is a smps ok for a pic

[Thunderchild]

I have a 5 V 1 A SMPS is it ok for powering pics or is a 7805 regulator best ?
the SMPS was supplied as a power unit for a portable HDD so I suppose its ok for digital applications its output actually measures 5.2 V according to my meter

 

[aussiepoof]

An SMPS is just fine. I have designed a PIC board that has a built-in SMPS using a Nat Semi Simple Switcher(tm) IC. I needed to be able to feed the board from a 15vac source and the 7805 was just getting way too hot dropping 13vdc.

Make sure you sprinkle .1uF monolythic capacitors (the little blue ones) at the PIC power pins to catch any high frequency noise from either the SMPS or the PIC. A tantalum cap of 22uF or 33uF might be a good idea too, though I've not needed one.

P.

 

[dknguyen]

As long as it's regulated and you have the proper decoupling capacitors. Just be aware it's a bit noisier and not as fast to respond to transient power demands so it may not be as good for analog circuits or may need a bit more decoupling capacitors than with a LM7805.

A LM7805 (or any linear regulator realy) is better when you don't need the efficiency and want it to be cheaper and smaller with less noise with faster transient response. The SMPS only wins out in efficiency (but sometimes that's a HUGE thing!).

 

[Thunderchild]

well as I alreadly have the SMPS its even cheaper than buying a 7805 that my supplyer is not stocking right now and yes I do like efficiency

 

[Hero999]

Is it a mains powered SMPs or one you've made with a transformer plus SMPS IC?

 

[Mike - K8LH]

An SMPS is just fine. I have designed a PIC board that has a built-in SMPS using a Nat Semi Simple Switcher(tm) IC. I needed to be able to feed the board from a 15vac source and the 7805 was just getting way too hot dropping 13vdc.

Make sure you sprinkle .1uF monolythic capacitors (the little blue ones) at the PIC power pins to catch any high frequency noise from either the SMPS or the PIC. A tantalum cap of 22uF or 33uF might be a good idea too, though I've not needed one.

P.

That's an appealing solution that would allow me to use almost any of my assortment of power bricks instead of just the one 9 volt brick that I have.

Can you elaborate, please? Is it a relatively small circuit that I could potentially use as a 'standard' regulator circuit on my future projects?

Mike

 

[aussiepoof]

That's an appealing solution that would allow me to use almost any of my assortment of power bricks instead of just the one 9 volt brick that I have.
Can you elaborate, please? Is it a relatively small circuit that I could potentially use as a 'standard' regulator circuit on my future projects?

Mike, it is a very small and simple circuit: it consists of the National Semiconductor Simple Switcher IC (in my case an LM2595, TO-220 with 5 pins), a schottky diode, an inductor and a capacitor.

The switcher IC will take a DC input of 2V greater than you want as an output, so about 5V for a 3.3V output and 7V for a 5V output, up to a input max of 40Vdc. It comes in 3.3V, 5V, 12V and adjustable versions (1.2V to 37V) which output 1A. Other members of the Simple Switcher family have outputs of 3A etc.

In my application with an input voltage of 18-21VDC and a 5V output drawing 200mA the device gets just above room temperature *without* a heatsink. Not needing a heatsink also saves quite a bit of space. The other great advantage is that cool electronics last a lot longer

So yes you could use any DC brick you have with an output between 7 and 40Vdc provided you make sure the polarity is right. I would put a bridge rectifier in front so that even when you forget to check the polarity it will still work! In that case with the diode drop in the rectifier you'd want to use a 9Vdc or greater brick, and of course you could also use AC bricks if you added a filter cap after the rectifier (as I have done.)

P.

 

[Mike - K8LH]

So it seems I'll be busy downloading some data sheets today (grin).

Thank you Sir.

Mike

 

[eng1]

In the National Semiconductor site there is an useful **broken link removed** that helps in designing a DC-DC converter with their chips and in selecting suitable parts. (you have to be registered).

Another option might be the MC34063A. I like it because it comes in a DIP8 package.

 

[Thunderchild]

its a wall wart smps it was meant to power an external HDD so I guess its made for ditital stuff I've also ordered a nation semiconductor SMPS IC that uses 2 condensers, 1 inductor and 1 skotky diode

 

[dknguyen]

That's an appealing solution that would allow me to use almost any of my assortment of power bricks instead of just the one 9 volt brick that I have.

Can you elaborate, please? Is it a relatively small circuit that I could potentially use as a 'standard' regulator circuit on my future projects?

Mike

You can buy something like it (a ~7805 footprint SMPS replacement in a fixed voltage, variable step-down voltage, or variable step down and up) from www.dimensionengineering.com for like $15-$20. But that's me. I'm lazy and don't like building tools to use in my projects of interest. I prefer working on the projects themselves.

 

[Thunderchild]

well for 5 £ (10 $) I get the components to do my own its still 1 A but if memory serves the 3 A version costs no more

 

[Tarsil]

About switching reg..see this:
https://www.romanblack.com/smps/smps.htm
might work...

 

[Thunderchild]

hm what power was that capable of ? I've seen it before but as far as I remeber it weren't powerful enough or something (nothing to do with pics)

 

[Tarsil]

like .5A claims Roman. I'l stay below 250mA thought. The picture of the scope's screen sugest that he built it and worked.

 

[Thunderchild]

yea it looks pretty good

 

[aussiepoof]

You can buy something like it (a ~7805 footprint SMPS replacement in a fixed voltage, variable step-down voltage, or variable step down and up) from www.dimensionengineering.com for like $15-$20. But that's me. I'm lazy and don't like building tools to use in my projects of interest. I prefer working on the projects themselves.

The folks at Sparkfun www.sparkfun.com also have such things. They are great for one-off projects.

I built mine because it's part of an overall design of which I'm currently making 60 units ;-)

P.