# Ampere requirements [duplicate]

I read question from someone who was having monitor with 90 W power, 19 V and 3.32 A rating. When calculated using volt and power the amp came out to be 4.74 A. I didn't understand why is there requirement to find amp rating. If we consider 3.32 A as a minimum then we can take any ampere rating higher than 3.32 A.

Is there any need of any calculation? By reading the posts what I understood is that there is no harm in having a higher amp rating as power supply will draw only the required and discard the rest.

## marked as duplicate by winny, laptop2d, Finbarr, Dwayne Reid, Edgar BrownJan 4 at 17:53

• If you have a power supply and draw less power than it can provide that is just fine. The extra power (or current) capability is NOT discarded, it is simply not used. – Jack Creasey Dec 25 '18 at 15:57
• Are the numbers quoted from the label on the monitor, or on the power supply ? – peufeu Dec 25 '18 at 17:04
• Is it a coincidence that the figures of 90 watts, 3.32 amps and 19 volts differ with $\sqrt{2}$? The only important thing is, if you use a power supply that delivers much more power, to protect the circuit in case of a fault, preferably with a suitable fuse or similar. – Tom Kuschel Dec 25 '18 at 17:04
• One other thing to note with overpowered power supplies, particularly switching regulators, is that their efficiency can suffer greatly at smaller load. A converter marketed at 92% efficiency likely has an efficiency curve in the datasheet that shows it only operates at 92% at rated current. PWM tends to lose efficiency at lower duty cycle due to switching losses, and PFM tends to have a flatter efficiency curve, sometimes with a spike at the high end. It's a good idea to check the efficiency of the power supply you want to use at the indended load. – K H Dec 25 '18 at 20:13
• Thank You Tom Kuschel , Jack Creasey , peufeu , Satish and KH ....Please see my question below .It is more related to ampere. Hello peufeu - it is a doubt that i got after I was going through someone else's post.I was doing some research related to current rating – Mohit Aggarwal Dec 25 '18 at 22:22

Over rated power supply for an under-rated load will effect in following ways

1. Efficiency -

most ac to dc converters now are SMPS devices... whose efficiency will vary based on load. Since they always designed at maximum efficiency for Rated loads.

2. Price

The price will increase with increase in rated current at constant voltage.

3. Safety

In case of failure ... over rated power supplies will damage load more extent .. if the load don't have internal protection...

most of the power supplies will shutdown at over load conditions.

If you use over rated power supply then it will not shutdown till it achieves its over current conditions.. but the load may get damages at that current.

But, it will work, you can use it, if the above cases are not an issue.

• Hello Everyone , My question was less related to power and more related to ampere..If I keep the power constant and exactly what I require .Also if I keep the voltage also constant and exactly what I require. In that case if I choose a high ampere rating supply then what happens? Will that cause harm to the equipment or is a fuse required? The reason I asked is because by my understanding we can choose any higher ampere rating supply then what is required. So the extra current won't be used But then why do I require a fuse ? – Mohit Aggarwal Dec 25 '18 at 21:47
• If you use an high current rated supply for a low current device, in the case of any fault occurs... the device or some of its sub-systems will draw more current and damages...since the power supply will not restricting the power or shutting down... yes, you need a fuse or an controllable power supply. – Satish Singupuram Dec 25 '18 at 21:50
• In that case how should we decide the ampere rating of a supply if we don't know which I was searching for a very long time.And based on whatever ampere rating we select what fuse should we use(ampere). It would be wonderful if someone can explain by an example. – Mohit Aggarwal Dec 25 '18 at 21:55