using pc smps to power a pic

[varunme]

can we use a PC smps (using 5V/3.3V pins) to power a pic, without a voltage regulator or a current limiter ?

 

[tpetar]

can we use a PC smps (using 5V/3.3V pins) to power a pic, without a voltage regulator or a current limiter ?

Why not, but in case that you have good quality of PC PSU. If you suspect in PSU quality use 12V output with 7805 or 3,3V regulators to get stable 5V or 3,3V. Some bad quality PSU products have bad noise and voltage variations.

 

[varunme]

with normal multimeter its steady

 

[tpetar]

Standard unimer have low sampling speed or bad display refresh time, but if PSU have very bad ripple you can see flickering led 7-seg display digits of your project, or such things. My opinion that is best way in that situation to use 12V from PSU and voltage regulators for 5V and 3,3V with appropriate capacitors in input and output stages, and of course standard usage of 100nF closer as possible to VDD/VSS pins as usually.

:wink:



I use very often PC PSU when working on various projects.

 

[Tahmid]

It's best to use the 12V supply and use a 5V regulator such as the 7805. For 3.3V you can use another regulator. For example you can use an LDO powered off the 5V line. You may use the LM2937.

Like mentioned before the power supply may suffer from noise issues and voltage variations. And obviously you don't want some "small" spike to damage your PIC or the voltage variations to affect the rest of the circuit. Better safe than sorry.

As for the current limiter, there's no need for that.

Hope this helps.
Tahmid.

 

[varunme]

One more question, can we charge a 12V, 7Ah battery from a PC smps, I have a lot of spare pc smps here, so i asked

 

[tpetar]

Its possible but on special way. You need to boost voltage from 12V to battery charging voltage.

See example:




My advice is better go to real battery charger.


Also if you plan to make battery charger, consult the manufacturer product datasheet about needed charging voltage and max allowed currents.

With that posted circuit I successfully charge car battery from another car battery (just for testing).

;-)

 

[Tahmid]

One more question, can we charge a 12V, 7Ah battery from a PC smps, I have a lot of spare pc smps here, so i asked

Boost up the voltage to a level where the battery can be charged. A simple way is to use float/trickle charging. For this you usually need around 13.5V, but do consult the manufacturer's datasheet if available. So you can use a circuit to boost up the voltage to 13.5V and you can keep the battery connected without worrying about damage. You can also implement a multi-step charging algorithm. However if you don't require relatively quick charge, this is unnecessary.

You can use either the 12V or 5V input for the boost converter (just an idea)! Just be careful with the circuit design.

Hope this helps.
Tahmid.