# power consumption calculation..

Status
Not open for further replies.

#### TKS

##### New Member
i'm building an device and i need to calculate the power consumption of it.

I know that it has one device wich uses 200ma.

Does this mean that this device consumpes 200ma per hour if i have it connected during one hour?

long time ago!

i use 4 batteries in series! each 950mah does this mean that the device could be on for 950/200 = 4.75hours?

Tks

#### warrenob1000

##### New Member
doesn't P = IV?

P = (0.2 A) x voltage

so if V = 5V, then

P = 1 W

A 950 mah battery theoretically should run a 200mA circuit for 4.75 h.

#### TKS

##### New Member
oww yes its right

Sow i use 5 mini penlight they have each 1,2volts @ 950mah..

in series means... 5*1.2= 6 volts... an 7805 will make it to 5volts....

sow i have 6x950 = 5.05watts...

How can i calculate when the voltage drops?

because the devices connected need 5volts...

Tks

#### Nigel Goodwin

##### Super Moderator
Re: oww yes its right

TKS said:
Sow i use 5 mini penlight they have each 1,2volts @ 950mah..

in series means... 5*1.2= 6 volts... an 7805 will make it to 5volts....

sow i have 6x950 = 5.05watts...

How can i calculate when the voltage drops?

because the devices connected need 5volts...
You can't use a 7805, it requires a much higher voltage input for the regulation to work. Assuming this is for powering a PIC based circuit?, just use 4 x 1.2V cells to give 4.8V - PIC's work fine like this!.

#### TKS

##### New Member
mhh tis true the datasheets states that it needs an minimum voltage of 7volts to work....

The problem is i also need to connect the scanner to the batteries..how could i do that?

if i use the voltage reg i need one wich closes if the voltage becomes down the 7 volts (else i would blow the pic / display / scanner..

Right?

Tks

come on guys there is an solution right for this?

how to regulate to 5 volts from an higher source..

my goal is that the device will work on the batteries....
and if there is no problems.. when it gets out of batteries etc.

Tks

http://www.st.com/stonline/products/literature/ds/2142/l4941.pdf

#### Nigel Goodwin

##### Super Moderator
TKS said:
mhh tis true the datasheets states that it needs an minimum voltage of 7volts to work....

The problem is i also need to connect the scanner to the batteries..how could i do that?
What does the scanner require?, if it's 5V then it will most probably be fine on 4.8V - try it and see!.

#### TKS

##### New Member
nigel

but what if the voltage drops?

it are batteries sow when the voltage drops to 4,2 volt?

maybe i don't get the purpose of the batteries right...

i thought that while you are using them they give an lower voltage??

i wanted to hook up for 3hrs using..including display led i need 400ma i estimate..

150 display and led
200 scanner
50 pic etc.

#### Nigel Goodwin

##### Super Moderator
Re: nigel

TKS said:
but what if the voltage drops?

it are batteries sow when the voltage drops to 4,2 volt?

maybe i don't get the purpose of the batteries right...

i thought that while you are using them they give an lower voltage??

i wanted to hook up for 3hrs using..including display led i need 400ma i estimate..

150 display and led
200 scanner
50 pic etc.
Try it and see! - BTW, if your PIC is drawing 50mA then there's something SERIOUSLY wrong 8)

NiCd and NiMh drop off fairly gradually, then suddenly drop at the end.

#### TKS

##### New Member
etc..

means an RS232 to!

have ordered some MAX639 ones...

hope they will do the job....

don't have the stuff yet to try it!

but i need it working before 30dec..

sow i need non time consuming solutions...

We will look and try..

Tks

#### ljcox

##### Well-Known Member
You can buy low dropout voltage regualtors such as the LP2950/LP2951.

There is a 5 V one - LP2950-5.0.

A quick scan of the data sheet indicated that it has a maximum dropout of 150 mV.

So a 6V supply should be adequate.

#### Oznog

##### Active Member
TKS said:
I know that it has one device wich uses 200ma.

Does this mean that this device consumpes 200ma per hour if i have it connected during one hour?
This depends. Sometimes they spec based on the max a device can pull for even a brief moment. This may be many times its average power consumption. In many devices the average power consumption depends on how the device is being used so it is difficult for the spec to provide a simple answer. However usually the spec sheet provides hints as to what it means. When in doubt you'll probably just want to take the current consumption figure provided at its face value.

200mA is a rate current flows at. 1 AH ("one amp-hour", be sure you do not read as "one amp per hour") is a current capacity. The correct terminology would be a 200mA load consumes 200mAH off the battery in one hour.

Status
Not open for further replies.