# EEVblog Electronics Community Forum

## Electronics => Beginners => Topic started by: SwampRat on March 14, 2016, 09:44:30 am

Title: What determines PSU wattage?
Post by: SwampRat on March 14, 2016, 09:44:30 am
Greetings!

Total noob here.  I'm currently in the "Wow! I have a heck of a lot to learn stage." In fact sometimes I feel like I'm thinking in circles.

I have been looking at regulated power supplies (not benchtop, I potentially need to power a DC motor for a project).  The thing that has me puzzled is wattage ratings. What determines the wattage rating in a power supply.  I mean what hardware inside changes to increase or decrease wattage.  I assume amp rating and wattage rating are related, but there does not seem to be a fixed correlation. Yes I said assume, I'm willing to accept the assumption could be dead wrong wrong.

Okay so given that voltage is the same:
What physical difference is there between an Xwatt PSU and a >Xwatt PSU.
Are watts and Amps direct functions of each other.
While I'm at it, what changes amp ratings?

Hopefully I am asking the correct questions...

Thanks for the patience

Swamp
Title: Re: What determines PSU wattage?
Post by: IanB on March 14, 2016, 10:14:25 am
All other things being equal, it is physical size.

You can make an analogy with many other things. For example, car engines. Bigger engines are more powerful than small engines when both engines are of the same type. However, if one engine is normally aspirated and the other engine is turbo-charged, then the smaller engine may be more powerful.

Thus it is with power supplies. If two PSUs are of the same basic design, then to get more power you have to make it bigger. But for example, if one PSU is based on a normal mains transformer and the other one is a switched mode supply, then the switched mode supply could easily be much smaller and lighter.

Designs can also become more advanced over time. So for instance, modern PSUs are often smaller than older ones, even if they are both switched mode designs. This can be achieved by using a higher switching frequency or other more advanced features.
Title: Re: What determines PSU wattage?
Post by: Shadetreeprops on March 14, 2016, 10:44:42 am
simple explination

Volts times Amps = Watts

12Vdc x 19.5A = 234 Watts

if you need to do it for Amps divide, same for finding volts..if you know Watts and amps, or watts or volts...
Title: Re: What determines PSU wattage?
Post by: SwampRat on March 16, 2016, 09:00:43 pm
Thanks. I understand Volts x Amps = Wattage.  I should have remembered that.  I'm pulling out the Noob card.  8)

No replacement for displacement! Not just for cars.  If I understand correctly, using a beefier transformer will allow for more amperage.  What I'm trying to understand is if you have two switching power supplies from the same company and both have the same voltage, what components will allow one to only put out say 10 amps and the other to put out 100 amps?  I'm guessing you will need bigger caps, better wiring, better heat dissipation, etc., to allow for more amps, but what determines/sets the output difference?
Title: Re: What determines PSU wattage?
Post by: joe 90 on March 16, 2016, 09:08:03 pm
If you try to suck too many amps out, one of 2 things can happen.
The fuse might blow.....eventually it will .
The voltage will drop.
A bench top power supply might be rated at 13.8 V and 10A...........138W
If you draw more amps then the voltage will drop a % below that 13.8....whatever that % is, it depends on how it's rated.
The internal electronics might get too hot over time, might fail before the fuse blows.
Title: Re: What determines PSU wattage?
Post by: tautech on March 16, 2016, 09:19:57 pm
Consider also LOSSES.

This depends on design, be it linear or SMPS.
Linear PSU losses are mostly heat from the regulating pass elements and this can amount to many Watts.
SMPS PSU's on the other hand have switching losses, how much they might be is determined by the design and its efficiency.

In order for losses to not affect intended usage you're well advised to overspec your PSU for voltage and current, IMO by 25%.
Title: Re: What determines PSU wattage?
Post by: hayatepilot on March 16, 2016, 09:24:45 pm
What I'm trying to understand is if you have two switching power supplies from the same company and both have the same voltage, what components will allow one to only put out say 10 amps and the other to put out 100 amps?  I'm guessing you will need bigger caps, better wiring, better heat dissipation, etc., to allow for more amps, but what determines/sets the output difference?
Yes, bigger caps, bigger coils, more/bigger switching transistors, bigger rectifiers, better cooling, etc.
The maximum wattage is usually limited by thermals. Rated wattage is the point at which the supply can safely operate without overheating.

Greetings
Title: Re: What determines PSU wattage?
Post by: SL4P on March 16, 2016, 11:37:10 pm
What I'm trying to understand is if you have two switching power supplies from the same company and both have the same voltage, what components will allow one to only put out say 10 amps and the other to put out 100 amps?  I'm guessing you will need bigger caps, better wiring, better heat dissipation, etc., to allow for more amps, but what determines/sets the output difference?
Yes, bigger caps, bigger coils, more/bigger switching transistors, bigger rectifiers, better cooling, etc.
The maximum wattage is usually limited by thermals. Rated wattage is the point at which the supply can safely operate without overheating.
This may also help you see why higher rated power supplies have fans blowing across or through the heat sinks, but this still depends on the ability to remove heat from the heat sink! - for a given 'close to rated output', the psu will 'last longer' in a cool environment than a hot one!