# power supply out put and total consumption in a switch.

Dec 16th, 2008
• Bronze, 100 points or more

Hi everybody!

Let say we have a Modular switch.I just make up the values to make my point.

The power supply out put= 100 watt

The total consumption in switch=70 watt

So what will happen to rest of 30 watt? Where did it go?

thanks a lot!

Hello Sarah,

let's keep it simple:

a combined chassis cannot stay with a single working Power supply

If you insert a single power supply and then set the switch to combined mode, the switch displays this error message:

Insufficient power supplies present for specified configuration.

a redundant yes

So the threshold is lower for redundant design: to be able to provide the power needed in case of failure of the other supply no more then 50% of max power can be used by chassis+linecards+POE (if used)

with combined the threshold is higher 83% but if one power supply fails something like a linecard has to be powered down

Hope to help

Giuseppe

Sarah

When using fixed wattage you don't need to match the switch power consumption exactly to the power input. What you do need to do is make sure that the power supply wattage is greater than the switch configuration needs.

So if the switch needs 10watts and your power supply is 100watts that doesn't mean the module in the switch will be fried, it will still only get 10Watts. It just means the switch draws 100watts from the power supply whether it needs it or not.

Variable power supplies only draw what they need.

Jon

Hello Sarah,

yes from a practical point of view it is so

Hope to help

Giuseppe

Overall Rating: 5 (3 ratings)

## Replies

Giuseppe Larosa Tue, 12/16/2008 - 13:14
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Hello Sarah,

a power supply can provide up to 100W at a given voltage level this means that a current is received by modules

P= V*I

if you use less power means the Power supply is sending a lower current to the modules

that 30W becomes available power: how much margin there is.

In real world power consumption is not constant and for example when a system is powered up it consumes more at the beginning during start time.

the show power or other show commands can give you this information.

then there is the Power over ethernet and you can find a way to consume the extra power by adding some phones ...or by inserting other linecards.

You need to check available power to understand if you can add an additional module.

Data sheets provide power consumption of each linecard

Hope to help

Giuseppe

sarahr202 Tue, 12/16/2008 - 14:21
• Bronze, 100 points or more

Thanks a lot Giuseppe!

"a power supply can provide up to 100W at a given voltage level "

I just quoted you above for easy reference.

It means power supply acts as variable power supply with max power =100 watt.

if there is one module in switch , requiring 10 watt, then power supply 10 watt, as we increase the load 1.e number of modules. power supply changes its output accordingly.

Say we add one more module which requires 10 watt, now the power supplly will supply 20 watt . But it can provide up to 100 watt at maximum.

Am I correct ?

Giuseppe Larosa Tue, 12/16/2008 - 14:25
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Founding Member

Hello Sarah,

yes from a practical point of view it is so

Hope to help

Giuseppe

sarahr202 Tue, 12/16/2008 - 18:46
• Bronze, 100 points or more

Thanks a lot Giuseppe!

I was going through the link on 4500' power supplies,forwarded by Edison.

I find 4500 switch supports two types of power supplies:

1) fixed wattage

2) variable wattage.

If 4500 switch uses fixed wattage power supply,we need to make sure the total consumption= power supply's wattage.

If fixed power supply' output=100 watt, and we have only one module which requires only 10 watt, then module will get toasted ,as power supply is providing fixed constant wattage .

Am i correct?

thanks a lot!

Jon Marshall Wed, 12/17/2008 - 05:54
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2017 LAN, WAN

Sarah

When using fixed wattage you don't need to match the switch power consumption exactly to the power input. What you do need to do is make sure that the power supply wattage is greater than the switch configuration needs.

So if the switch needs 10watts and your power supply is 100watts that doesn't mean the module in the switch will be fried, it will still only get 10Watts. It just means the switch draws 100watts from the power supply whether it needs it or not.

Variable power supplies only draw what they need.

Jon

sarahr202 Wed, 12/17/2008 - 11:56
• Bronze, 100 points or more

Just one more question about redundant mode in 6500 switch

"When running in Redundant Mode, each power supply provides approximately 50% of its capacity to the chassis."

I am little confused about" 50 percent of its capacity" phrase in the excerpt.

Let say i have power supplies with max output given below:

p1=100 watt

p2=100 watt

Would p1 and p2 supply 50%of their total capacity i.e

p1=50

p2=50,

if yes, the combined output is way too much than load on switch p1+P2>load or 100> 50.

OR

p1 will supply 50 % of total load wattage i.e p1=25 watts

p2 ill supply 50%of total load wattage=25,

thanks a lot!

Giuseppe Larosa Wed, 12/17/2008 - 12:45
• Super Silver, 17500 points or more
• Hall of Fame,

Founding Member

Hello Sarah,

let's keep it simple:

a combined chassis cannot stay with a single working Power supply

If you insert a single power supply and then set the switch to combined mode, the switch displays this error message:

Insufficient power supplies present for specified configuration.

a redundant yes

So the threshold is lower for redundant design: to be able to provide the power needed in case of failure of the other supply no more then 50% of max power can be used by chassis+linecards+POE (if used)

with combined the threshold is higher 83% but if one power supply fails something like a linecard has to be powered down

Hope to help

Giuseppe