem of hot.


Hi Embromation,

The simple and chippest is still the 7805  voltage regulator. The spec said, it can regulated for 5 Volts at one Amp. In your case, you use 13 volts input and get 5 volts out put; that is 8 volts drops accross the 7805. If you draw only 100 mA than the power disipated on the 7805 is 8 V x 0.1 = 800 mWatts. Which is not much heat at all. Remember, the 7805 can operate up to 125 degree C (junction temperature). If heat is your concern, use 9 volts unregulated input instead of 13 volts, that will drop down the wattage on the 7805 (4 V x .1 = 0.4 Watts). As you can see, the more current you draw out of the 7805, the more heat it will dissipate to air. You will not destroy the 7805 with 1Amp, just use heat sink. The 78xx have the Thermal Resistance Junction-Cases (TO-220) of 5 C^/Watt, and Thermal Resistance Junction-Air (TO-220) is 65 C^/W.

I used 22 ohms for the LCD back light on 5 volts (very bright, Hantronix 16x2, blue).

Just to make you feel safe, here is a quick calculation for the case temperature:

13 Volts input - 5 Volts output = 8 Volts drops
8 x 1 = 8 watts (assume the circuit used 1 Amp)

Case temperature = 25^C + (8W x 5^C/W) + 3^C = 68 ^C

68^C is very warm but still safe. If we don't put a heat sink on the 7805 and use the circuit for a long time, heat will build up because the body of the 7805 is small so the heat will not dissipated to air fast enought and thermal run away will occursed; and the 7805 will be destroy. If we use a heat sink than the temperature will keep at 68^C (more or less depends on room temperature).

note: ^C is degree C

Regards,

Tom
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Dembo
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« Reply #7 on: September 07, 2008, 10:47:07 10:47 »

I use 12 to 5 V vehicle charging adapter for Nokia cellular, it's MC34063 based, it have good efficiency and can keep up to 1A. I changed the inductor to larger size, but probably original one is also OK.
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Silent_Thunder
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« Reply #8 on: September 07, 2008, 11:38:21 11:38 »

Hi Embromation;

Attached to this post a simple circuit that u can use, if your circuit does not comsume more than 100ma use 78L05 if u consume more use 7805.

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« Reply #9 on: September 07, 2008, 03:19:20 15:19 »

If you make some system for noisy environment, it is better to use switching power supply than linear with transformer. Switching power supply has filters and are projected to suppres frequences that come from switching, so they will suppres external noise that come via power lines by itself, you can put some extra LC factors after supply unit to ensure stability. 100nF decoupling capacitors are necesary near Vdd (even with LCD display, because LCD controller can suffer because noise).

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zed
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« Reply #10 on: September 07, 2008, 03:20:09 15:20 »

Thank you, Grin
however would you have a circuit that uses few components efeciente for power suplly 5Volts?

I need convert 13Volts to 5Volts with few components, withou problem of hot.


You are asking for efficiency and low dissipation. That is switching power supply by default :-) I myself use MC34063 with very good result. I just used the original schematics found in the datasheet, it is very easy and only need a few component for 5V/500ma, look at page 7:

http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF


OTOH, I found this on ebay, maybe it is of your interest, as it is cheap:
http://cgi.ebay.es/MC34063-Based-Switching-Regulator-Adapter-Step-Down_W0QQitemZ350095978700QQihZ022QQcategoryZ66990QQssPageNameZWDVWQQrdZ1QQcmdZViewItem



Regards.

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« Reply #11 on: September 08, 2008, 03:19:27 03:19 »

I also use MC34063 - never have problem - and with buck/boost circuit, you can have a good variety of input voltages too.
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« Reply #12 on: September 08, 2008, 08:36:23 20:36 »

Today I found on the Microchip Site this files:

Tranformerless Power Supply 5v - for PICs

http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf

http://ww1.microchip.com/downloads/en/AppNotes/91008C.pdf
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pickit2
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« Reply #13 on: September 14, 2008, 11:24:34 23:24 »

Today I found on the Microchip Site this files:

Tranformerless Power Supply 5v - for PICs

http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf

http://ww1.microchip.com/downloads/en/AppNotes/91008C.pdf
please take extreme care if you use these, in a project that user can touch live side of circuit.

you could also look here >>
http://www.powerint.com/en/products/product-archive/linkswitch

you can get the transformers from a dead panasonic microwave control pcb.
local repair shop throw them at you Smiley (thats where I get mine)
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Wizpic
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« Reply #14 on: September 14, 2008, 11:35:47 23:35 »

Yes I would be very careful on building the microchip PSU I've read some bad things about them

you can find a lot about one from here which is sold in a product.

http://www.picbasic.co.uk/forum/showthread.php?t=3265&highlight=transformerless
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« Reply #15 on: September 15, 2008, 04:13:28 16:13 »

Linear voltage regulators have very low efficiency 20% or less.
Better choice is MC34063 as mentioned above or LM2675. Or to feed directly from mains - LNK306G -
very good , i've implemented it in several projects , highly recommended  Smiley.
« Last Edit: September 15, 2008, 04:17:57 16:17 by jenya7 » Logged
beque
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« Reply #16 on: September 15, 2008, 07:51:42 19:51 »

The LM2940 (Dropout) replaces LM78XX series - pin compatible

http://www.national.com/mds/LM/MNLM2940-12-X.pdf
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« Reply #17 on: September 17, 2008, 09:27:56 09:27 »

This is obvious, but did you check the backlight requirements?

maybe you can limit the current consumptions to below 100mA ?
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« Reply #18 on: September 17, 2008, 07:11:28 19:11 »

I usually use the LDO LM2940 or LM1117. So far so good to me.
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« Reply #19 on: January 02, 2009, 01:13:06 13:13 »

Hi,
     Instead of 7805 which is linear type and very inefficient, you can use a switching regulator, such as L4971, which is a 1.5A switching regulator in an 8-pin DIP package from ST Microelectronics. If you require less power, you can use L4976, which is also an 8-pin DIP. The circuit with L4971 is shown below:


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« Reply #20 on: January 12, 2009, 12:28:58 12:28 »

Maybe look out for regulator boards as kits. Here in Germany such where sold at about 5$ complete a while ago..board including parts. 

I also have seen ideas on adding a serial resistor to the 7805 input. In my AVR designs the also get quite hot, as they have to convert the 7 - 8 Volts into heat somehow. If you add a resistor ( killing 5 volts at your current level ) this one gets hot..not the 7805.

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« Reply #21 on: March 21, 2009, 01:25:50 01:25 »

I didn't see that anyone mentioned these:

http://focus.ti.com/lit/ds/symlink/78ht205.pdf

The switching equivelant of the 7805.  Expensive, but handy.
« Last Edit: March 22, 2009, 02:56:55 02:56 by oldvan » Logged

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