Can one power a Linear regulator from a SMPS output

[StudentSA]

Hi All,

I have a nice 50W Enclosed type SMPS unit that outputs 12V DC. Would there be a problem if I wanted to tap off a 5V supply using a 7805 from this output?

Kind Regards,
StudentSA

 

[Reloadron]

No, just make sure you use adequate heat sinking. Remember 12 - 5 = 7 volts so you have 7 volts times the current for the power dissipated by the regulator. I would also add a .1 uF in and out plus maybe 10 uF in and out depending on your game plan.

Ron

 

[MrAl]

Hi,

I can be done though, but it will waste a bit of power there. For 5 watts used 7 watts will be wasted.
For low output currents however not that much will be wasted. For 0.5 watts used 0.7 watts will be wasted.

A down converter (buck) will help with this if that's what you want to do. Like that for every 5 watts used about 1 watt will be wasted.

 

[StudentSA]

Thanks, so basically the normal rules about power loss in applies.

Lastly, would it be a problem to use another SMPS to step down the voltage to 5V?

So I would have AC -> SMPS(12V) -> SMPS(5V)... better to make sure and avoid an incident.

 

[JimB]

So I would have AC -> SMPS(12V) -> SMPS(5V)... better to make sure and avoid an incident.

If you have an SMPS with a 12volt input, yes you can do that.

JimB

 

[MrAl]

Thanks, so basically the normal rules about power loss in applies.

Lastly, would it be a problem to use another SMPS to step down the voltage to 5V?

So I would have AC -> SMPS(12V) -> SMPS(5V)... better to make sure and avoid an incident.

Yes that's what i meant by a 'buck' circuit which is also called a "down converter". It would convert 12v to 5v with less loss in most cases.
Your buck circuit should also have good input filtering.
You can look up a 'buck' circuit or ask about it here too.

If i was going to power a 100ma circuit with 5v i might just slap a linear on there, but at 500ma to 1 amp i'd have to think about if i wanted to waste that power or not. The additional problem of the linear at higher output currents is the heat sink required would be much larger than with a buck circuit. For the linear you might need a 3x2x1 inch heat sink but for the buck only 2x1x0.5 inch or something little like that.

There's an added benefit to using a buck like this too. That is, you get true power conversion. True power conversion means that you can utilize almost the full power ability of the original power supply.
For example, for a 12v 36 watt power supply using an appropriate linear 5v regulator, the max output current available is 3 amps. With a 5v buck regulator however, you can get about twice that (around 6 amps) with an 80 percent efficient design.

 

[StudentSA]

I don't know why I have not started using SMPS a long time ago! They are pretty awesome!
Thanks for all the invaluable advice it is much appreciated.
Regards,
StudentSA

 

[MrAl]

Hello again,


I think i speak for everyone who replied in this thread when i say, "You're welcome"

We also do buck circuits and related here.