Author Topic: How to calculate total amps required for various voltages?  (Read 1202 times)

0 Members and 1 Guest are viewing this topic.

Offline shaiTopic starter

  • Contributor
  • Posts: 20
  • Country: us
How to calculate total amps required for various voltages?
« on: January 19, 2019, 08:50:15 am »
Say I have 12V for power, but I have a PCB with loads that use 3v, 5v and 12v. How do I figure out the total amps all the loads consume? Do I do 3V x amps = watts and then add up all the watts and divide by 12v to figure out the total amps? Is watts used as a common denominator? Thanks!
 

Offline Rerouter

  • Super Contributor
  • ***
  • Posts: 4694
  • Country: au
  • Question Everything... Except This Statement
Re: How to calculate total amps required for various voltages?
« Reply #1 on: January 19, 2019, 09:03:46 am »
in this case it would be (3V x amps) / efficiency = input watts, then input watts / 12 = input amps.

your probably using switchmode regulators, but they are not 100% efficient so they will pull a bit extra than just the load watts would say.
 

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 12806
Re: How to calculate total amps required for various voltages?
« Reply #2 on: January 19, 2019, 02:05:38 pm »
If there are any linear regulators, their input current is their output current + their quiescent current (usually negligible).  The difference between their input and output powers (calculated from V*I) is thrown away as heat.
 

Offline shaiTopic starter

  • Contributor
  • Posts: 20
  • Country: us
Re: How to calculate total amps required for various voltages?
« Reply #3 on: January 20, 2019, 12:02:12 am »
Hi, thank you for the help, but I am confused what you mean by efficiency? I'm using a linear regulator, specifically the LM1117. Can you please post an example calculation on how I can figure out how much 12V amps I need to feed the PCB if I have a 12V 2A component and a 3V 0.5A component?
 

Offline sleemanj

  • Super Contributor
  • ***
  • Posts: 3020
  • Country: nz
  • Professional tightwad.
    • The electronics hobby components I sell.
Re: How to calculate total amps required for various voltages?
« Reply #4 on: January 20, 2019, 12:05:57 am »
Since you are only using linear regulators, just sum all three (load) currents.  Input current = output current for a linear regulator, ignoring quiescent.

« Last Edit: January 20, 2019, 12:07:33 am by sleemanj »
~~~
EEVBlog Members - get yourself 10% discount off all my electronic components for sale just use the Buy Direct links and use Coupon Code "eevblog" during checkout.  Shipping from New Zealand, international orders welcome :-)
 

Offline Rerouter

  • Super Contributor
  • ***
  • Posts: 4694
  • Country: au
  • Question Everything... Except This Statement
Re: How to calculate total amps required for various voltages?
« Reply #5 on: January 20, 2019, 12:07:12 am »
Linear regulators essentially burn off the voltage difference as heat, so a 0.5A load on 3V draws 0.5A at 12V, and burns 4.5W of heat in the process.

Be aware this will put your regulator into thermal shutdown if your not careful, that is a lot of heat.
 

Offline james_s

  • Super Contributor
  • ***
  • Posts: 21611
  • Country: us
Re: How to calculate total amps required for various voltages?
« Reply #6 on: January 20, 2019, 04:17:16 am »
Think of linear regulators as automatic series resistors that adjust their resistance as needed to maintain the specified output voltage into whatever load that is on it. The current through any point in a series circuit is the same, therefore the current at the input of a linear regulator must equal the current drawn from the output.
 

Offline bson

  • Supporter
  • ****
  • Posts: 2265
  • Country: us
Re: How to calculate total amps required for various voltages?
« Reply #7 on: January 20, 2019, 07:19:25 am »
Hi, thank you for the help, but I am confused what you mean by efficiency? I'm using a linear regulator, specifically the LM1117. Can you please post an example calculation on how I can figure out how much 12V amps I need to feed the PCB if I have a 12V 2A component and a 3V 0.5A component?
Currents add regardless of voltage, so you need 2+0.5 = 2.5A.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf